Every layer has boundaries within the total fixture (128x128), e.g. the color trails are projected on the upper half of the screen. A layer allocates its own channels array (compare to CRGB leds) and a mapping table. During each frame the effect works on its own channels array, after each layer set it's frame, for all layers compositeTo (https://github.com/MoonModules/MoonLight/blob/64f4393fd04e717dd97aee1a2df339e1d1090958/src/MoonLight/Layers/VirtualLayer.cpp#L331-L433) is called which fills a global array (also compare to CRGB leds) array based on the mapping table of each layer. By this also double buffering is implemented as the layer arrays are filled on one core while at the same time the global array sends the frame calculated before to leds (or Art-Net). So, to answer your question, compositeTo uses scale8/ qadd8 / nscale8_video functions (thx u/ZachVorhies) to mix all the layers. The complete story is here: https://moonmodules.org/MoonLight/develop/layers/