Add OpenXR bring-up: render the game as a floating quad in an immersive Quest session

- New android/app/src/main/cpp/xr_session.c (+.h): owns the OpenXR
  instance/session/local-space/swapchain and the per-frame
  xrWaitFrame/xrBeginFrame/xrEndFrame loop, submitting the game's
  existing flat render as a single head-tracked XrCompositionLayerQuad.
  No stereo rendering or controller input yet - that's steps B/C/D of
  the plan.
- 11-android-openxr-cmake.patch: links the OpenXR loader as a build
  dependency, staged into jniLibs the same way as gl4es/SDL2.
  build-image/Dockerfile and prepare-android-project.sh build and stage
  it.
- 12-android-openxr-present.patch: redirects OpenGL.cc's final present
  step (opengl_swap_and_restore / SDLDraw's software path) into the XR
  swapchain FBO instead of the window, on Android only. Two real gl4es
  bugs had to be worked around to get pixels on screen at all:
    - gl4es's own glBindFramebuffer errors on an FBO id it didn't create
      itself, even though the real driver-level bind succeeds - fixed by
      creating the swapchain FBO container via gl4es's own
      glGenFramebuffers/glBindFramebuffer, and only using the real
      (dlsym'd) driver call for attaching OpenXR's foreign swapchain
      texture, which gl4es's own attach can't handle.
    - gl4es's glBegin/glVertexAttrib/glVertex3f immediate-mode emulation
      only captures a fresh per-vertex value for attribute 0 (position,
      driven directly by glVertex3f) - custom attributes like this
      shader's texcoords/light are GLES2's *constant*-attribute API and
      applied once for the whole draw, not per vertex, silently
      collapsing the UI-overlay quad to a single sampled texel. Fixed by
      switching that one draw call to real vertex arrays
      (glVertexAttribPointer/glDrawArrays).
  Also flips the V texcoord to match SDL's top-down row order against
  GL's texture convention, and reuses a persistent texture object
  instead of a fresh gen/upload/delete every frame.
- AndroidManifest.xml: declares the immersive-HMD intent category and
  focus-aware metadata, drops the 2D-panel layout hint.
- README: documents the new OpenXR immersive mode.

README.md

@@ -6,6 +6,16 @@ An open source project to play the classic 1994 System Shock on a VR
 headset, built on top of [Shockolate](https://github.com/Interrupt/systemshock),
 a cross-platform port of the original game.
 
+## Design principles
+
+- **Standalone-headset-only.** The game must run entirely on the headset
+  itself - no PC required, whether tethered or streamed (not PCVR). Any
+  feature or dependency that assumes a host PC is out of scope.
+- **OpenXR-first.** Favor OpenXR-standard APIs over vendor-specific ones
+  (e.g. Meta's Oculus Mobile SDK) wherever there's a choice, so the port
+  isn't locked to Meta Quest and can work across other standalone,
+  Android-based OpenXR headsets (e.g. Pico) too.
+
 ## Layout
 
 - `engine/` - a vendored snapshot of the Shockolate engine source. Built
@@ -88,11 +98,14 @@ resulting tarball to dl.ladkau.de.
 
 ## Playing on Meta Quest
 
-`make apk` builds `dist/questshock-<version>-android-arm64.apk` - a plain
-(non-VR) Android app that runs as a flat, floating panel in the Quest's
-Home environment, same as any other sideloaded Android app. It's not a
-head-tracked 6DoF VR port (that's a much larger, separate undertaking);
-play with a Bluetooth mouse/keyboard connected to the headset.
+`make apk` builds `dist/questshock-<version>-android-arm64.apk` - an
+immersive OpenXR app (see `android/app/src/main/cpp/xr_session.c`): the
+game's own rendering is unchanged (still a flat, 2D render, no stereo 3D
+scene), but instead of running as a Home-hosted 2D panel it's now shown as
+a single head-tracked quad floating in front of the viewer in an otherwise
+empty space. There's no controller-driven interaction yet (that's ongoing
+work - a laser-pointer-driven menu and on-screen keyboard); for now, play
+with a Bluetooth mouse/keyboard connected to the headset same as before.
 
 1. Install the APK with [SideQuest](https://sidequestvr.com/) (or `adb
    install`).
@@ -121,7 +134,7 @@ To have Android Studio compile and deploy `android/` itself instead:
 ```
 
 This stages everything `make apk` normally stages (a scratch, patched
-copy of `engine/`; the Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es
+copy of `engine/`; the Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es/openxr
 prebuilts; bundled assets) - the same as `build-apk.sh`'s own prep step -
 except it writes `android/engine.properties` with paths that resolve on
 your host filesystem, and additionally exports the prebuilt libraries
@@ -151,6 +164,10 @@ The Quest build also bundles [GL4ES](https://github.com/ptitSeb/gl4es)
 image), which translates the engine's desktop-style OpenGL calls into
 GLES/EGL and is MIT-licensed.
 
+It also bundles the Khronos Group's own [OpenXR-SDK loader](
+https://github.com/KhronosGroup/OpenXR-SDK) (`lib/arm64-v8a/libopenxr_loader.so`,
+prebuilt unmodified into the build image), which is Apache 2.0-licensed.
+
 The vendored engine snapshot in `engine/` is
 [Shockolate](https://github.com/Interrupt/systemshock), which is licensed
 under the **GNU GPLv3** (see `engine/LICENSE`) - it is included unchanged

android/app/build.gradle

@@ -115,15 +115,23 @@ android {
                 // build-image/Dockerfile's ANDROID_16KB_LDFLAGS, which does
                 // the same for the prebuilt SDL2/SDL2_mixer/fluidsynth-lite/
                 // gl4es .so's this links against.
+                // ANDROID_EXTRA_SOURCES is a semicolon-separated CMake list,
+                // not shell-split - AGP passes each "arguments" string
+                // straight through as one argv element to cmake, so the
+                // ';' below survives intact. ANDROID_EXTRA_INCLUDE_DIRS lets
+                // engine/'s own OpenGL.cc find "xr_session.h" (see
+                // android/engine-patches/11-android-openxr-cmake.patch,
+                // which adds it to include_directories()).
                 arguments "-DENABLE_SDL2=ON", "-DENABLE_SOUND=BUNDLED", "-DENABLE_FLUIDSYNTH=BUNDLED", \
                     "-DANDROID_PREBUILT_DIR=${prebuiltDir}", \
+                    "-DANDROID_EXTRA_INCLUDE_DIRS=${projectDir}/src/main/cpp", \
                     "-DCMAKE_PREFIX_PATH=${prebuiltDir}/sdl2", \
                     "-DCMAKE_FIND_ROOT_PATH=${prebuiltDir}/sdl2", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=BOTH", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=BOTH", \
                     "-DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=BOTH", \
                     "-DCMAKE_SHARED_LINKER_FLAGS=-Wl,-z,max-page-size=16384", \
-                    "-DANDROID_EXTRA_SOURCES=${projectDir}/src/main/cpp/questshock_native.c"
+                    "-DANDROID_EXTRA_SOURCES=${projectDir}/src/main/cpp/questshock_native.c;${projectDir}/src/main/cpp/xr_session.c"
                 abiFilters 'arm64-v8a'
             }
         }

android/app/src/main/AndroidManifest.xml

@@ -12,6 +12,11 @@
     <!-- External mouse input events -->
     <uses-feature android:name="android.hardware.type.pc" android:required="false" />
 
+    <!-- Immersive OpenXR app now (see android/app/src/main/cpp/xr_session.c),
+         not a 2D Home panel - vr.headtracking is what actually declares that
+         to Horizon OS. -->
+    <uses-feature android:name="android.hardware.vr.headtracking" android:version="1" android:required="true" />
+
     <!-- Plain, unrestricted /sdcard access for the res/data,res/sound the
          user drops in and the shaders/soundfont this app extracts there on
          first run - see res/GET_ASSETS_QUEST.txt and QuestShockActivity.
