Clearing the alpha channel of an NSImage

Question

It can be done by mallocing a temporary bitmap with 32bits per pixel and then clearing the alpha component with a for loop and and finally turn it back into a NSImage again.

I suspect is can be done in a simpler way using a clever combination of NSColor and NSCompositingOperation. Or perhaps the image needs to be composited with itself using drawAtPoint.

My code looks like this.

NSImage* img = some image with RGB and Alpha;
NSRect rect  = some rect inside the image;
[img lockFocus];
[[NSColor clearColor] set];
NSRectFillUsingOperation(rect, NSCompositeXOR);
[img unlockFocus];

NOTE: Setting the alpha channel to 1 can be done by using a blackColor with NSCompositePlusLighter.

What is the secret in clearing the alpha channel?

Answer 1

It won't be fast but this will work as well:

NSImage *newImage = [[NSImage alloc] initWithSize:[srcImage size]];
[newImage lockFocus];
[[NSColor whiteColor] set];
NSRectFill(NSMakeRect(0,0,[newImage size].width, [newImage size].height));
[srcImage compositeToPoint:NSZeroPoint operation:NSCompositeCopy];
[newImage unlockFocus];