The fuzz effect, or partial invisibility effect, creates a shimmering translucency effect by replacing the actor's pixels by darker versions of one of their neighbors. In addition to actors, decals can also be drawn with the fuzz effect, by using the fuzzy keyword in DECALDEF.
An accurate implementation of the fuzz effect in an OpenGL renderer is difficult. GZDoom provides different shaders to emulate the look, available through the gl_fuzztype console variable accessible from the OpenGL preferences menu; with "shadow" as a backup option in case the hardware used does not support pixel shaders.
Actors with the Fuzzy or OptFuzzy render styles are displayed with the partial invisibility effect. However, those with the OptFuzzy render style can be displayed with a different method depending on the value of the r_drawfuzz console variable, accessible through the display options menu. Alternatives to the fuzzy style include "translucent" and "shadow".
The gallery below shows one of the first spectres encountered with various settings in ZDoom and GZDoom.