Combining two pictures into one can be done in many ways. The most useful methods are provided here:
The top picture is drawn on top of the bottom picture. Parts of the bottom picture are hidden.
This gives the same visual result as two transparent pieces of film placed on top of each other in front of a light source (e.g. two slides on a light box). The resulting image is always darker than the input pictures.
This multiplies the negative of one picture by the other picture. The visual effect is the same as a simple addition (e.g. two images projected on a screen) but the result is better than a simple addition because the highlights don't burn. The resulting image is always brighter than the input pictures.
The two images are compared, the darkest or lightest pixel of each input picture is used for the composite image.
- Overlay/Hard Light
Multiplies or screens the colors, depending on the pixel values of the input picture.
Creates a rich and colorful result using subtraction. The resulting image is a mix of positive and negative images.
Invert the top input picture. This gives a simple result with flat black areas.
This uses any of the previous mode in a random way. This is the default value.
If you want to know more about mixing/blending modes you can get good information at: