al_set_target_bitmap(3)al_set_target_bitmap(3)NAMEal_set_target_bitmap - Allegro 5 API
SYNOPSIS
#include <allegro5/allegro.h>
void al_set_target_bitmap(ALLEGRO_BITMAP *bitmap)
DESCRIPTION
This function selects the bitmap to which all subsequent drawing opera‐
tions in the calling thread will draw to. To return to drawing to a
display, set the backbuffer of the display as the target bitmap, using
al_get_backbuffer(3). As a convenience, you may also use al_set_tar‐
get_backbuffer(3).
Each video bitmap is tied to a display. When a video bitmap is set to
as the target bitmap, the display that the bitmap belongs to is auto‐
matically made "current" for the calling thread (if it is not current
already). Then drawing other bitmaps which are tied to the same dis‐
play can be hardware accelerated.
A single display cannot be current for multiple threads simultaneously.
If you need to release a display, so it is not current for the calling
thread, call al_set_target_bitmap(NULL);
Setting a memory bitmap as the target bitmap will not change which dis‐
play is current for the calling thread.
OpenGL note:
Framebuffer objects (FBOs) allow OpenGL to directly draw to a bitmap,
which is very fast. When using an OpenGL display, if all of the fol‐
lowing conditions are met an FBO will be created for use with the bit‐
map:
· The GL_EXT_framebuffer_object OpenGL extension is available.
· The bitmap is not a memory bitmap.
· The bitmap is not currently locked.
In Allegro 5.0.0, you had to be careful as an FBO would be kept around
until the bitmap is destroyed or you explicitly called
al_remove_opengl_fbo(3) on the bitmap, wasting resources. In newer
versions, FBOs will be freed automatically when the bitmap is no longer
the target bitmap, unless you have called al_get_opengl_fbo(3) to
retrieve the FBO id.
In the following example, no FBO will be created:
lock = al_lock_bitmap(bitmap);
al_set_target_bitmap(bitmap);
al_put_pixel(x, y, color);
al_unlock_bitmap(bitmap);
The above allows using al_put_pixel(3) on a locked bitmap without cre‐
ating an FBO.
In this example an FBO is created however:
al_set_target_bitmap(bitmap);
al_draw_line(x1, y1, x2, y2, color, 0);
An OpenGL command will be used to directly draw the line into the bit‐
map's associated texture.
SEE ALSOal_get_target_bitmap(3), al_set_target_backbuffer(3)Allegro reference manualal_set_target_bitmap(3)