@@ -28,6 +33,11 @@
         android:theme="@android:style/Theme.NoTitleBar.Fullscreen"
         android:hardwareAccelerated="true" >
 
+        <!-- Tells Horizon OS this activity wants the compositor for itself
+             (immersive VR), not the Home 2D-panel path - required for an
+             OpenXR session to actually start. -->
+        <meta-data android:name="com.oculus.vr.focusaware" android:value="true" />
+
         <activity android:name="de.ladkau.questshock.QuestShockActivity"
             android:label="@string/app_name"
             android:alwaysRetainTaskState="true"
@@ -40,23 +50,19 @@
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
+                <!-- The vendor-neutral, Khronos-defined way to declare an
+                     immersive OpenXR entry point (vs. e.g. Meta's own
+                     com.oculus.intent.category.VR) - see this project's
+                     "favor OpenXR" rule in the README. If Horizon OS still
+                     launches this as a flat panel on-device, the next thing
+                     to try is adding a com.oculus.supportedDevices meta-data
+                     listing the target Quest models - not confirmed
+                     necessary from static analysis alone. -->
+                <category android:name="org.khronos.openxr.intent.category.IMMERSIVE_HMD" />
             </intent-filter>
             <intent-filter>
                 <action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" />
             </intent-filter>
-            <!-- Without this, Quest's Home shell picks its own default 2D
-                 panel shape for this activity - a tall portrait rectangle,
-                 which crops a 4:3 landscape game like System Shock's classic
-                 640x480 down to a sliver. defaultWidth/defaultHeight (the
-                 standard Android multi-window sizing hint, which Quest's
-                 panel system honors for 2D apps) requests a properly
-                 landscape, 4:3-ish panel instead; minWidth/minHeight keep it
-                 from being resized below the game's native resolution. -->
-            <layout android:defaultWidth="1280dp"
-                android:defaultHeight="960dp"
-                android:minWidth="640dp"
-                android:minHeight="480dp"
-                android:gravity="center" />
         </activity>
     </application>
 

android/app/src/main/cpp/xr_session.c

@@ -0,0 +1,456 @@
+#include "xr_session.h"
+
+#include <dlfcn.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <android/log.h>
+#include <jni.h>
+
+#include <EGL/egl.h>
+#include <GLES3/gl3.h>
+
+#define XR_USE_PLATFORM_ANDROID 1
+#define XR_USE_GRAPHICS_API_OPENGL_ES 1
+#include <openxr/openxr.h>
+#include <openxr/openxr_platform.h>
+
+#include <SDL.h>
+
+#define TAG "QuestShock"
+#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, TAG, __VA_ARGS__)
+#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, TAG, __VA_ARGS__)
+
+// Fixed pose/size for the single game quad - 2m in front of the local space
+// origin, sized to preserve the game's own aspect ratio at a comfortable
+// visual angle. No per-eye view/projection work is needed for a quad layer
+// at all - the compositor handles reprojecting it per eye itself, which is
+// exactly why this milestone doesn't need xrLocateViews or any stereo
+// rendering (see the plan's step A notes).
+#define QUAD_DISTANCE_METERS 2.0f
+#define QUAD_WIDTH_METERS 1.6f
+
+// Up to this many swapchain images/FBOs - real runtimes report small counts
+// (2-4); this is just a fixed upper bound for the cache arrays below.
+#define MAX_SWAPCHAIN_IMAGES 8
+
+static XrInstance g_instance = XR_NULL_HANDLE;
+static XrSystemId g_system_id = XR_NULL_SYSTEM_ID;
+static XrSession g_session = XR_NULL_HANDLE;
+static XrSpace g_local_space = XR_NULL_HANDLE;
+static XrSessionState g_session_state = XR_SESSION_STATE_UNKNOWN;
+// Distinct from g_session_state - true from a successful xrBeginSession
+// until xrEndSession/loss/exit. Many runtimes only advance the *state*
+// (e.g. READY -> SYNCHRONIZED -> VISIBLE -> FOCUSED) in response to the
+// app actually calling xrWaitFrame/xrBeginFrame/xrEndFrame continuously
+// after xrBeginSession - gating frame submission on already having
+// reached SYNCHRONIZED+ (as this used to) is a chicken-and-egg deadlock,
+// since the runtime never progresses past READY without the frame loop
+// running in the first place. Frame calls just need the session to have
+// begun, per the standard OpenXR sample pattern - not any particular
+// later state.
+static bool g_session_running = false;
+
+static XrSwapchain g_game_swapchain = XR_NULL_HANDLE;
+static int g_game_width = 0;
+static int g_game_height = 0;
+static GLuint g_swapchain_fbos[MAX_SWAPCHAIN_IMAGES];
+static uint32_t g_swapchain_image_count = 0;
+
+static XrTime g_predicted_display_time = 0;
+static bool g_frame_should_render = false;
+static bool g_have_acquired_image = false;
+
+// This file's GL calls otherwise resolve to gl4es (the only GL symbol
+// provider linked into this binary - see android/engine-patches/
+// 02-android-opengl-es.patch), which is fine for anything shared with the
+// engine's own gl4es-routed rendering. But the swapchain images OpenXR
+// hands us are real driver texture objects gl4es never created itself,
+// and gl4es's own glFramebufferTexture2D can't attach a texture it has no
+// tracked metadata for. So the FBO *container* is created via gl4es's own
+// glGenFramebuffers/glBindFramebuffer (so gl4es recognizes the id as its
+// own and its own per-frame glBindFramebuffer succeeds), while the
+// texture-attach step - the specifically foreign part - goes through the
+// real driver directly, via dlsym against libGLESv2.so.
