What's the difference between an CGImageRef that is a mask, and one that is not a mask?

I discovered that CGBitmapContextCreateImage() creates an image, which is not neccessarily always a mask compatible with CGContextClipToMask(). But when using CGImageMaskCreate(), the CGImageRef is always a mask that works with 开发者_C百科CGContextClipToMask(). Now, what is so special about the mask VS. the "normal" image?

My guess is that the mask is grayscale only, where as an CGImageRef created with CGBitmapContextCreateImage() may have RGBA values which irritate CGContextClipToMask(). I couldn't find the spot in the documentation where the exact difference between masks and CG images is explained.

But it seems that an Core Graphics image != a mask, while a mask == a Core Graphics Image

Every value in an image, be it RGB, CMYK or Greyscale, represents a position in a particular colorspace. It is meaningful to ask "What would this value be in colour-space 'x'?" - and the result would, if possible, be the same colour, but could be a different numerical value.

eg (simplistically). A pixel with value (255,255,255) is White in an RGB colorspace but Black in a (hypothetical) CMY colour-space. Converting the White RGB pixel to the CMY colorspace would give the value (0,0,0). In other words an Image must have a colorspace, it only makes sense given a colorspace.

On the contrary, an 8bit mask represents absolute values between 0-255. There is no colorspace and it makes no sense to think of a mask in a particular colorspace.

In that way images and masks are fundamentally different, even though we often think of masks as greyscale images.

An image mask in Core Graphics is a special kind of image. From the CGImageMaskCreate reference:

A Quartz bitmap image mask is used the same way an artist uses a silkscreen, or a sign painter uses a stencil. The bitmap represents a mask through which a color is transferred. The bitmap itself does not have a color. It gets its color from the fill color currently set in the graphics state.

When you draw into a context with a bitmap image mask, Quartz uses the mask to determine where and how the current fill color is applied to the image rectangle. Each sample value in the mask specifies how much of the current fill color is masked out at a specific location. Effectively, the sample value specifies the opacity of the mask. Larger values represent greater opacity and hence less color applied to the page.

See more here: http://developer.apple.com/library/mac/#documentation/GraphicsImaging/Reference/CGImage/Reference/reference.html