GPUImageMovie 不支持 Alpha 通道？

Question

我像这样通过 GPUImage 创建视频效果

self.overlayerView = [[GPUImageView alloc] init];
self.overlayerView.frame = self.view.frame;

dispatch_queue_t queue = dispatch_queue_create("queue", NULL);
dispatch_async(queue, ^{
    
    NSURL *sourceURL = [[NSBundle mainBundle] URLForResource:@"212121" withExtension:@"mp4"];
    GPUImageMovie *sourceMovie = [[GPUImageMovie alloc] initWithURL:sourceURL];
    sourceMovie.playAtActualSpeed = YES;
    sourceMovie.shouldRepeat = YES;
    
    sourceMovie.shouldIgnoreUpdatesToThisTarget = YES;
    
    NSURL *maskURL = [[NSBundle mainBundle] URLForResource:@"rose" withExtension:@"mp4"];
    GPUImageMovie *maskMovie = [[GPUImageMovie alloc] initWithURL:maskURL];
    maskMovie.playAtActualSpeed = YES;
    maskMovie.shouldRepeat = YES;
    
    
    NSURL *alphaURL = [[NSBundle mainBundle] URLForResource:@"rose_alpha" withExtension:@"mp4"];
    GPUImageMovie *alphaMovie = [[GPUImageMovie alloc] initWithURL:alphaURL];
    alphaMovie.playAtActualSpeed = YES;
    alphaMovie.shouldRepeat = YES;
    
    
    NSURL *topURL = [[NSBundle mainBundle] URLForResource:@"screen" withExtension:@"mp4"];
    GPUImageMovie *topMovie = [[GPUImageMovie alloc] initWithURL:topURL];
    topMovie.playAtActualSpeed = YES;
    topMovie.shouldRepeat = YES;
    
    filter0 = [[GPUImageThreeInputFilter alloc] initWithFragmentShaderFromString:@"precision highp float;uniform sampler2D inputImageTexture;uniform sampler2D inputImageTexture2;uniform sampler2D inputImageTexture3;varying vec2 textureCoordinate;void main(){vec4 video=texture2D(inputImageTexture,textureCoordinate);vec4 mv=texture2D(inputImageTexture2, textureCoordinate);vec4 alpha = texture2D(inputImageTexture3, textureCoordinate);gl_FragColor = video * (1.0 - alpha.r) + mv;}"];
    

    filter1 = [[GPUImageTwoInputFilter  alloc] initWithFragmentShaderFromString:@"\nprecision highp float;\nuniform sampler2D inputImageTexture; //video\nuniform sampler2D inputImageTexture2; //screen\nvarying vec2 textureCoordinate;\nvoid main()\n{\nvec4 video = texture2D(inputImageTexture, textureCoordinate);\nvec4 screen = texture2D(inputImageTexture2, textureCoordinate);\nmediump vec4 whiteColor = vec4(1.0);\ngl_FragColor = whiteColor - ((whiteColor - screen) * (whiteColor - video));\n}"];


    [sourceMovie addTarget:filter0];
    [maskMovie addTarget:filter0];
    [alphaMovie addTarget:filter0];
    [filter0 addTarget:filter1];
    [topMovie addTarget:filter1];

    [sourceMovie startProcessing];
    [alphaMovie startProcessing];
    [maskMovie startProcessing];
    [topMovie startProcessing];

    [filter0 forceProcessingAtSize:CGSizeMake(480,480)];
    [filter1 forceProcessingAtSize:CGSizeMake(480,480)];
    
    dispatch_async(dispatch_get_main_queue(), ^{
        [filter1 addTarget:self.overlayerView];
    });
});

代码可以运行，我得到了这样的视频效果

视频有黑色背景是因为 alphaMovie 与 maskMovie 不同时播放？

这就是我想要创建的

效果视频无黑色背景

问题：

1：如何去除黑色背景？

2：为什么我的效果视频有黑色背景？

Answer 1

GPUImage 框架不包含对此类 Alpha 通道功能的支持。有一个绿屏功能，所以如果您预先制作了绿屏视频，则可以将视频从绿屏背景中分离出来。但是，您在这里描述的是一个 Alpha 通道视频和一个第二个视频，并且它无法正常工作，因为您同时从两个不同的视频源中提取并且它们不会保持同步。请注意，即使使用绿屏功能，精确边缘也存在问题，如this blog post 中所述（包括源代码）。基本问题是，接近绿色但不完全是绿色的边缘可能会被过滤器斜坡以奇怪的方式处理。您可能要考虑的另一种方法是在尝试使用 iOS 视频逻辑播放视频之前将 N 帧预组合成 1 个视频。