+typedef void (*PFNQSFRAMEBUFFERTEXTURE2D)(GLenum, GLenum, GLenum, GLuint, GLint);
+typedef GLenum (*PFNQSCHECKFRAMEBUFFERSTATUS)(GLenum);
+
+static PFNQSFRAMEBUFFERTEXTURE2D real_glFramebufferTexture2D;
+static PFNQSCHECKFRAMEBUFFERSTATUS real_glCheckFramebufferStatus;
+
+static bool xr_load_real_gles(void) {
+    void *lib = dlopen("libGLESv2.so", RTLD_NOW | RTLD_LOCAL);
+    if (lib == NULL) {
+        LOGE("XR: dlopen(libGLESv2.so) failed: %s", dlerror());
+        return false;
+    }
+    real_glFramebufferTexture2D = (PFNQSFRAMEBUFFERTEXTURE2D)dlsym(lib, "glFramebufferTexture2D");
+    real_glCheckFramebufferStatus =
+        (PFNQSCHECKFRAMEBUFFERSTATUS)dlsym(lib, "glCheckFramebufferStatus");
+    return real_glFramebufferTexture2D && real_glCheckFramebufferStatus;
+}
+
+static bool xr_check(XrResult result, const char *what) {
+    if (XR_SUCCEEDED(result))
+        return true;
+    char resultString[XR_MAX_RESULT_STRING_SIZE] = {0};
+    if (g_instance != XR_NULL_HANDLE)
+        xrResultToString(g_instance, result, resultString);
+    LOGE("XR: %s failed: %s (%d)", what, resultString[0] ? resultString : "?", (int)result);
+    return false;
+}
+
+static PFN_xrVoidFunction xr_get_proc(const char *name) {
+    PFN_xrVoidFunction fn = NULL;
+    // xrGetInstanceProcAddr itself works with a NULL instance for the
+    // handful of functions (like xrInitializeLoaderKHR) that must be called
+    // before an instance exists.
+    if (!xr_check(xrGetInstanceProcAddr(g_instance, name, &fn), name))
+        return NULL;
+    return fn;
+}
+
+// The Android OpenXR loader needs the JavaVM/context before anything else -
+// separate from XR_KHR_android_create_instance below, which only affects
+// xrCreateInstance itself. Skipping this is a common cause of
+// xrCreateInstance silently failing to find a runtime on Android.
+static bool xr_initialize_android_loader(JavaVM *vm, jobject activity) {
+    PFN_xrInitializeLoaderKHR initializeLoader =
+        (PFN_xrInitializeLoaderKHR)xr_get_proc("xrInitializeLoaderKHR");
+    if (initializeLoader == NULL) {
+        LOGE("XR: xrInitializeLoaderKHR not available - no Android OpenXR loader present?");
+        return false;
+    }
+
+    XrLoaderInitInfoAndroidKHR loaderInitInfo = {XR_TYPE_LOADER_INIT_INFO_ANDROID_KHR};
+    loaderInitInfo.applicationVM = vm;
+    loaderInitInfo.applicationContext = activity;
+    return xr_check(initializeLoader((const XrLoaderInitInfoBaseHeaderKHR *)&loaderInitInfo),
+                     "xrInitializeLoaderKHR");
+}
+
+// eglGetCurrentDisplay()/eglGetCurrentContext() hand us the display/context,
+// but not the EGLConfig they were created with - eglQueryContext's
+// EGL_CONFIG_ID plus eglChooseConfig is the standard way to recover it (the
+// same trick Khronos' own hello_xr sample uses).
+static EGLConfig xr_get_current_egl_config(EGLDisplay display, EGLContext context) {
+    EGLint configId = 0;
+    eglQueryContext(display, context, EGL_CONFIG_ID, &configId);
+
+    EGLint attribs[] = {EGL_CONFIG_ID, configId, EGL_NONE};
+    EGLConfig config = NULL;
+    EGLint numConfigs = 0;
+    eglChooseConfig(display, attribs, &config, 1, &numConfigs);
+    return config;
+}
+
+static bool xr_create_instance_and_session(int game_width, int game_height) {
+    JNIEnv *env = (JNIEnv *)SDL_AndroidGetJNIEnv();
+    jobject activity = (jobject)SDL_AndroidGetActivity();
+    if (env == NULL || activity == NULL) {
+        LOGE("XR: no JNIEnv/Activity from SDL - can't init OpenXR");
+        return false;
+    }
+    JavaVM *vm = NULL;
+    (*env)->GetJavaVM(env, &vm);
+
+    if (!xr_initialize_android_loader(vm, activity))
+        return false;
+
+    const char *extensions[] = {
+        XR_KHR_ANDROID_CREATE_INSTANCE_EXTENSION_NAME,
+        XR_KHR_OPENGL_ES_ENABLE_EXTENSION_NAME,
+    };
+
+    XrInstanceCreateInfoAndroidKHR androidInfo = {XR_TYPE_INSTANCE_CREATE_INFO_ANDROID_KHR};
+    androidInfo.applicationVM = vm;
+    androidInfo.applicationActivity = activity;
+
+    XrInstanceCreateInfo createInfo = {XR_TYPE_INSTANCE_CREATE_INFO};
+    createInfo.next = &androidInfo;
+    createInfo.enabledExtensionCount = sizeof(extensions) / sizeof(extensions[0]);
+    createInfo.enabledExtensionNames = extensions;
+    strncpy(createInfo.applicationInfo.applicationName, "QuestShock",
+            XR_MAX_APPLICATION_NAME_SIZE - 1);
+    strncpy(createInfo.applicationInfo.engineName, "Shockolate", XR_MAX_ENGINE_NAME_SIZE - 1);
+    createInfo.applicationInfo.applicationVersion = 1;
+    createInfo.applicationInfo.engineVersion = 1;
+    createInfo.applicationInfo.apiVersion = XR_CURRENT_API_VERSION;
+
+    if (!xr_check(xrCreateInstance(&createInfo, &g_instance), "xrCreateInstance"))
+        return false;
+
+    XrSystemGetInfo systemInfo = {XR_TYPE_SYSTEM_GET_INFO};
+    systemInfo.formFactor = XR_FORM_FACTOR_HEAD_MOUNTED_DISPLAY;
+    if (!xr_check(xrGetSystem(g_instance, &systemInfo, &g_system_id), "xrGetSystem"))
+        return false;
+
+    // Required by spec before creating an OpenGL ES-backed session, even
+    // though we don't gate on the returned min/max API version here.
