FlySketch Graphic Compositing Modes

If you are running FlySketch 1.5 on Mac OS X 10.4 or later, you will notice an extra pallet with various "Compositing modes". What the heck are those?

Well, direct from Apple, here's what you get:

• Clear
  Transparent.

• Copy
  Source image.

• Destination Atop
  Destination image wherever both images are opaque, source image wherever source image is opaque but destination image is transparent, and transparent elsewhere.

• Destination In
  Destination image wherever both images are opaque, and transparent elsewhere.

• Destination Out
  Destination image wherever destination image is opaque but source image is transparent, and transparent elsewhere.

• Destination Over
  Destination image wherever destination image is opaque, and source image elsewhere.

• Plus Darker
  Sum of source and destination images, with color values approaching 0 as a limit.

• Plus Lighter
  Sum of source and destination images, with color values approaching 1 as a limit.

• Source Atop
  Source image wherever both images are opaque, destination image wherever destination image is opaque but source image is transparent, and transparent elsewhere.

• Source In
  Source image wherever both images are opaque, and transparent elsewhere.

• Source Out
  Source image wherever source image is opaque but destination image is transparent, and transparent elsewhere.

• Source Over
  Source image wherever source image is opaque, and destination image elsewhere.

• XOR
  Exclusive OR of source and destination images. Works only with black and white images and is not recommended for color contexts.

Shamelessly stolen from: http://developer.apple.com/documentation/Cocoa/Reference/ApplicationKit/ObjC_classic/Classes/NSImage.html