cocoa: Read pixel color of NSImage

Question

I have an NSImage. I would like to read the NSColor for a pixel at some x and y. Xcode seems to thing that there is a colorAtX:y: method on NSImage, but this causes a crash saying that there is no such method for NSImage. I have seen some examples where you create an NSBitmapImageRep and call the same method on that, but I have not been able to successfully convert my NSImage to an NSBitmapImageRep. The pixels on the NSBitmapImageRep are different for some reason.

There must be a simple way to do this. It cannot be this complicated.

Answer 1

Without seeing your code it's difficult to know what's going wrong.

You can draw the image to an NSBitmapImageRep using the initWithData: method and pass in the image's TIFFRepresentation.

You can then get the pixel value using the method colorAtX:y:, which is a method of NSBitmapImageRep, not NSImage:

NSBitmapImageRep* imageRep = [[NSBitmapImageRep alloc] initWithData:[yourImage TIFFRepresentation]];
NSSize imageSize = [yourImage size];
CGFloat y = imageSize.height - 100.0;
NSColor* color = [imageRep colorAtX:100.0 y:y];
[imageRep release];

Note that you must make an adjustment for the y value because the colorAtX:y method uses a coordinate system that starts in the top left of the image, whereas the NSImage coordinate system starts at the bottom left.

Alternatively, if the pixel is visible on-screen then you can use the NSReadPixel() function to get the color of a pixel in the current coordinate system.

Answer 2

Function colorAtX of NSBitmapImageRep seems not to use the device color space, which may lead to color values that are slightly different from what you actually see. Use this code to get the correct color in the current device color space:

[yourImage lockFocus]; // yourImage is just your NSImage variable
NSColor *pixelColor = NSReadPixel(NSMakePoint(1, 1)); // Or another point
[yourImage unlockFocus];