+    PFN_xrGetOpenGLESGraphicsRequirementsKHR getGLESRequirements =
+        (PFN_xrGetOpenGLESGraphicsRequirementsKHR)xr_get_proc("xrGetOpenGLESGraphicsRequirementsKHR");
+    if (getGLESRequirements == NULL)
+        return false;
+    XrGraphicsRequirementsOpenGLESKHR glesRequirements = {XR_TYPE_GRAPHICS_REQUIREMENTS_OPENGL_ES_KHR};
+    if (!xr_check(getGLESRequirements(g_instance, g_system_id, &glesRequirements),
+                  "xrGetOpenGLESGraphicsRequirementsKHR"))
+        return false;
+
+    EGLDisplay display = eglGetCurrentDisplay();
+    EGLContext context = eglGetCurrentContext();
+    if (display == EGL_NO_DISPLAY || context == EGL_NO_CONTEXT) {
+        LOGE("XR: no current EGL display/context - call xr_init() after init_opengl()");
+        return false;
+    }
+
+    XrGraphicsBindingOpenGLESAndroidKHR graphicsBinding = {
+        XR_TYPE_GRAPHICS_BINDING_OPENGL_ES_ANDROID_KHR};
+    graphicsBinding.display = display;
+    graphicsBinding.config = xr_get_current_egl_config(display, context);
+    graphicsBinding.context = context;
+
+    XrSessionCreateInfo sessionInfo = {XR_TYPE_SESSION_CREATE_INFO};
+    sessionInfo.next = &graphicsBinding;
+    sessionInfo.systemId = g_system_id;
+    if (!xr_check(xrCreateSession(g_instance, &sessionInfo, &g_session), "xrCreateSession"))
+        return false;
+
+    XrReferenceSpaceCreateInfo spaceInfo = {XR_TYPE_REFERENCE_SPACE_CREATE_INFO};
+    spaceInfo.referenceSpaceType = XR_REFERENCE_SPACE_TYPE_LOCAL;
+    spaceInfo.poseInReferenceSpace.orientation.w = 1.0f;
+    if (!xr_check(xrCreateReferenceSpace(g_session, &spaceInfo, &g_local_space),
+                  "xrCreateReferenceSpace"))
+        return false;
+
+    // Prefer a plain linear 8-bit format over GL_SRGB8_ALPHA8: the source
+    // SDL surface pixels are already sRGB-encoded (as ordinary 8-bit image
+    // data conventionally is) and get uploaded/sampled as plain linear
+    // GL_RGBA with no decode step anywhere in this path, matching the
+    // desktop SDL_RenderCopy path this replaces - an sRGB swapchain format
+    // would auto-gamma-encode on write and double-encode already-encoded
+    // data.
+    uint32_t formatCount = 0;
+    xrEnumerateSwapchainFormats(g_session, 0, &formatCount, NULL);
+    int64_t *formats = (int64_t *)malloc(sizeof(int64_t) * formatCount);
+    xrEnumerateSwapchainFormats(g_session, formatCount, &formatCount, formats);
+    int64_t chosenFormat = formats[0];
+    for (uint32_t i = 0; i < formatCount; i++) {
+        LOGI("XR: swapchain format[%u] = 0x%llx", i, (unsigned long long)formats[i]);
+        if (formats[i] == GL_RGBA8) {
+            chosenFormat = GL_RGBA8;
+            break;
+        }
+    }
+    free(formats);
+
+    XrSwapchainCreateInfo swapchainInfo = {XR_TYPE_SWAPCHAIN_CREATE_INFO};
+    swapchainInfo.usageFlags = XR_SWAPCHAIN_USAGE_COLOR_ATTACHMENT_BIT | XR_SWAPCHAIN_USAGE_SAMPLED_BIT;
+    swapchainInfo.format = chosenFormat;
+    swapchainInfo.sampleCount = 1;
+    swapchainInfo.width = (uint32_t)game_width;
+    swapchainInfo.height = (uint32_t)game_height;
+    swapchainInfo.faceCount = 1;
+    swapchainInfo.arraySize = 1;
+    swapchainInfo.mipCount = 1;
+    if (!xr_check(xrCreateSwapchain(g_session, &swapchainInfo, &g_game_swapchain),
+                  "xrCreateSwapchain"))
+        return false;
+    g_game_width = game_width;
+    g_game_height = game_height;
+
+    uint32_t imageCount = 0;
+    xrEnumerateSwapchainImages(g_game_swapchain, 0, &imageCount, NULL);
+    if (imageCount > MAX_SWAPCHAIN_IMAGES) {
+        LOGE("XR: swapchain reports %u images, only room for %d", imageCount,
+             MAX_SWAPCHAIN_IMAGES);
+        return false;
+    }
+    // Zero-initialized, not just `.type` set per element - these structs
+    // also carry a `next` field the runtime may read, and an
+    // uninitialized stack array would leave it as garbage.
+    XrSwapchainImageOpenGLESKHR images[MAX_SWAPCHAIN_IMAGES] = {0};
+    for (uint32_t i = 0; i < imageCount; i++)
+        images[i].type = XR_TYPE_SWAPCHAIN_IMAGE_OPENGL_ES_KHR;
+    if (!xr_check(xrEnumerateSwapchainImages(g_game_swapchain, imageCount, &imageCount,
+                                              (XrSwapchainImageBaseHeader *)images),
+                  "xrEnumerateSwapchainImages"))
+        return false;
+    g_swapchain_image_count = imageCount;
+
+    if (!xr_load_real_gles()) {
+        LOGE("XR: couldn't resolve real GLES FBO functions via dlsym");
+        return false;
+    }
+
+    // Wraps each swapchain-provided texture in its own framebuffer, matching
+    // how OpenGL.cc's own CreateFrameBuffer() wraps backupBuffer - just
+    // without a depth/stencil attachment, since the final composite draw
+    // (see opengl_swap_and_restore) never needs one. The Gen/Bind calls are
+    // gl4es's own (linked, not dlsym'd) - see the comment above
+    // real_glFramebufferTexture2D for why.
