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    <p>You really owe it to yourself to play around with writing custom shaders. It's extremely approachable, and can very quickly become powerful if you invest the effort. </p> <p>That said, I think you're trying for something like this result: <img src="https://i.stack.imgur.com/OQN1U.png" alt="Edge Detection on a pile of Lego&#39;s"></p> <p>There are many acceptable ways you could get here, but writing a custom shader for a subclass of GPUImageTwoInputFilter then targeting it with both the original image AND the edgeDetection image is how I accomplished the picture you see here.</p> <p>The subclass would look something like this:</p> <pre><code>#import "OriginalColorEdgeMixer.h" //Assumes you have targeted this filter with the original image first, then with an edge detection filter that returns white pixels on edges //We are setting the threshold manually here, but could just as easily be a GLint which is dynamically fed at runtime #if TARGET_IPHONE_SIMULATOR || TARGET_OS_IPHONE NSString *const kOriginalColorEdgeMixer = SHADER_STRING ( varying highp vec2 textureCoordinate; varying highp vec2 textureCoordinate2; uniform sampler2D inputImageTexture; uniform sampler2D inputImageTexture2; lowp float threshold; mediump float resultingRed; mediump float resultingGreen; mediump float resultingBlue; void main() { mediump vec4 textureColor = texture2D(inputImageTexture, textureCoordinate); mediump vec4 textureColor2 = texture2D(inputImageTexture2, textureCoordinate2); threshold = step(0.3, textureColor2.r); resultingRed = threshold * textureColor.r; resultingGreen = threshold * textureColor.g; resultingBlue = threshold *textureColor.b; gl_FragColor = vec4(resultingRed, resultingGreen, resultingBlue, textureColor.a); } ); #else NSString *const kGPUImageDifferenceBlendFragmentShaderString = SHADER_STRING ( varying vec2 textureCoordinate; varying vec2 textureCoordinate2; uniform sampler2D inputImageTexture; uniform sampler2D inputImageTexture2; float threshold; float resultingRed; float resultingGreen; float resultingBlue; void main() { vec4 textureColor = texture2D(inputImageTexture, textureCoordinate); vec4 textureColor2 = texture2D(inputImageTexture2, textureCoordinate2); threshold = step(0.3,textureColor2.r); resultingRed = threshold * textureColor.r; resultingGreen = threshold * textureColor.g; resultingBlue = threshold *textureColor.b; gl_FragColor = vec4(resultingRed, resultingGreen, resultingBlue, textureColor.a); } ); #endif @implementation OriginalColorEdgeMixer - (id)init; { if (!(self = [super initWithFragmentShaderFromString:kOriginalColorEdgeMixer])) { return nil; } return self; } @end </code></pre> <p>As I've written this, we're expecting the edgeDetection filter's output to be the second input of this custom filter.</p> <p>I arbitrarily chose a threshold value of 0.3 for intensities on the edgeDetection image to enable the original color to show through. This could easily be made dynamic by tying it to a GLint fed from a UISlider in your app (many examples of this in Brad's sample code)</p> <p>For the sake of clarity for people just starting out with GPUImage, using that custom filter you wrote is really easy. I did it like this:</p> <pre><code>[self configureCamera]; edgeDetection = [[GPUImageSobelEdgeDetectionFilter alloc] init]; edgeMixer = [[OriginalColorEdgeMixer alloc] init]; [camera addTarget:edgeDetection]; [camera addTarget:edgeMixer]; [edgeDetection addTarget:edgeMixer]; [edgeMixer addTarget:_previewLayer]; [camera startCameraCapture]; </code></pre> <p>In summary, don't be scared to start writing some custom shaders! The learning curve is brief, and the errors thrown by the debugger are extremely helpful in letting you know exactly where you f**d up the syntax.</p> <p>Lastly, <a href="http://www.opengl.org/sdk/docs/" rel="noreferrer">this is a great place for documentation of the syntax and usage of OpenGL specific functions</a></p>
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    1. POHow to find edges of image with its colors in iphone?
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    1. USryan cumley
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    1. COIt works for me and it work abjectly what i want. I tried to create custom filter but didn't succeed. You solve my problem. So Thanks a lot Ryan cumley.
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      1. USSurjeet
    2. COMy pleasure! Programming is exceptionally tolerant of repeated try/fail cycles, so don't be discouraged when your first few attempts don't succeed. Tweak and try again until you succeed, it's how we all do it...
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