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iosgpuimage

GPUImageAlphaBlendFilter realtime processing from GPUImageStillCamera source

I am using the GPUImage library and I'm trying to blend two images in realtime, and display them on a GPUImageView. I am trying to alpha-blend plain camera input, with a filtered version of it. Here is what I'm trying to do:

          ----------------->----v
--camera--|                     alpha blend ----> image view
          -----> color filter --^

I've found some posts about using the blend filters, but they don't seem to be methods for realtime processing. I've found https://github.com/BradLarson/GPUImage/issues/319, GPUImage: blending two images, and https://github.com/BradLarson/GPUImage/issues/751 (but they either aren't for realtime processing, (first and the second), or doesn't work (third one).

I've tried almost everything, but all I'm getting is a white image in the GPUImageView. If I don't use the alpha blend filter, say, just use a false color filter or something similar, it works perfectly. Here is my code:

blendFilter = [[GPUImageAlphaBlendFilter alloc] init];
blendFilter.mix = 0.5;
[blendFilter prepareForImageCapture];
[blendFilter addTarget:imageView];

passThrough = [[GPUImageFilter alloc] init];
[passThrough prepareForImageCapture];
[passThrough addTarget:blendFilter];

selectedFilter = [[GPUImageFalseColorFilter alloc] init];
[selectedFilter prepareForImageCapture];
[selectedFilter addTarget:blendFilter];

stillCamera = [[GPUImageStillCamera alloc] init];
stillCamera.outputImageOrientation = UIInterfaceOrientationPortrait;

[stillCamera addTarget:passThrough];
[stillCamera addTarget:selectedFilter];
[stillCamera startCameraCapture];

All I'm getting is a white, blank screen. If I change [selectedFilter addTarget:blendFilter]; to [selectedFilter addTarget:imageView]; then false color filter gets displayed on the image.

There seems to be something wrong with the alpha blend filter. I've read that in some posts that I need to call processImage on the inputs, but those posts are all for non-realtime inputs as far as I understand. How can I get GPUImageAlphaBlendFilter to work in realtime?

Solution

  • Ok, after investigating the issue further over the internet and on project's issue list (https://github.com/BradLarson/GPUImage/issues) and found a workaround. While setting the blend filter as the target, I needed to specify the texture index specifically. For some reason (probably a bug), adding the target blend filter two times doesn't add the second texture correctly at the next index. So setting the texture indices explicitly as 0 and 1 did work:

    [passThrough addTarget:blendFilter atTextureLocation:0];
[selectedFilter addTarget:blendFilter atTextureLocation:1];

    For the filters that are targets of single sources, addTarget: is enough though such as [stillCamera addTarget:selectedFilter];.