+    bool all_complete = true;
+    for (uint32_t i = 0; i < imageCount; i++) {
+        glGenFramebuffers(1, &g_swapchain_fbos[i]);
+        glBindFramebuffer(GL_FRAMEBUFFER, g_swapchain_fbos[i]);
+        real_glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D,
+                                     images[i].image, 0);
+        GLenum status = real_glCheckFramebufferStatus(GL_FRAMEBUFFER);
+        if (status != GL_FRAMEBUFFER_COMPLETE) {
+            LOGE("XR: swapchain FBO %u incomplete: 0x%x", i, status);
+            all_complete = false;
+        }
+    }
+    glBindFramebuffer(GL_FRAMEBUFFER, 0);
+    if (!all_complete)
+        return false;
+
+    LOGI("XR: instance/session/swapchain ready (%dx%d, %u images)", game_width, game_height,
+         imageCount);
+    return true;
+}
+
+bool xr_init(int game_width, int game_height) {
+    if (!xr_create_instance_and_session(game_width, game_height)) {
+        LOGE("XR: bring-up failed - falling back to the normal window present path");
+        xr_shutdown();
+        return false;
+    }
+    return true;
+}
+
+void xr_poll_events(void) {
+    if (g_instance == XR_NULL_HANDLE)
+        return;
+
+    while (true) {
+        XrEventDataBuffer event = {XR_TYPE_EVENT_DATA_BUFFER};
+        XrResult result = xrPollEvent(g_instance, &event);
+        if (result == XR_EVENT_UNAVAILABLE)
+            break;
+        if (!xr_check(result, "xrPollEvent"))
+            break;
+
+        if (event.type == XR_TYPE_EVENT_DATA_SESSION_STATE_CHANGED) {
+            XrEventDataSessionStateChanged *stateEvent = (XrEventDataSessionStateChanged *)&event;
+            g_session_state = stateEvent->state;
+            LOGI("XR: session state -> %d", (int)g_session_state);
+
+            if (g_session_state == XR_SESSION_STATE_READY) {
+                XrSessionBeginInfo beginInfo = {XR_TYPE_SESSION_BEGIN_INFO};
+                beginInfo.primaryViewConfigurationType = XR_VIEW_CONFIGURATION_TYPE_PRIMARY_STEREO;
+                g_session_running = xr_check(xrBeginSession(g_session, &beginInfo), "xrBeginSession");
+            } else if (g_session_state == XR_SESSION_STATE_STOPPING) {
+                xr_check(xrEndSession(g_session), "xrEndSession");
+                g_session_running = false;
+            } else if (g_session_state == XR_SESSION_STATE_EXITING ||
+                       g_session_state == XR_SESSION_STATE_LOSS_PENDING) {
+                g_session_running = false;
+            }
+        }
+    }
+}
+
+bool xr_is_session_running(void) { return g_session_running; }
+
+bool xr_frame_begin(void) {
+    g_have_acquired_image = false;
+    if (!xr_is_session_running())
+        return false;
+
+    XrFrameWaitInfo waitInfo = {XR_TYPE_FRAME_WAIT_INFO};
+    XrFrameState frameState = {XR_TYPE_FRAME_STATE};
+    if (!xr_check(xrWaitFrame(g_session, &waitInfo, &frameState), "xrWaitFrame"))
+        return false;
+    g_predicted_display_time = frameState.predictedDisplayTime;
+    g_frame_should_render = frameState.shouldRender;
+
+    XrFrameBeginInfo beginInfo = {XR_TYPE_FRAME_BEGIN_INFO};
+    if (!xr_check(xrBeginFrame(g_session, &beginInfo), "xrBeginFrame"))
+        return false;
+
+    if (!g_frame_should_render)
+        return false;
+
+    uint32_t imageIndex = 0;
+    XrSwapchainImageAcquireInfo acquireInfo = {XR_TYPE_SWAPCHAIN_IMAGE_ACQUIRE_INFO};
+    if (!xr_check(xrAcquireSwapchainImage(g_game_swapchain, &acquireInfo, &imageIndex),
+                  "xrAcquireSwapchainImage"))
+        return false;
+
+    XrSwapchainImageWaitInfo waitImageInfo = {XR_TYPE_SWAPCHAIN_IMAGE_WAIT_INFO};
+    waitImageInfo.timeout = XR_INFINITE_DURATION;
+    if (!xr_check(xrWaitSwapchainImage(g_game_swapchain, &waitImageInfo), "xrWaitSwapchainImage"))
+        return false;
+
+    // gl4es's own (linked, not dlsym'd) bind - this targets an FBO id
+    // gl4es itself created (see xr_create_instance_and_session()), so its
+    // own "current FBO" bookkeeping updates correctly and its immediate-
+    // mode draw calls in android_draw_surface_as_quad() land in the right
+    // place.
+    glBindFramebuffer(GL_FRAMEBUFFER, g_swapchain_fbos[imageIndex]);
+    glViewport(0, 0, g_game_width, g_game_height);
+
+    g_have_acquired_image = true;
+    return true;
+}
+
+void xr_frame_end(void) {
+    if (g_session == XR_NULL_HANDLE)
+        return;
+
+    if (g_have_acquired_image) {
+        XrSwapchainImageReleaseInfo releaseInfo = {XR_TYPE_SWAPCHAIN_IMAGE_RELEASE_INFO};
+        xr_check(xrReleaseSwapchainImage(g_game_swapchain, &releaseInfo),
+                 "xrReleaseSwapchainImage");
+    }
+
+    if (!xr_is_session_running())
+        return;
+
+    XrCompositionLayerQuad quad = {XR_TYPE_COMPOSITION_LAYER_QUAD};
+    quad.space = g_local_space;
+    quad.eyeVisibility = XR_EYE_VISIBILITY_BOTH;
+    quad.subImage.swapchain = g_game_swapchain;
+    quad.subImage.imageRect.extent.width = g_game_width;
+    quad.subImage.imageRect.extent.height = g_game_height;
+    quad.pose.orientation.w = 1.0f;
+    quad.pose.position.z = -QUAD_DISTANCE_METERS;
+    quad.size.width = QUAD_WIDTH_METERS;
+    quad.size.height = QUAD_WIDTH_METERS * (float)g_game_height / (float)g_game_width;
+
+    const XrCompositionLayerBaseHeader *layers[1] = {(XrCompositionLayerBaseHeader *)&quad};
+
+    XrFrameEndInfo endInfo = {XR_TYPE_FRAME_END_INFO};
+    endInfo.displayTime = g_predicted_display_time;
+    endInfo.environmentBlendMode = XR_ENVIRONMENT_BLEND_MODE_OPAQUE;
+    endInfo.layerCount = g_have_acquired_image ? 1 : 0;
+    endInfo.layers = g_have_acquired_image ? layers : NULL;
+    xr_check(xrEndFrame(g_session, &endInfo), "xrEndFrame");
+}
+
+void xr_shutdown(void) {
+    for (uint32_t i = 0; i < g_swapchain_image_count; i++) {
+        if (g_swapchain_fbos[i] != 0)
+            glDeleteFramebuffers(1, &g_swapchain_fbos[i]);
+    }
+    g_swapchain_image_count = 0;
+
+    if (g_game_swapchain != XR_NULL_HANDLE)
+        xrDestroySwapchain(g_game_swapchain);
+    g_game_swapchain = XR_NULL_HANDLE;
+
+    if (g_local_space != XR_NULL_HANDLE)
+        xrDestroySpace(g_local_space);
+    g_local_space = XR_NULL_HANDLE;
+
+    if (g_session != XR_NULL_HANDLE)
+        xrDestroySession(g_session);
+    g_session = XR_NULL_HANDLE;
+
+    if (g_instance != XR_NULL_HANDLE)
+        xrDestroyInstance(g_instance);
+    g_instance = XR_NULL_HANDLE;
+
+    g_session_state = XR_SESSION_STATE_UNKNOWN;
+    g_session_running = false;
+}

android/app/src/main/cpp/xr_session.h

@@ -0,0 +1,56 @@
+// Minimal OpenXR bring-up for questshock's immersive Quest build. Keeps the
+// game's own rendering untouched (see android/engine-patches/
+// 12-android-openxr-present.patch) - this module only owns the OpenXR
+// instance/session/swapchain and the final "submit a quad layer instead of
+// presenting to a window" step. No stereo rendering, no controller input
+// yet (see the plan's steps B/C/D for those) - the game composite is shown
+// as a single flat quad floating in front of the viewer.
+#ifndef QUESTSHOCK_XR_SESSION_H
+#define QUESTSHOCK_XR_SESSION_H
+
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Call once, right after init_opengl() (OpenGL.cc) has created and made
+// current the GL context SDL/gl4es already use - creates the OpenXR
+// instance/session sharing that same EGL display/context, plus one
+// swapchain sized to the game's logical resolution (game_width/height, i.e.
