Documentation

bm Blend Mode

Blend Modes set how the blend color or image set with blend will transfer onto your main image. For example, multiply will multiply the the pixel values from the two images resulting in a darker composite image. screen will make lighter pixels stand out. Acceptable operations for bm are: color, burn, dodge, darken, difference, exclusion, hardlight, hue, lighten, luminosity, multiply, overlay, saturation, screen, softlight, and normal.

Setting bm=normal turns off blending, causing the blend image to be superimposed. This behavior is intended for use with watermarks, similar to the mark parameter.