Convert a black & white image to a transparent & white image in GPUImage? RGB -> Alpha basically

Question

I got a black and white image from the edge detection filter in GPUImage, where white represents the edges, and the majority of the image contents is black, nontransparent. The problem is, I want to overlay this image on top of another image, to show how the edges are aligned with the image below.

But the non-edge area is not transparent but black, so basically I want the black to be transparent, or changing the intensity of blackness and whiteness to the intensity of transparency.

I did not find a direct GPUImage filter for this conversion task. GPUImageChromaKeyFilter seems to be a way of doing this, but it's more than RGB to Alpha channel conversion, and is not intended for this task.

Answer 1

Edit: Thanks to Brad's comment, a more efficient solution without swapping the channel is replacing "gl_FragColor = vec4(vec3(mag), 1.0);" with "gl_FragColor = vec4(mag);" in the edge detection filter source.

You can play with the parameters in GPUImageColorMatrixFilter, and map the channels from one to another. Sample code:

GPUImagePicture *gpuImage = [[GPUImagePicture alloc] initWithImage:image];
GPUImageSobelEdgeDetectionFilter *edgeFilter = [[GPUImageSobelEdgeDetectionFilter alloc] init];

GPUImageColorMatrixFilter *conversionFilter = [[GPUImageColorMatrixFilter alloc] init];
conversionFilter.colorMatrix = (GPUMatrix4x4){
    {0.0, 0.0, 0.0, 1.0},
    {0.0, 0.0, 0.0, 1.0},
    {0.0, 0.0, 0.0, 1.0},
    {1.0,0.0,0.0,0.0},
};

[gpuImage addTarget:edgeFilter];
[edgeFilter addTarget:conversionFilter];
[gpuImage processImage];

return [conversionFilter imageFromCurrentlyProcessedOutputWithOrientation:orientation];

The values specified in colorMatrix basically says using the previous Alpha channel (all 255 for a non transparent image) to replace the RGB channels, and using the previous R channel (black and white images have the same values in RGB) to replace the Alpha channel.