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我正在尝试对通过 UIView 播放的 mp4 视频进行色度键控。

我每帧都得到一个纹理，如下所示：

var fullTex:CVMetalTexture?
let w = CVPixelBufferGetWidthOfPlane(pixelBuffer, 0);
let h = CVPixelBufferGetHeightOfPlane(pixelBuffer, 0);

CVMetalTextureCacheCreateTextureFromImage(kCFAllocatorDefault,
                                          videoTextureCache!,
                                          pixelBuffer,
                                          nil,
                                                      MTLPixelFormat.bgra8Unorm,
                                                      w, h, 0,
                                                      &fullTex)
let fullTexture:MTLTexture? = CVMetalTextureGetTexture(fullTex!)

像这样将它传递给我的着色器：

let commandBuffer = commandQueue.makeCommandBuffer()!
let commandEncoder = commandBuffer.makeComputeCommandEncoder()!
            commandEncoder.setComputePipelineState(pipelineState)
            commandEncoder.setTexture(fullTexture, index: 0)
            commandEncoder.setTexture(fullTexture, index: 1)
            commandEncoder.dispatchThreadgroups(threadgroupsPerGrid, threadsPerThreadgroup: threadsPerThreadgroup)
            commandEncoder.endEncoding()
            commandBuffer.commit();
            commandBuffer.waitUntilCompleted()

// This proves the shader created transparency, but it doesn't render as such on screen
let i = UIImage(pixelBuffer: pixelBuffer, context: CIContext())

//This writes to a AVSampleBufferDisplayLayer
self.displayPixelBuffer(pixelBuffer: pixelBuffer, atTime: outputItemTime)

着色器如下所示：

kernel void GreenScreen(texture2d<float, access::read> inTexture [[texture(0)]],
                    texture2d<float, access::write> outTexture [[texture(1)]],
                    uint2 gid [[thread_position_in_grid]]) {

float4 color = inTexture.read(gid);
float4 combinedColor = float4(color.rgb,1.0);
if (color.r < 0.5)
    combinedColor = float4(0.0,0,0,0);

outTexture.write(combinedColor, gid);
}

显示如下：

func displayPixelBuffer(pixelBuffer: CVPixelBuffer, atTime outputTime: CMTime) {
    var err: OSStatus = noErr

    if videoInfo == nil || false == CMVideoFormatDescriptionMatchesImageBuffer(videoInfo!, pixelBuffer) {
        if videoInfo != nil {
            videoInfo = nil
        }
        err = CMVideoFormatDescriptionCreateForImageBuffer(nil, pixelBuffer, &videoInfo)
        if (err != noErr) {
            DLog("Error at CMVideoFormatDescriptionCreateForImageBuffer \(err)")
        }
    }

    var sampleTimingInfo = CMSampleTimingInfo(duration: kCMTimeInvalid, presentationTimeStamp: outputTime, decodeTimeStamp: kCMTimeInvalid)
    var sampleBuffer: CMSampleBuffer?
    err = CMSampleBufferCreateForImageBuffer(nil, pixelBuffer, true, nil, nil, videoInfo!, &sampleTimingInfo, &sampleBuffer)
    if (err != noErr) {
        DLog("Error at CMSampleBufferCreateForImageBuffer \(err)")
    }

    if sampleLayer.isReadyForMoreMediaData {
        sampleLayer.enqueue(sampleBuffer!)
    }
}

示例层添加如下：

private func setupVideoPlaybackLayer() {

    self.view.isOpaque = false
    self.view.layer.isOpaque = false

    sampleLayer = AVSampleBufferDisplayLayer()
    sampleLayer.backgroundColor = UIColor.green.cgColor
    sampleLayer.frame = view.layer.frame
    self.view.layer.addSublayer(sampleLayer)
}

但是当电影回放时，我期望透明的区域是黑色的，它后面的 UIView 是蓝色的。

有任何想法吗？

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