iPhone UIImage from texture has gray instead of alpha - iphone

Question

I'm using the following code to get UIImage from screen:
CGSize s = [self getSize];
int tx = s.width;
int ty = s.height;
int bitsPerComponent = 8;
int bitsPerPixel = 32;
int bytesPerPixel = (bitsPerComponent * 4)/8;
int bytesPerRow = bytesPerPixel * tx;
NSInteger myDataLength = bytesPerRow * ty;
NSMutableData *buffer = [[NSMutableData alloc] initWithCapacity:myDataLength];
NSMutableData *pixels = [[NSMutableData alloc] initWithCapacity:myDataLength];
if( ! (buffer && pixels) ) {
[buffer release];
[pixels release];
return nil;
}
glReadPixels(0,0,tx,ty,GL_RGBA,GL_UNSIGNED_BYTE, [buffer mutableBytes]);
// make data provider with data.
CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast | kCGBitmapByteOrderDefault;
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, [buffer mutableBytes], myDataLength, NULL);
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGImageRef iref = CGImageCreate(tx, ty,
bitsPerComponent, bitsPerPixel, bytesPerRow,
colorSpaceRef, bitmapInfo, provider,
NULL, false,
kCGRenderingIntentDefault);
CGContextRef context = CGBitmapContextCreate([pixels mutableBytes], tx,
ty, CGImageGetBitsPerComponent(iref),
CGImageGetBytesPerRow(iref), CGImageGetColorSpace(iref),
bitmapInfo);
CGContextTranslateCTM(context, 0.0f, ty);
CGContextScaleCTM(context, 1.0f, -1.0f);
CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, tx, ty), iref);
CGImageRef outputRef = CGBitmapContextCreateImage(context);
UIImage* image = [[UIImage alloc] initWithCGImage:outputRef];
CGImageRelease(iref);
CGContextRelease(context);
CGColorSpaceRelease(colorSpaceRef);
CGDataProviderRelease(provider);
CGImageRelease(outputRef);
[pixels release];
[buffer release];
return [image autorelease];
What I get is an image that whenever there should be transparency (there is in opengl), there is just color (255, 255, 255, 226). How can I fix this?

Answer 1

CGContextDrawImage doesn't just copy data over, it draws the source image onto the destination image. So at that point, it'll be using your alpha channel as an alpha channel as an alpha channel rather than merely copying it across.
To be honest, I'm a bit confused as to the point of context and image, if you don't actually want to modify the data that comes back from OpenGL. I'd recommend you chop to just:
CGImageRef iref = CGImageCreate(tx, ty,
bitsPerComponent, bitsPerPixel, bytesPerRow,
colorSpaceRef, bitmapInfo, provider,
NULL, false,
kCGRenderingIntentDefault);
/* ... lots of stuff cut out here ... */
UIImage* image = [[UIImage alloc] initWithCGImage:iref];