iPhone: Changing CGImageAlphaInfo of CGImage
alpha, cgbitmapcontextcreate, cgimage, iphone, png
Solution
Yeah, I had problems with 8 bit (indexed) .PNGs. I had to convert it to a more native image to perform graphics operations. I essentially did something like this:
- (UIImage *) normalize {
CGColorSpaceRef genericColorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef thumbBitmapCtxt = CGBitmapContextCreate(NULL,
self.size.width,
self.size.height,
8, (4 * self.size.width),
genericColorSpace,
kCGImageAlphaPremultipliedFirst);
CGColorSpaceRelease(genericColorSpace);
CGContextSetInterpolationQuality(thumbBitmapCtxt, kCGInterpolationDefault);
CGRect destRect = CGRectMake(0, 0, self.size.width, self.size.height);
CGContextDrawImage(thumbBitmapCtxt, destRect, self.CGImage);
CGImageRef tmpThumbImage = CGBitmapContextCreateImage(thumbBitmapCtxt);
CGContextRelease(thumbBitmapCtxt);
UIImage *result = [UIImage imageWithCGImage:tmpThumbImage];
CGImageRelease(tmpThumbImage);
return result;
}
Problem
I have a PNG image that has an unsupported bitmap graphics context pixel format. Whenever I attempt to resize the image, `CGBitmapContextCreate()` chokes on the unsupported format I receive the following error (error formatted for easy reading): ``` CGBitmapContextCreate: unsupported parameter combination: 8 integer bits/component; 32 bits/pixel; 3-component colorspace; kCGImageAlphaLast; 1344 bytes/row. ``` The list of supported pixel formats definitely does not support this combination. It appears I need to redraw the image and move the alpha channel information to kCGImageAlphaPremultipliedFirst or kCGImageAlphaPremultipliedLast. I have no idea how to go about doing this. There is nothing unusual about the PNG file and it isn't corrupted. It works in all other context just fine. I encountered this error just by chance but obviously my users might have similarly formatted files so I will have to check my app's imported images and correct for this problem.