How to Implement GPUImageMaskFilter using GPUImage

Question

I need to Cut from the Full Image using the mask and created the masked Image.

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Answer 1

The mask is the second target, as can been seen in the filter shader code (textureColor2).

//Averages mask's the RGB values, and scales that value by the mask's alpha
//
//The dot product should take fewer cycles than doing an average normally
//
//Typical/ideal case, R,G, and B will be the same, and Alpha will be 1.0

 lowp float newAlpha = dot(textureColor2.rgb, vec3(.33333334, .33333334, .33333334)) * textureColor2.a;

 gl_FragColor = vec4(textureColor.xyz, newAlpha);

Then you need to "invert" your mask : white heart on black background, as the filter uses the "weight" of the RGB pixel value to set the alpha value on the target image.

So your code should be

// Image first, Mask next
[FullGpuImage addTarget:maskingFilter];
[FullGpuImage processImage];

[maskingFilter useNextFrameForImageCapture];

[maskGpuImage addTarget:maskingFilter];
[maskGpuImage processImage];    

and your mask (ok I did an ugly quick test, use a proper image) like

for the expected result.

Answer 2

You don't need a mask filter but an alpha mask blend.

I implemented one here like this:

// GPUImage shader strings
NSString * const kNBUAlphaMaskShaderString = SHADER_STRING
(
 varying highp vec2 textureCoordinate;
 varying highp vec2 textureCoordinate2;

 uniform sampler2D inputImageTexture;
 uniform sampler2D inputImageTexture2;

 void main()
 {
     lowp vec4 textureColor = texture2D(inputImageTexture, textureCoordinate);
     lowp vec4 textureColor2 = texture2D(inputImageTexture2, textureCoordinate2);

     gl_FragColor = vec4(textureColor.xyz, textureColor2.a);
 }
 );

Simply keeps the colors from one image and the alpha channel of the second one.

Then create a GPUImageTwoInputFilter:

GPUImageTwoInputFilter * alphaMask = [[GPUImageTwoInputFilter alloc] initWithFragmentShaderFromString:kNBUAlphaMaskShaderString];

I'm not sure if a similar blend filter has been added to GPUImage since.

---

Just checked again to see if there's a built-in blend that does it, and there isn't. But the blend filter I used as inspiration is still there (GPUImageAlphaBlendFilter). It merges two images using the alpha mask to mix them. The filter mentioned above doesn't require a second "empty" image.