OpenGL clipping on sub bitmaps

no-reply@allegro.cc (Matthew Leverton) — Mon, 13 Dec 2010 12:32:06 +0000

Perhaps related to this thread. It doesn't look like OpenGL honors clipping on sub bitmaps. Test program:

#SelectExpand
  1int main()
  2{
  3    al_init();
  4    al_init_primitives_addon();
  5    al_set_new_display_flags(ALLEGRO_OPENGL);
  6
  7    ALLEGRO_DISPLAY *display = al_create_display(320, 240);
  8    ALLEGRO_BITMAP *sub = al_create_sub_bitmap(al_get_backbuffer(display), 0, 0, 320, 240);
  9
 10    // clear to black
 11    al_set_target_bitmap(sub);
 12    al_clear_to_color(al_map_rgb(0,0,0));
 13
 14    // 64x64 clip with filled white rectangle
 15    al_set_clipping_rectangle(0, 0, 64, 64);
 16    al_draw_filled_rectangle(0, 0, 320, 240, al_map_rgb_f(1,1,1));
 17
 18    al_flip_display();
 19
 20    al_rest(5);
 21    return 0;
 22}

My initial test was clipping a sub bitmap of a regular (non backbuffer) bitmap; it had the same problem.

Expected result (D3D):

{"name":"602854","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/7\/d\/7d70960c3c746fc2e5e21cbe8ad29116.png","w":336,"h":278,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/7\/d\/7d70960c3c746fc2e5e21cbe8ad29116"}

OpenGL:

{"name":"602855","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/5\/e\/5ee6bba9dd6ca61d2d6945ec46283bb2.png","w":336,"h":278,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/5\/e\/5ee6bba9dd6ca61d2d6945ec46283bb2"}

It works fine when the sub bitmap is removed.

no-reply@allegro.cc (Elias) — Mon, 13 Dec 2010 17:53:22 +0000

Can you try adding this to ogl_update_clipping_rectangle (in src/opengl/ogl_bitmap.c):

if (bitmap->parent) ogl_bitmap = bitmap->parent;

Basically the check if the bitmap is the opengl_target looks wrong to me as it would fail for sub-bitmaps. Can't test it myself right now.

no-reply@allegro.cc (Matthew Leverton) — Mon, 13 Dec 2010 22:26:22 +0000

The attached patch seems to work.

Index: ogl_bitmap.c
===================================================================
--- ogl_bitmap.c	(revision 14083)
+++ ogl_bitmap.c	(working copy)
@@ -530,6 +530,10 @@
    ALLEGRO_DISPLAY *ogl_disp = al_get_current_display();
    ALLEGRO_BITMAP_OGL *ogl_bitmap = (void *)bitmap;
 
+   if (bitmap->parent) {
+      ogl_bitmap = (ALLEGRO_BITMAP_OGL *)bitmap->parent;
+   }
+
    if (ogl_disp->ogl_extras->opengl_target == ogl_bitmap) {
       _al_ogl_setup_bitmap_clipping(bitmap);
    }