+// grd_cap->w/h - see Shock.c's InitSDL()). Returns false if OpenXR bring-up
+// failed (e.g. no runtime installed) - callers should fall back to the
+// existing window-present path in that case.
+bool xr_init(int game_width, int game_height);
+
+// Pumps XR session-state events. Call once per frame, before
+// xr_frame_begin(). Must still be called even when xr_init() returned
+// false (no-op in that case).
+void xr_poll_events(void);
+
+// True once the session has reached a state where frames are expected
+// (XR_SESSION_STATE_SYNCHRONIZED or later) - i.e. it's safe/required to
+// start calling xr_frame_begin()/xr_frame_end() each frame.
+bool xr_is_session_running(void);
+
+// Begins the XR frame (xrWaitFrame/xrBeginFrame) and, if the runtime wants
+// this frame rendered, acquires the next swapchain image and binds its
+// framebuffer as the current render target - ready for the caller to draw
+// into exactly as it would have drawn to the default framebuffer. Returns
+// true if the caller should draw this frame; xr_frame_end() must be called
+// unconditionally afterward either way (a begun XR frame must always be
+// ended, rendered or not).
+bool xr_frame_begin(void);
+
+// Releases the swapchain image (if one was acquired this frame) and
+// submits it as a single XrCompositionLayerQuad positioned in front of the
+// local reference space's origin, then ends the XR frame.
+void xr_frame_end(void);
+
+void xr_shutdown(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

android/engine-patches/11-android-openxr-cmake.patch

@@ -0,0 +1,39 @@
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -64,6 +64,19 @@
+ 	endif(WIN32)
+ endif(ENABLE_OPENGL)
+ 
++# OpenXR loader (Android only for now - see android/app/src/main/cpp/
++# xr_session.c, which submits the game's existing render as a composition
++# layer instead of presenting to a normal window/surface). EGL is linked
++# explicitly too - xr_session.c calls eglGetCurrentDisplay/Context/
++# QueryContext/ChooseConfig directly (to share the GL context gl4es/SDL
++# already made), and unlike GL itself (where gl4es's own libGL.so already
++# provides every symbol the engine or xr_session.c call), there's no other
++# EGL provider in this link.
++if(ANDROID)
++	set(OPENXR_INCLUDE_DIRS ${ANDROID_PREBUILT_DIR}/openxr/include)
++	set(OPENXR_LIBRARIES ${ANDROID_PREBUILT_DIR}/openxr/lib/libopenxr_loader.so EGL)
++endif(ANDROID)
++
+ if(ENABLE_SDL2 MATCHES "ON")
+ 	find_package(SDL2 REQUIRED)
+ 	if(SDL2_FOUND)
+@@ -109,6 +122,8 @@
+ 	${SDL2_MIXER_INCLUDE_DIRS}
+ 	${FLUIDSYNTH_INCLUDE_DIRS}
+ 	${OPENGL_INCLUDE_DIRS}
++	${OPENXR_INCLUDE_DIRS}
++	${ANDROID_EXTRA_INCLUDE_DIRS}
+ )
+ 
+ if(NOT WIN32)
+@@ -439,6 +454,7 @@
+ 	${SDL2_MIXER_LIBRARIES}
+ 	${FLUIDSYNTH_LIBRARIES}
+ 	${OPENGL_LIBRARIES}
++	${OPENXR_LIBRARIES}
+ 	${ALSA_LIBRARIES}
+ 	$<$<BOOL:${ANDROID}>:log>
+ )

android/engine-patches/12-android-openxr-present.patch

@@ -0,0 +1,252 @@
+--- a/src/MacSrc/OpenGL.cc	2026-07-25 07:10:06.189412183 +0200
++++ b/src/MacSrc/OpenGL.cc	2026-07-25 07:12:05.356301052 +0200
+@@ -36,6 +36,7 @@
+     if (loc >= 0)
+         glUniform1f(loc, size);
+ }
++#include "xr_session.h"
+ #endif // __ANDROID__
+ 
+ extern "C" {
+@@ -382,6 +383,21 @@
+ #endif
+     opengl_resize(width, height);
+ 
++#ifdef __ANDROID__
++    // Bring up the OpenXR session now that the shared GL context above is
++    // current (xr_init() reuses it via eglGetCurrentContext/Display - see
++    // xr_session.c) - sized to the game's own logical resolution, not the
++    // physical window, since the swapchain is composited by the XR runtime
++    // rather than upscaled by us into a physical-window-sized viewport (see
++    // opengl_swap_and_restore()/android_composite_software_frame() below).
++    // Failure here (e.g. no OpenXR runtime installed) just means the
++    // immersive path never activates - SDLDraw() falls back to the plain
++    // window-present path for the rest of the run.
++    int xr_logical_width, xr_logical_height;
++    SDL_RenderGetLogicalSize(renderer, &xr_logical_width, &xr_logical_height);
++    xr_init(xr_logical_width, xr_logical_height);
++#endif
++
+     // Now make the palettes
+     opaquePalette = SDL_AllocPalette(256);
+     transparentPalette = SDL_AllocPalette(256);
+@@ -498,7 +514,109 @@
+     *y_scale = output_height / screen_height;
+ }
+ 
++#ifdef __ANDROID__
++// Shared by opengl_swap_and_restore()'s UI-overlay blit and
++// android_composite_software_frame() below - draws an SDL_Surface as a
++// fullscreen textured quad via gl4es. SDL's own renderer can't be used
++// here: it's tied to the window's own EGL surface, not whatever XR
++// swapchain framebuffer the caller already bound via xr_frame_begin().
++static void android_draw_surface_as_quad(SDL_Surface *ui, bool blend) {
++    SDL_Surface *uiRgba = SDL_ConvertSurfaceFormat(ui, SDL_PIXELFORMAT_RGBA32, 0);
++    if (uiRgba == nullptr)
++        return;
++
++    glUseProgram(textureShaderProgram.shaderProgram);
++    GLint tcAttrib = textureShaderProgram.tcAttrib;
++    GLint lightAttrib = textureShaderProgram.lightAttrib;
++    glUniform1i(textureShaderProgram.uniNightSight, false);
++    glUniformMatrix4fv(textureShaderProgram.uniView, 1, false, IdentityMatrix);
++    glUniformMatrix4fv(textureShaderProgram.uniProj, 1, false, IdentityMatrix);
++
++    // Persistent texture, reused every call instead of a fresh
++    // gen/upload/delete each frame.
++    static GLuint s_uiTexture = 0;
++    if (s_uiTexture == 0)
++        glGenTextures(1, &s_uiTexture);
++    bind_texture(s_uiTexture);
++    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
++    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
++    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, uiRgba->w, uiRgba->h, 0, GL_RGBA, GL_UNSIGNED_BYTE,
++                 uiRgba->pixels);
++
++    set_blend_mode(blend);
++
++    // Real per-vertex arrays, not glBegin/glVertexAttrib/glVertex3f
++    // immediate mode: glVertexAttrib*() is GLES2's *constant*-attribute
++    // API - without an enabled vertex array it sets one value for the
++    // whole draw call, not a fresh value per vertex. gl4es's immediate-mode
++    // emulation only reproduces desktop GL's per-vertex-capture behavior
++    // for attribute 0 (position, driven directly by glVertex3f), not for
++    // custom attributes like this shader's texcoords/light.
