GPUImage not support alpha channel ?

[AllanChen]

Here is my code, I want to create video effect. But i got black background.
GPUImage Not Support Alpha Channel ?
Description : "http://stackoverflow.com/questions/34989942/gpuimagemovie-not-support-alpha-channel"

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;

    GPUImageAddBlendFilter *addFilter = [[GPUImageAddBlendFilter alloc] init];
    [maskMovie addTarget:addFilter];
    [alphaMovie addTarget:addFilter];


    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 atTextureLocation:0];
    [maskMovie addTarget:filter0 atTextureLocation:1];
    [alphaMovie addTarget:filter0 atTextureLocation:2];
    [filter0 addTarget:filter1 atTextureLocation:0];
    [topMovie addTarget:filter1 atTextureLocation:1];

    [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];
    });
});