al_orthographic_transform - Allegro 5 API
#include <allegro5/allegro.h> void al_orthographic_transform(ALLEGRO_TRANSFORM *trans,
float left, float top, float n,
float right, float bottom, float f)
Combines the given transformation with an orthographic transformation which maps the screen rectangle to the given left/top and right/bottom coordinates.
near/far is the z range, coordinates outside of that range will get clipped. Normally -1/1 is fine because all 2D graphics will have a z coordinate of 0. However if you for example do al_draw_rectangle(0, 0, 100, 100) and rotate around the x axis (“towards the screen”) make sure your z range allows values from -100 to 100 or the rotated rectangle will get clipped.
Also, if you are using a depth buffer the z range decides the depth resolution. For example if you have a 16 bit depth buffer there are only 65536 discrete depth values. So if your near/far is set to -1000000/1000000 most of the z positions would not result in separate depth values which could lead to artifacts.
The result of applying this transformation to coordinates will be to normalize visible coordinates into the cube from -1/-1/-1 to 1/1/1. Such a transformation is mostly useful for passing it to al_use_projection_transform(3alleg5) - see that function for an example use.
|Allegro reference manual|