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Is there any way on the iOS SDK to overlay the non-transparent pixels in an image with colored pixels?

Thanks very much to both who answered.

The final solution I implemented used the code mentioned in the accepted answer within the drawRect method of a subclassed UIView, I used the following code to overlay the color:

CGContextSetFillColor(context, CGColorGetComponents([UIColor colorWithRed:0.5 green:0.5 blue:0 alpha:1].CGColor));

CGContextFillRect(context, area);
Tobias M
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2 Answers2

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I think you are probably looking for the blend mode kCGBlendModeSourceAtop. First, draw the UIImage into your view. Then obtain the current graphics context with

CGContext currentCtx = UIGraphicsGetCurrentContext();

Then save the context state, and set the blend mode:

CGContextSaveGState(currentCtx);
CGContextSetBlendMode(currentCtx, kCGBlendModeSourceAtop);

Then you can draw whatever you want to be overlaid over the opaque pixels of the UIImage. Afterward, just reset the context state:

CGContextRestoreGState(currentCtx);

Hope this helps!

voidzm
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You can modify how something is drawn using the blend mode. See http://developer.apple.com/library/ios/#documentation/GraphicsImaging/Reference/CGContext/Reference/reference.html%23//apple_ref/doc/c_ref/CGBlendMode for a full list of the blend modes supported by Core Graphics on iOS. From your description, I think you would be interested in either kCGBlendModeSourceIn, which draws the new content using the old content's alpha value as a mask, or kCGBlendModeSourceAtop, which draws the new content using the old content's alpha as a mask on top of the old content using the new content's alpha value as a mask. You can set the blend mode for all drawing using CGContextSetBlendMode, or you can draw a UIImage with a certain blend mode using -[UIImage drawAtPoint:blendMode:alpha:].

ughoavgfhw
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