MTKView - Draw on to Two Views at Once

Question

What I got

I am following Apple sample code AVCamPhotoFilter to display camera feed on a MTKView.

What I am trying to do

In addition to above MTKView, I need to display a second MTKView. However, the second one will be displaying exactly the same content as the first one. So I do not want to duplicate the code and do work twice.

Current drawing method

override func draw(_ rect: CGRect) {
        var pixelBuffer: CVPixelBuffer?
        var mirroring = false
        var rotation: Rotation = .rotate0Degrees

        syncQueue.sync {
            pixelBuffer = internalPixelBuffer
            mirroring = internalMirroring
            rotation = internalRotation
        }

        guard let drawable = currentDrawable,
            let currentRenderPassDescriptor = currentRenderPassDescriptor,
            let previewPixelBuffer = pixelBuffer else {
                return
        }

        // Create a Metal texture from the image buffer
        let width = CVPixelBufferGetWidth(previewPixelBuffer)
        let height = CVPixelBufferGetHeight(previewPixelBuffer)

        if textureCache == nil {
            createTextureCache()
        }
        var cvTextureOut: CVMetalTexture?
        CVMetalTextureCacheCreateTextureFromImage(kCFAllocatorDefault,
                                                  textureCache!,
                                                  previewPixelBuffer,
                                                  nil,
                                                  .bgra8Unorm,
                                                  width,
                                                  height,
                                                  0,
                                                  &cvTextureOut)
        guard let cvTexture = cvTextureOut, let texture = CVMetalTextureGetTexture(cvTexture) else {
                print("Failed to create preview texture")

                CVMetalTextureCacheFlush(textureCache!, 0)
                return
        }

        if texture.width != textureWidth ||
            texture.height != textureHeight ||
            self.bounds != internalBounds ||
            mirroring != textureMirroring ||
            rotation != textureRotation {
            setupTransform(width: texture.width, height: texture.height, mirroring: mirroring, rotation: rotation)
        }

        // Set up command buffer and encoder
        guard let commandQueue = commandQueue else {
            print("Failed to create Metal command queue")
            CVMetalTextureCacheFlush(textureCache!, 0)
            return
        }

        guard let commandBuffer = commandQueue.makeCommandBuffer() else {
            print("Failed to create Metal command buffer")
            CVMetalTextureCacheFlush(textureCache!, 0)
            return
        }

        guard let commandEncoder = commandBuffer.makeRenderCommandEncoder(descriptor: currentRenderPassDescriptor) else {
            print("Failed to create Metal command encoder")
            CVMetalTextureCacheFlush(textureCache!, 0)
            return
        }

        commandEncoder.label = "Preview display"
        commandEncoder.setRenderPipelineState(renderPipelineState!)
        commandEncoder.setVertexBuffer(vertexCoordBuffer, offset: 0, index: 0)
        commandEncoder.setVertexBuffer(textCoordBuffer, offset: 0, index: 1)
        commandEncoder.setFragmentTexture(texture, index: 0)
        commandEncoder.setFragmentSamplerState(sampler, index: 0)
        commandEncoder.drawPrimitives(type: .triangleStrip, vertexStart: 0, vertexCount: 4)
        commandEncoder.endEncoding()

        commandBuffer.present(drawable) // Draw to the screen
        commandBuffer.commit()
    }

Question

Is there a way I can simply pass on the texture to the second MTKView and draw without doing work twice?

Answer 1

If you set the framebufferOnly property of the first MTKView to false, you can submit commands which read from its drawable texture. Then, you can use a blit command encoder to copy from the first drawable's texture to the second's, if they are compatible. Otherwise, you can draw a quad to the second drawable's texture with the first drawable's texture as the source for texturing the quad.

Personally, I think I would prefer all of the rendering to go to a texture of your own creation (not any drawable's texture). Then, copy/draw that to both of the drawable textures.

In any case, if you need the two views to update in perfect sync, you should set presentsWithTransaction to true for both views, synchronously wait (using -waitUntilScheduled) for the command buffer that does (at least) the copy/draw to the drawable textures, and then call -present directly on both drawables. (That is, don't use -presentDrawable: on the command buffer.)