no-reply@allegro.cc (Mark Oates) — Mon, 10 Aug 2015 13:05:04 +0000

Ok, so when creating an ALLEGRO_BITMAP with al_create_bitmap(), then drawing some content onto it and then using that bitmap, it exhibits some strange behavior in certain circumstances. Specifically, if you have set al_set_bitmap_flags(ALLEGRO_MIPMAP) and are in ALLEGRO_OPENGL. I think the odd behavior alludes to the possibility that a mipmap is just not created.

Here is an example program that illustrated the odd behavior:

#SelectExpand
  1
  2
  3
  4#include 
  5#include 
  6#include 
  7
  8
  9
 10ALLEGRO_BITMAP *create_generated_bitmap()
 11{
 12  ALLEGRO_STATE prev;
 13  al_store_state(&prev, ALLEGRO_STATE_TARGET_BITMAP);
 14
 15
 16  // create our bitmap
 17  ALLEGRO_BITMAP *generated_bitmap = al_create_bitmap(512, 512);
 18  al_set_target_bitmap(generated_bitmap);
 19
 20  // clear the color to "ugly computer teal"
 21    al_clear_to_color(al_map_rgba_f(0, 0.25, 0.25, 1.0));
 22
 23  // just draw some lines and junk
 24  al_draw_line(20/2, 20/2, 960/2, 724/2, al_color_name("dodgerblue"), 20);
 25  al_draw_line(20/2, 700/2, 660/2, 234/2, al_color_name("deeppink"), 30);
 26  al_draw_line(700/2, 20/2, 860/2, 1024/2, al_color_name("brown"), 40);
 27  al_draw_line(900/2, 70/2, 760/2, 824/2, al_color_name("orange"), 30);
 28
 29  // put a border on that shit
 30  al_draw_rectangle(0, 0, al_get_bitmap_width(generated_bitmap), al_get_bitmap_height(generated_bitmap),
 31    al_color_name("red"), 20);
 32
 33
 34  // restore our previous drawing state
 35  al_restore_state(&prev);
 36
 37  return generated_bitmap;
 38}
 39
 40
 41
 42int main(int argc, char* argv[])
 43{
 44  // init stuff
 45  al_init();
 46  al_init_primitives_addon();
 47
 48
 49  // set the mode and create the display
 50  al_set_new_display_flags(ALLEGRO_OPENGL);
 51  ALLEGRO_DISPLAY *display = al_create_display(960, 600);
 52
 53
 54  // generate our bitmap
 55  al_set_new_bitmap_flags(ALLEGRO_MIPMAP);
 56  ALLEGRO_BITMAP *bitmap = create_generated_bitmap();
 57
 58
 59  // draw at 1.0 scale (normal)
 60  al_draw_bitmap(bitmap, 20, 20, 0);
 61  // draw at 0.8 scale
 62  al_draw_scaled_bitmap(bitmap, 0, 0, 512, 512, 300, 60, 512*0.8, 512*0.8, 0);
 63  // draw at 0.6 scale
 64  al_draw_scaled_bitmap(bitmap, 0, 0, 512, 512, 600, 120, 512*0.6, 512*0.6, 0);
 65
 66
 67  // flip it
 68  al_flip_display();
 69
 70
 71  // give it some time
 72  al_rest(2);
 73}

Running the program should look something like this:
{"name":"609698","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/3\/c3f404614a867f26efcfda574960d499.png","w":995,"h":662,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/3\/c3f404614a867f26efcfda574960d499"}

But it looks like this (presumably because when scaling the bitmap, the hardware is trying to crossfade between a smaller mipmap that just isn't there, so it starts to look transparent)
{"name":"609699","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/8\/0\/809e9e8e9fb8f801ee2d324df854264b.png","w":992,"h":658,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/8\/0\/809e9e8e9fb8f801ee2d324df854264b"}

I can "fix" the issue by cloning the bitmap after drawing everything to it, (and then destroying the original for good measure), like this:

  // GENERATED BITMAP in OPENGL with ALLEGRO_MIPMAP fix:
  ALLEGRO_BITMAP *cloned = al_clone_bitmap(generated_bitmap);
  al_destroy_bitmap(generated_bitmap);
  generated_bitmap = cloned;

Extra notes:

It may also be related to these 2 threads:
https://www.allegro.cc/forums/thread/615520
https://www.allegro.cc/forums/thread/613196

My hardware is an the Intel card, as Bruce Pascoe mentions in the 2nd thread. I haven't tried it on other hardware.

[a5] al_create_bitmap() not creating mipmap in OPENGL