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POCorrupt pixels when combining images in iOS
 

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    <p>I'm trying to combine two photos into one image (think a body with a hole in the face on top of a picture of a different person's face). The top image has some semi-transparent pixels and some fully transparent pixels and I want to overlay it on top of a solid image.</p> <p>Here's what I'm doing: I have a Context with the right size and I draw the bottom image on it, without any alpha (faceImage). On top of that I draw an image that has a transparent hole in it, with various levels of transparencies (coverImage): </p> <pre><code>UIGraphicsBeginImageContext(view.bounds.size); [faceImage drawAtPoint:CGPointMake(0, 0) blendMode:kCGBlendModeNormal alpha:1]; [coverImage drawAtPoint:CGPointMake(0, 0) blendMode:kCGBlendModeNormal alpha:1]; UIImage *combinedImage = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); </code></pre> <p>The results in the final image are quite unexpected:</p> <p>In pixels where the <strong>top image has no transparency</strong> the final image is saved properly and shows the pixel from the top image. (<strong>OK</strong>)</p> <p>In pixels where the <strong>top image has full transparency</strong> the final image is saved properly and shows the pixel from the bottom image. (<strong>OK</strong>)</p> <p>In pixels where the <strong>top image has semi-transparent pixels and the brightness is very light</strong> (the brightness of the final pixel) I suddenly get entirely <strong>transparent pixels</strong> (instead of a final pixel with no transparency that's a blend between the top pixel and the bottom pixel). (WTF?)</p> <p>In the image below you can see weird blotches of white. Those are the pixels that became entirely transparent pixels (you see the white background through them): <img src="https://i.stack.imgur.com/B4zaN.png" alt="The combined image with the weird transparent pixels"></p> <p>This is the image that I'm putting on top:</p> <p><img src="https://i.stack.imgur.com/cVAiV.png" alt="The top image with the semi transparent pixels"></p> <p>This is the face image on the bottom:</p> <p><img src="https://i.stack.imgur.com/4039z.png" alt="The face image on the bottom"></p> <p>Any ideas what could be causing this?</p> <p>TIA</p>
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