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POHow to pull out the ARGB component from BitmapContext on iPhone?
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copied!<p>I'm trying to get ARGB components from CGBitmapContext with the following codes:</p> <pre><code> -(id) initWithImage: (UIImage*) image //create BitmapContext with UIImage and use 'pixelData' as the pointer { CGContextRef context = NULL; CGColorSpaceRef colorSpace; int bitmapByteCount; int bitmapBytesPerRow; bitmapBytesPerRow = (image.size.width * 4); bitmapByteCount = (bitmapBytesPerRow * image.size.height); colorSpace = CGColorSpaceCreateDeviceRGB(); pixelData = malloc( bitmapByteCount ); //unsigned char* pixelData is defined in head file context = CGBitmapContextCreate (pixelData, image.size.width, image.size.height, 8, // bits per component bitmapBytesPerRow, colorSpace, kCGImageAlphaPremultipliedFirst ); CGColorSpaceRelease( colorSpace ); CGContextDrawImage(context, CGRectMake(0, 0, image.size.width, image.size.height), image.CGImage); pixelData = CGBitmapContextGetData(context); return self; } </code></pre> <p><pre><code> -(float) alphaAtX:(int)x y:(int)y //get alpha component using the pointer 'pixelData' { return pixelData[(y *width + x) *4 + 3]; //+0 for red, +1 for green, +2 for blue, +3 for alpha }</p> <p>-(void)viewDidLoad { [super viewDidLoad]; UIImage *img = [UIImage imageNamed:@"MacDrive.png"]; //load image</p> <pre><code>[self initWithImage:img]; //create BitmapContext with UIImage float alpha = [self alphaAtX:20 y:20]; //store alpha component </code></pre> <p>} </pre></code></p> <p>When I try to store red/green/blue, they turn out to be always 240. And alpha is always 255.</p> <p>So I think maybe something is wrong with the pointer. It could not return the correct ARGB data I want. Any ideas about what's wrong with the code?</p>

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