Note that most gpu’s generally dither if the display driver decides that is the right thing to do for a given framebuffer color depth, gamma table, display cable, display device and video mode. Essentially whenever the pixel color data has a higher bit precision (bpc) than what the display cable + device can consume at a given video mode, the driver will set the gpu to dither to emulate the given bpc on the < bpc precision video link.
Psychtoolbox has special setup code to get the driver to use identity pixel passthrough and disable dithering on X11 with native XOrg X-Server, in operation modes that require this, e.g., running visual stimulators from CRS or VPixx in some op-modes. This does not mean that just because you don’t have dithering when using Psychtoolbox, that the gpu will generally not dither, e.g., while using a desktop GUI, or while using any of the other more primitive vision science toolkits.
The code is assumed to work on AMD gpu’s on Linux with native X-Server and AMD on Windows, and Intel gpu’s on Linux. NVidia gpu’s may or may not have their own controls depending on gpu and driver and OS in use.