al_draw_bitmap fails to run on threads other than the parent thread

iam_donald

I'm thought it'd be wise to have the drawing code of my game run on a separate thread for obvious performance reasons. After several attempts at this i decided to experiment and just run calls of al_draw_bitmap on a separate thread. Here is the code I used for this:

#SelectExpand
  1void draw_video_frame(ALLEGRO_BITMAP * frame) { al_draw_bitmap(frame, 0, 0, 0); }
  2
  3void * t_draw_video_frame(ALLEGRO_THREAD * thread, void * arg)
{
  ALLEGRO_BITMAP * frame  = (ALLEGRO_BITMAP *) arg;
  draw_video_frame(frame);
  return NULL;
}
  9
 10void stream_video(Video * video)
{
  load_frame(video);
  video_frame_step(video);
  ALLEGRO_THREAD * thread  = al_create_thread(t_draw_video_frame, (void *) video->last_stepped_frame);
  al_start_thread(thread);
  al_join_thread(thread, NULL);
  destroy_video_frame(video->prev_stepped_frame);
  al_destroy_thread(thread);
}

This fails with the error: Assertion failed: (src), function al_copy_transform, file /Users/iam_donald/Downloads/allegro/src/transformations.c, line 32.

Calling draw_video_frame() works. My question is: Is this behavior by design or I am I doing something silly here? Currently this behavior doesn't allow running any chunk of code with calls to al_draw_bitmap on a thread other than parent thread.

Matthew Leverton

See al_set_target_bitmap().

Quote:

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);

So set the parent's target bitmap to NULL and set the child's to the display's back buffer.

iam_donald

That explains it. Thanks.

Thread #610506. Printed from Allegro.cc