plucky (3) al_set_target_bitmap.3alleg5.gz

NAME

       al_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 operations 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(3alleg5).  As a convenience, you may also use al_set_target_backbuffer(3alleg5).

       Each allegro bitmap maintains two transformation matrices associated with it for drawing onto the bitmap.
       There is a view matrix and a projection matrix.  When you call al_set_target_bitmap, these will  be  made
       current for the bitmap, affecting global OpenGL and DirectX states depending on the driver in use.

       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 automatically made “current” for the calling thread (if it is  not  current
       already).  Then drawing other bitmaps which are tied to the same display 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 display is  current  for  the  calling
       thread.

       On  some  platforms,  Allegro  automatically  backs  up the contents of video bitmaps because they may be
       occasionally lost (see discussion in al_create_bitmap(3alleg5)’s documentation).   If  you’re  completely
       recreating  the  bitmap  contents  often (e.g.  every frame) then you will get much better performance by
       creating the target bitmap with ALLEGRO_NO_PRESERVE_TEXTURE flag.

       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  following  conditions are met an FBO will be created for use with the
       bitmap:

       • 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(3alleg5) 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(3alleg5) 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(3alleg5) on a locked bitmap without creating 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 bitmap’s associated texture.

SEE ALSO

       al_get_target_bitmap(3alleg5), al_set_target_backbuffer(3alleg5)