qml-metalangle

Example of mixing MetalANGLE with Qt's QML.

• This example is based on Qt's metaltextureimport example.
• This example uses a shared Metal texture between MetalANGLE & Qt's Metal.
  • Unlike Qt's original example, this shared texture is drawn by MetalANGLE's GL commands.
• To ensure correct order of rendering (on MetalANGLE side) and consuming (on Qt's Metal side), a shared semaphore also needs to be used. See GL_EXT_semaphore.

In-depth explanation

• Create an MTLTexture.
• Create an MTLSharedEvent.
• Import MTLTexture to OpenGL as a GLuint texture object. Attach it to an FBO.
• Import MTLSharedEvent to OpenGL as GLuint semaphore object.
• Initialize an integer semaphoreCounter to zero.
• Rendering loop:

  // Wait for Qt's Metal side to finish using the texture
  glSemaphoreParameterui64vEXT(..., GL_TIMELINE_SEMAPHORE_VALUE_MGL, &semaphoreCounter);
  glWaitSemaphoreEXT(...);
  
  // Draw to texture
  glBindFramebuffer(...);
  glClear(...)
  ...
  
  // Notify Qt's Metal side that the texture is safe to be used now
  semaphoreCounter++;
  glSemaphoreParameterui64vEXT(..., GL_TIMELINE_SEMAPHORE_VALUE_MGL, &semaphoreCounter);
  glSignalSemaphoreEXT(...);
  
  // On Qt's Metal side, wait for the semaphore's signal
  QSGRendererInterface *rif = m_window->rendererInterface();
  id<MTLCommandBuffer> cb = (id<MTLCommandBuffer>) rif->getResource(m_window, QSGRendererInterface::CommandListResource);
  [cb encodeWaitForEvent:... value:semaphoreCounter];
  
  // Display the texture on the screen (mostly done internally by QML scene graph)
  ...
  
  // Notify MetalANGLE side that the texture is safe to be rendered into now
  semaphoreCounter++;
  [cb encodeSignalEvent:... value:semaphoreCounter];