metal:offscreen

(metal:offscreen &key width height clear depth)

A metal:context with no window at all: frames are drawn into a texture of its own and read back with metal:pixels. :width and :height are pixels (there is no backing scale without a screen to have one), :clear is the (r g b a) a frame starts from and :depth asks for a depth attachment. Everything else -- metal:library, metal:pipeline, metal:depth-state, metal:buffer, metal:uniform, metal:frame -- is the same function a layer-backed context takes, which is the point: what this renders is what the window renders, not a second path that resembles it. The device comes from a throwaway CAMetalLayer's preferredDevice exactly as metal:attach's does, and no display is needed for that property to answer. Part of the metal package, a Metal drawing surface written in rontolisp over objc and loaded on first use: macOS (java -jar, the rontolisp binary, or a compiled .class / .jar; never a .wasm). See the macOS GUI guide.

CL-USER> (defvar *ctx* (metal:offscreen :width 256 :height 192 :depth t))
CL-USER> (length (metal:pixels *ctx*))
196608