ALLEGRO_SHADER_TYPE man page
ALLEGRO_SHADER_TYPE — Allegro 5 API
#include <allegro5/allegro.h> typedef enum ALLEGRO_SHADER_TYPE ALLEGRO_SHADER_TYPE;
Used with al_attach_shader_source(3) and al_attach_shader_source_file(3) to specify how to interpret the attached source.
A vertex shader is executed for each vertex it is used with. The program will output exactly one vertex at a time.
When Allegro's graphics are being used then in addition to all vertices of primitives from the primitives addon, each drawn bitmap also consists of four vertices.
A pixel shader is executed for each pixel it is used with. The program will output exactly one pixel at a time - either in the backbuffer or in the current target bitmap.
With Allegro's builtin graphics this means the shader is for example called for each destination pixel of the output of an al_draw_bitmap(3) call.
A more accurate term for pixel shader would be fragment shader since one final pixel in the target bitmap is not necessarily composed of only a single output but of multiple fragments (for example when multi-sampling is being used).