++    //
++    // V is flipped (1 at the bottom, 0 at the top): GL texture v=0
++    // addresses the first row of data passed to glTexImage2D, but uiRgba
++    // (an SDL surface) stores its rows top-down.
++    struct QuadVertex {
++        float x, y, z;
++        float u, v;
++        float light;
++    };
++    static const QuadVertex quadVerts[4] = {
++        {1.0f, -1.0f, 0.0f, 1.0f, 1.0f, 1.0f},
++        {1.0f, 1.0f, 0.0f, 1.0f, 0.0f, 1.0f},
++        {-1.0f, -1.0f, 0.0f, 0.0f, 1.0f, 1.0f},
++        {-1.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f},
++    };
++    glEnableVertexAttribArray(0);
++    glEnableVertexAttribArray(tcAttrib);
++    glEnableVertexAttribArray(lightAttrib);
++    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(QuadVertex), &quadVerts[0].x);
++    glVertexAttribPointer(tcAttrib, 2, GL_FLOAT, GL_FALSE, sizeof(QuadVertex), &quadVerts[0].u);
++    glVertexAttribPointer(lightAttrib, 1, GL_FLOAT, GL_FALSE, sizeof(QuadVertex),
++                           &quadVerts[0].light);
++    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
++    // The engine's other rendering uses glBegin/glVertexAttrib/glVertex3f
++    // immediate mode for attribute 0, which depends on it not having an
++    // enabled array bound.
++    glDisableVertexAttribArray(0);
++    glDisableVertexAttribArray(tcAttrib);
++    glDisableVertexAttribArray(lightAttrib);
++    glFlush();
++
++    SDL_FreeSurface(uiRgba);
++}
++
++void android_composite_software_frame(SDL_Surface *ui) {
++    // Deliberately no SDL_GL_MakeCurrent() here - see the comment in
++    // opengl_swap_and_restore() below for why.
++    glClear(GL_COLOR_BUFFER_BIT);
++    android_draw_surface_as_quad(ui, false);
++}
++#endif
++
+ void opengl_swap_and_restore(SDL_Surface *ui) {
++#ifdef __ANDROID__
++    // No SDL_GL_MakeCurrent() here on Android, deliberately - unlike the
++    // desktop path below. init_opengl() already made (window, context)
++    // current once; re-asserting it every frame turned out to reset
++    // gl4es's own internal "current framebuffer" bookkeeping back to
++    // whatever it associates with that context/surface pair (observed
++    // on-device: the XR swapchain image reliably received our real
++    // glClear/bind from xr_frame_begin(), but every actual gl4es-routed
++    // draw call after a fresh SDL_GL_MakeCurrent() here kept landing
++    // somewhere else - the quad stayed permanently black). Confirmed this
++    // exact class of gl4es/OpenXR interaction against RTCWQuest (Team
++    // Beef Studios), a shipped gl4es+Khronos-OpenXR-loader Quest port:
++    // it makes its EGL context current exactly once at init and never
++    // again per-frame, for the same reason.
++    //
++    // xr_frame_begin() (called from SDLDraw()) already bound the XR
++    // swapchain's framebuffer and a full-size viewport - no physical-
++    // window letterboxing needed here either, unlike the desktop/2D-panel
++    // path below, since the swapchain is sized to exactly the game's
++    // logical resolution (see xr_init()).
++    glClear(GL_COLOR_BUFFER_BIT);
++#else
+     // restore the view backup (without HUD overlay) for incremental
+     // updates in the subsequent frame
+     SDL_GL_MakeCurrent(window, context);
+@@ -510,6 +628,7 @@
+     // Set the drawable area for the 3d view
+     glViewport(phys_offset_x * x_hdpi_scale, phys_offset_y * y_hdpi_scale, phys_width * x_hdpi_scale,
+                phys_height * y_hdpi_scale);
++#endif
+     set_blend_mode(false);
+ 
+     // Bind and setup our general shader program
+@@ -552,6 +671,11 @@
+     if (err != GL_NO_ERROR)
+         ERROR("OpenGL error: %i", err);
+ 
++#ifdef __ANDROID__
++    // Blit the UI canvas over the 3d view (see android_draw_surface_as_quad()
++    // above - SDL's renderer can't target the XR swapchain framebuffer).
++    android_draw_surface_as_quad(ui, true);
++#else
+     // Blit the UI canvas over the 3d view
+     SDL_Texture *texture = SDL_CreateTextureFromSurface(renderer, ui);
+     SDL_SetTextureBlendMode(texture, SDL_BLENDMODE_BLEND);
+@@ -561,6 +685,7 @@
+ 
+     // Finally, swap to the screen
+     SDL_RenderPresent(renderer);
++#endif
+ }
+ 
+ void toggle_opengl() {
+--- a/src/MacSrc/OpenGL.h	2026-07-19 13:11:51.000000000 +0200
++++ b/src/MacSrc/OpenGL.h	2026-07-25 07:10:06.353410581 +0200
+@@ -22,6 +22,15 @@
+ void opengl_swap_and_restore(SDL_Surface *ui);
+ void opengl_change_palette();
+ 
++#ifdef __ANDROID__
++// Composites a plain software-rendered frame (the same content SDLDraw's
++// non-OpenGL fallback would otherwise present via SDL_RenderCopy) into the
++// XR swapchain image already bound by xr_frame_begin() - see Shock.c's
++// SDLDraw(), which needs this for the splash screen/cutscenes/menus that
++// render this way instead of through opengl_swap_and_restore().
++void android_composite_software_frame(SDL_Surface *ui);
++#endif
++
+ void opengl_set_viewport(int x, int y, int width, int height);
+ int opengl_draw_tmap(int n, g3s_phandle *vp, grs_bitmap *bm);
+ int opengl_light_tmap(int n, g3s_phandle *vp, grs_bitmap *bm);
+--- a/src/MacSrc/Shock.c	2026-07-25 07:10:06.181412261 +0200
++++ b/src/MacSrc/Shock.c	2026-07-25 07:10:06.353410581 +0200
+@@ -30,6 +30,10 @@
+ #include <math.h>
+ #include <SDL.h>
+ 
++#ifdef __ANDROID__
++#include "xr_session.h"
++#endif
++
+ #include "InitMac.h"
+ #include "Modding.h"
+ #include "OpenGL.h"
+@@ -301,6 +305,21 @@
+ }
+ 
+ void SDLDraw() {
++#ifdef __ANDROID__
++    // The true once-a-frame present hook for the immersive OpenXR path -
++    // unlike opengl_swap_and_restore() below, this runs every frame
++    // regardless of should_opengl_swap() (e.g. also during the splash
++    // screen/cutscenes/menus, which render through the plain software
++    // path below, not the OpenGL one). xr_poll_events() needs exactly that
++    // - called unconditionally - to ever observe the session reach a
++    // running state in the first place.
