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POPNG Image with BGRA CgBI premultiplied alpha
 

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  1. POPNG Image with BGRA CgBI premultiplied alpha
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    <p>I have PNGs in Apple's iOS optimized BGRA PNG format (what I get using <a href="https://github.com/basuke/OptimizedPNG" rel="nofollow">OptimizedPNG</a>) and want to draw them in a way that tells CoreGraphics NOT to ignore the alpha component of the image. I'm drawing to a <code>CGContextRef</code> in <code>drawRect:</code></p> <p>Edit: the rendered image shows black where it should be fully transparent (sometimes other random artifacts). The opaque areas are rendered normally. <code>CGImageAlphaInfo</code> I get from the image is <code>kCGImageAlphaNoneSkipLast</code>, which <em>seems</em> to indicate there is a problem in the way the image is saved by OptimizedPNG. I think this should be <code>kCGImageAlphaPremultipliedLast</code>.</p> <p>Perhaps the PNG chunks are wrong, but I don't see anything wrong with IHDR, and there is very little I can find about the CgBI chunk. </p> <p>This is how OptimizedPNG saves the color data:</p> <pre><code>// IDAT int size = width*height*4; unsigned char *buffer = malloc(size); CGContextRef context = CGBitmapContextCreate(buffer, width, height, 8, width*4, CGImageGetColorSpace(originalImage.CGImage), kCGImageAlphaPremultipliedLast); CGRect rect = CGRectMake(0.0, 0.0, (CGFloat)width, (CGFloat)height); CGContextDrawImage(context, rect, originalImage.CGImage); CGContextRelease(context); int size_line = 1 + width*4; int size_in = height*size_line; unsigned char *buffer_in = malloc(size_in); for(int y = 0; y &lt; height; ++y){ unsigned char *src = &amp;buffer[y*width*4]; unsigned char *dst = &amp;buffer_in[y*size_line]; *dst++ = 1; unsigned char r = 0, g = 0, b = 0, a = 0; for(int x = 0; x &lt; width; ++x){ dst[0] = src[2] - b; dst[1] = src[1] - g; dst[2] = src[0] - r; dst[3] = src[3] - a; r = src[0], g = src[1], b = src[2], a = src[3]; src += 4; dst += 4; } } </code></pre>
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    1. COCore Graphics honors the image's alpha channel, if it has one. Core Graphics transparency layers actually have little to do with image alpha. Please post a screen shot of the result you're getting, and describe how you want it to be different.
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    2. CO@robmayoff edited with more information.
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