Question

I’m trying to get ARGB components from CGBitmapContext with the following codes:

 -(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; } 

 -(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 }
  
-(void)viewDidLoad {     [super viewDidLoad];     UIImage *img = [UIImage imageNamed:@'MacDrive.png'];    //load image
  
[self initWithImage:img];   //create BitmapContext with UIImage float alpha = [self alphaAtX:20 y:20];  //store alpha component 
} 
When I try to store red/green/blue, they turn out to be always 240. And alpha is always 255.
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?
												

Answer 1

First of all, you’re leaking memory, the pixelData memory never gets freed.

For your usage, it’s better to let the CGContext calls manage the memory. Simply pass NULL i.s.o. pixelData, and as long as you keep the reference count up on your context, the CGBitmapContextGetData(context) will remain valid.

You’re using alpha as if it was the last entry (RGBA, not ARGB), use it as such in the creation of your context, or adapt the alphaAtX code. I’m not sure if you want premultiplied, if it’s just to check alpha values, you don’t.

All in all something like:

[...] CGContextRelease(context); context = CGBitmapContextCreate (NULL,                                                                                          image.size.width,                                     image.size.height,                                     8,    // bits per component                                     bitmapBytesPerRow,                                     colorSpace,                                     kCGImageAlphaLast                                     ); CGColorSpaceRelease( colorSpace );                                                                        CGContextDrawImage(context, CGRectMake(0, 0, image.size.width, image.size.height), image.CGImage); pixelData = CGBitmapContextGetData(context); [...] 

with context being a member variable, initialized to NULL, will already be a step in the right direction.