Icon-Compositor 0.0
How to create a Drop Shadow

A drop shadow is used to give the illusion that there is some distance between the background and an icon.

By convention, the fictional light source comes from the top left — ignoring this convention, dropping shadows in the wrong direction, can have the opposite effect to the one you were after. The shadow we create will therefore be drawn below and to the right of our “flat” icon.

icomp 'd = dragon.png ; d over offset(set_alpha(black * d, 50), 3, 3)' -o drop_shadow_dragon.png
   
The set_alpha function is used to make the black dragon be mostly transparent, resulting in a gentle darkening of the background. The offset function is used to move the shadow out from under the icon, in the direction that an upper left light source would throw the shadow.

Don't space the shadow too far from the shadow source or you risk making to look too fake.