Now this I say, he who sows sparingly will also reap sparingly, and he who sows bountifully will also reap bountifully. Each one must do just as he has purposed in his heart, not grudgingly or under compulsion, for God loves a cheerful giver. And God is able to make all grace abound to you, so that always having all sufficiency in everything, you may have an abundance for every good deed. - 2 Corinthians 9:6-8
Composite Operator
Over
Overlay second image over the first
Overlay
Multiplies or screens the colors, dependent on the destination color. Source colors overlay the destination whilst preserving its highlights and shadows. The destination color is not replaced, but is mixed with the source color to reflect the lightness or darkness of the destination.
In
The part of the source lying inside of the destination replaces the destination.
Out
The part of the source lying outside of the destination replaces the destination.
Atop
The part of the source lying inside of the destination is composited onto the destination.
XOR
The part of the source that lies outside of the destination is combined with the part of the destination that lies outside the source.
Plus
The source is added to the destination and replaces the destination. This operator is useful for animating a dissolve between two images.
Minus
The source is subtracted to the destination and replaces the destination.
Difference
Subtracts the darker of the two constituent colors from the lighter. Painting with white inverts the destination color. Painting with black produces no change.
Multiply
The source is multiplied by the destination and replaces the destination. The resultant color is always at least as dark as either of the two constituent colors. Multiplying any color with black produces black. Multiplying any color with white leaves the original color unchanged.
Bumpmap
The same as Multiply, except the source is converted to greyscale first.
Copy
Simply place the source on top of the destination.