al_use_projection_transform man page
al_use_projection_transform — Allegro 5 API
#include <allegro5/allegro.h> void al_use_projection_transform(const ALLEGRO_TRANSFORM *trans)
Sets the projection transformation to be used for the the drawing operations on the target bitmap (each bitmap maintains its own projection transformation). Every drawing operation after this call will be transformed using this transformation. To return default behavior, call this function with an orthographic transform like so:
ALLEGRO_TRANSFORM trans; al_identity_transform(&trans); al_orthographic_transform(&trans, 0, 0, -1.0, al_get_bitmap_width(bitmap), al_get_bitmap_height(bitmap), 1.0); al_set_target_bitmap(bitmap); al_use_projection_transform(&trans);
The orthographic transformation above is the default projection transform.
This function does nothing if there is no target bitmap. This function also does nothing if the bitmap is a memory bitmap (i.e. memory bitmaps always use an orthographic transform like the snippet above). Note that the projection transform will be reset to default if a video bitmap is converted to a memory bitmap. Additionally, if the bitmap in question is the backbuffer, it's projection transformation will be reset to default if it is resized. Lastly, when you draw a memory bitmap to a video bitmap with a custom projection transform, this transformation will be ignored (i.e. it'll be as if the projection transform of the target bitmap was temporarily reset to default).
The parameter is passed by reference as an optimization to avoid the overhead of stack copying. The reference will not be stored in the Allegro library so it is safe to pass references to local variables.
al_acknowledge_resize(3), al_get_current_projection_transform(3), al_orthographic_transform(3), al_perspective_transform(3), al_scale_transform_3d(3), al_transform_coordinates_3d_projective(3), al_translate_transform_3d(3).