What is the absolute lowest level of "drawing" abstraction in GNU/Linux?

Question

Motivation - Write a program in C (and Assembly, if required) to color a rectangular area in the screen red.

STRICT requirements - GNU/Linux running with the bare minimum utilities and interfaces in text/console mode. So no X (or equivalent like Wayland/Mir), no non-default (outside POSIX, LSB, etc. provided by the kernel) library or interface and no extra assumptions except the presence of the device driver for the monitor.

Effectively, what I am looking for is information on how to write a program which will eventually send a signal through the VGA port and cable to the monitor to color a particular portion of the screen red.

Apologies if this sounds rude, but no "Why do you want to do this?" or "Why don't you use ABC library?" answer. I am trying to understand how to write an implementation of the X server or a kernel framebuffer (/dev/fb0) library for example. It is ok to provide a link to the source of a C library.

Answer 1

no extra assumptions except the presence of the device driver for the monitor.

That means you can use X or Wayland, because those are the graphics driver infrastructure on Linux.

Linux (the Kernel) by itself doesn't contain any graphics primitives. It provides some interfaces to talk to the GPU, allocate memory on it and configure the on-screen framebuffer. But except raw framebuffer memory access the Linux kernel does have no way to perform drawing operations. For that you need some infrastructure in userspace.

Wayland builds on top of DRI2, which in turn talks to the DRM Kernel-API. Then you require GPU dependent state tracker. Mesa has state trackers for a number of GPUs and provides OpenGL and OpenVG frontends.

The NVidia and ATI propiatary, closed source graphics drivers are designed to work with X only. So with those to make use of the GPU you must use X. That's the way it is.

Outside of that you can manipulate the on-screen framebuffer memory through /dev/fbdev, but that's mere pixel pushing, without any GPU acceleration.

Answer 2

Once upon a time we we had svgalib (or was it called vgalib?) which did exactly what you are trying to do. I would recomend that you look at its source code. I don't know if it is still possible to find it anywhere, or if it would actually work with a modern kernel. Just whatever you do, be prepared to reboot often.