Fix Metal backend crash when rendering external image before set (#1847)

RELEASE_NOTES.md

@@ -9,6 +9,7 @@ A new header is inserted each time a *tag* is created.
 - Added support for the latest version of emscripten.
 - gltfio: fixed blackness seen with default material.
 - Added ETC2 and BC compressed texture support to Metal backend.
+- Rendering a SAMPLER_EXTERNAL texture before setting an external image no longer results in GPU errors.
 
 ## v1.4.2
 

filament/backend/src/metal/MetalContext.h

@@ -85,6 +85,9 @@ struct MetalContext {
     // External textures.
     CVMetalTextureCacheRef textureCache = nullptr;
 
+    // Empty texture used to prevent GPU errors when a sampler has been bound without a texture.
+    id<MTLTexture> emptyTexture = nil;
+
     MetalBlitter* blitter = nullptr;
 
     // Fences.
@@ -101,6 +104,8 @@ id<CAMetalDrawable> acquireDrawable(MetalContext* context);
 
 id<MTLCommandBuffer> acquireCommandBuffer(MetalContext* context);
 
+id<MTLTexture> getOrCreateEmptyTexture(MetalContext* context);
+
 } // namespace metal
 } // namespace backend
 } // namespace filament

filament/backend/src/metal/MetalContext.mm

@@ -63,6 +63,29 @@ id<MTLCommandBuffer> acquireCommandBuffer(MetalContext* context) {
     return commandBuffer;
 }
 
+id<MTLTexture> getOrCreateEmptyTexture(MetalContext* context) {
+    if (context->emptyTexture) {
+        return context->emptyTexture;
+    }
+
+    MTLTextureDescriptor* textureDescriptor = [MTLTextureDescriptor new];
+    textureDescriptor.pixelFormat = MTLPixelFormatBGRA8Unorm;
+    textureDescriptor.width = 1;
+    textureDescriptor.height = 1;
+    id<MTLTexture> texture = [context->device newTextureWithDescriptor:textureDescriptor];
+
+    MTLRegion region = {
+        { 0, 0, 0 },    // MTLOrigin
+        { 1, 1, 1 }     // MTLSize
+    };
+    uint8_t imageData[4] = {0, 0, 0, 0};
+    [texture replaceRegion:region mipmapLevel:0 withBytes:imageData bytesPerRow:4];
+
+    context->emptyTexture = texture;
+
+    return context->emptyTexture;
+}
+
 } // namespace metal
 } // namespace backend
 } // namespace filament

filament/backend/src/metal/MetalDriver.mm

@@ -73,6 +73,7 @@ MetalDriver::MetalDriver(backend::MetalPlatform* platform) noexcept
 
 MetalDriver::~MetalDriver() noexcept {
     mContext->device = nil;
+    mContext->emptyTexture = nil;
     CFRelease(mContext->textureCache);
     delete mContext->bufferPool;
     delete mContext->blitter;
@@ -908,6 +909,12 @@ void MetalDriver::draw(backend::PipelineState ps, Handle<HwRenderPrimitive> rph)
             texturesToBind[binding] = metalTexture->externalImage.getMetalTextureForDraw();
         }
 
+        if (!texturesToBind[binding]) {
+            utils::slog.w << "Warning: no texture bound at binding point " << (size_t) binding
+                    << "." << utils::io::endl;
+            texturesToBind[binding] = getOrCreateEmptyTexture(mContext);
+        }
+
         id <MTLSamplerState> samplerState = mContext->samplerStateCache.getOrCreateState(sampler->s);
         samplersToBind[binding] = samplerState;
     });