++    xr_poll_events();
++    if (!xr_frame_begin()) {
++        xr_frame_end();
++        return;
++    }
++#endif
++
+     if (should_opengl_swap()) {
+         // We want the UI background to be transparent!
+         sdlPalette->colors[255].a = 0x00;
+@@ -310,14 +329,27 @@
+ 
+         // Set the palette back, and we are done
+         sdlPalette->colors[255].a = 0xff;
++#ifdef __ANDROID__
++        xr_frame_end();
++#endif
+         return;
+     }
+ 
++#ifdef __ANDROID__
++    // Same software-rendered frame the desktop path below would present
++    // via SDL_RenderCopy/SDL_RenderPresent, composited into the XR
++    // swapchain image xr_frame_begin() bound above instead - SDL's
++    // renderer is tied to the window's own EGL surface, not that FBO.
++    android_composite_software_frame(drawSurface);
++    xr_frame_end();
++    return;
++#endif
++
+     // Clear the screen!
+     SDL_RenderClear(renderer);
+     SDL_Texture *texture = SDL_CreateTextureFromSurface(renderer, drawSurface);
+ 
+-    // Blit to the screen by drawing the surface 
++    // Blit to the screen by drawing the surface
+     SDL_Rect srcRect = {0, 0, gScreenWide, gScreenHigh};
+     SDL_RenderCopy(renderer, texture, &srcRect, NULL);
+     SDL_DestroyTexture(texture);

build-image/Dockerfile

@@ -57,6 +57,14 @@ ARG ANDROID_16KB_LDFLAGS="-Wl,-z,max-page-size=16384"
 # desktop-style immediate-mode OpenGL calls into real GLES/EGL calls, so
 # engine/src/MacSrc/OpenGL.cc needs no immediate-mode rewrite on Android.
 ARG ANDROID_GL4ES_VERSION=1.1.6
+# Khronos' own OpenXR loader (https://github.com/KhronosGroup/OpenXR-SDK) -
+# the vendor-neutral loader, not Meta's redistributed copy, per this
+# project's "favor OpenXR" rule (see README's Design principles). Only the
+# loader is built (BUILD_LOADER=ON, everything else off) - the actual XR
+# runtime implementation is resolved on-device by Horizon OS at load time,
+# nothing else needs bundling. Verify this is still the latest release tag
+# at https://github.com/KhronosGroup/OpenXR-SDK/releases when bumping.
+ARG ANDROID_OPENXR_VERSION=1.1.42
 
 ENV DEBIAN_FRONTEND=noninteractive
 
@@ -272,6 +280,26 @@ RUN git clone --branch "v${ANDROID_GL4ES_VERSION}" --depth 1 \
     && cp -a gl4es-android/include/. /opt/prebuilt/android/gl4es/include/ \
     && rm -rf gl4es-android build-gl4es-android
 
+# OpenXR loader, for the immersive VR mode (android/app/src/main/cpp/
+# xr_session.c and friends) - see android/engine-patches/
+# 11-android-openxr-cmake.patch for how the engine links it.
+RUN git clone --branch "release-${ANDROID_OPENXR_VERSION}" --depth 1 \
+        https://github.com/KhronosGroup/OpenXR-SDK.git openxr-sdk-android \
+    && rm -rf openxr-sdk-android/.git \
+    && cmake -S openxr-sdk-android -B build-openxr-android \
+        -DCMAKE_TOOLCHAIN_FILE="${ANDROID_NDK_TOOLCHAIN}" \
+        -DANDROID_ABI="${ANDROID_ABI}" -DANDROID_PLATFORM="android-${ANDROID_PLATFORM_VERSION}" \
+        -DCMAKE_INSTALL_PREFIX=/opt/prebuilt/android/openxr \
+        -DBUILD_LOADER=ON -DBUILD_TESTS=OFF -DBUILD_API_LAYERS=OFF \
+        -DBUILD_CONFORMANCE_TESTS=OFF -DBUILD_WITH_SYSTEM_JSONCPP=OFF \
+        -DDYNAMIC_LOADER=ON \
+        -DCMAKE_SHARED_LINKER_FLAGS="${ANDROID_16KB_LDFLAGS}" \
+    && cmake --build build-openxr-android -j"$(nproc)" --target openxr_loader \
+    && cmake --install build-openxr-android \
+    && mkdir -p /opt/prebuilt/android/openxr/include \
+    && cp -a openxr-sdk-android/include/. /opt/prebuilt/android/openxr/include/ \
+    && rm -rf openxr-sdk-android build-openxr-android
+
 COPY build-image/docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
 COPY build-image/build-engine.sh /usr/local/bin/build-engine.sh
 RUN chmod +x /usr/local/bin/docker-entrypoint.sh /usr/local/bin/build-engine.sh

build-image/prepare-android-project.sh

@@ -1,8 +1,8 @@
 #!/usr/bin/env bash
 # Prepares android/ to be built: a scratch, patched copy of engine/ (see
 # android/engine-patches/ - engine/ itself is never modified), the
-# Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es prebuilts staged into
-# jniLibs, bundled assets, and android/engine.properties (engineDir/
+# Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es/openxr prebuilts staged
+# into jniLibs, bundled assets, and android/engine.properties (engineDir/
 # prebuiltDir, read by android/app/build.gradle). Always run inside this
 # image (via ../run-image.sh, or directly if already inside it) - it
 # reads from /opt/prebuilt/android/*, which only exists there.
@@ -75,12 +75,12 @@ cp -a "$ANDROID_DIR/gles-shaders/." "$ASSETS_DIR/shaders/"
 cp "/opt/prebuilt/soundfont/default.sf2" "$ASSETS_DIR/res/soundfont.sf2"
 cp "$REPO_ROOT/res/assets/GET_ASSETS_QUEST.txt" "$ASSETS_DIR/GET_ASSETS_QUEST.txt"
 
-echo "== Staging prebuilt Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es .so's into jniLibs =="
+echo "== Staging prebuilt Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es/openxr .so's into jniLibs =="
 JNI_LIBS_DIR="$ANDROID_DIR/app/src/main/jniLibs/arm64-v8a"
 rm -rf "$ANDROID_DIR/app/src/main/jniLibs"
 mkdir -p "$JNI_LIBS_DIR"
 find /opt/prebuilt/android/sdl2/lib /opt/prebuilt/android/sdl2_mixer/lib /opt/prebuilt/android/fluidsynth-lite/lib \
-    /opt/prebuilt/android/gl4es/lib \
+    /opt/prebuilt/android/gl4es/lib /opt/prebuilt/android/openxr/lib \
     -name '*.so' -exec cp -a {} "$JNI_LIBS_DIR/" \;
 
 echo "== Done =="