no-reply@allegro.cc (Polybios) — Fri, 05 Aug 2011 03:26:11 +0000

I don't know if this is a bug in Allegro 5.0.3, but on my system, the alpha value of pixels isn't read back properly when using Direct3D and locking.

The simple example below fails to produce the expected output, which is:

a(al_get_pixel)  == 40
a(locked region) == 40

Instead, it prints:

a(al_get_pixel)  == 255
a(locked region) == 0

Red, green and blue values seem to work fine, it also works correctly when not using locking.
With OpenGL, there's no problem at all.

It would be nice if someone could try to reproduce this

#SelectExpand
  1#include 
  2#include "common.c"
  3
  4
  5int main(void)
  6{
  7   ALLEGRO_DISPLAY *display;
  8   ALLEGRO_COLOR c;
  9   unsigned char r, g, b, a;
 10   uint32_t pixel;
 11
 12   if (!al_init()) {
 13      abort_example("Could not init Allegro.\n");
 14      return 1;
 15   }
 16
 17//   al_set_new_display_flags(ALLEGRO_OPENGL);  // use OpenGL
 18
 19   display = al_create_display(640, 480);
 20   if (!display) {
 21      abort_example("Could not create display.\n");
 22      return 1;
 23   }
 24
 25   ALLEGRO_BITMAP *bmp = al_create_bitmap(32, 32);
 26   if (!bmp) {
 27      abort_example("Could not create bitmap.\n");
 28      return 1;
 29   }
 30
 31   al_set_target_bitmap(bmp);
 32   al_put_pixel(0, 0, al_map_rgba(10, 20, 30, 40));
 33   al_set_target_bitmap(al_get_backbuffer(display));
 34
 35   ALLEGRO_LOCKED_REGION *lr =
 36      al_lock_bitmap(bmp, ALLEGRO_LOCK_READONLY, ALLEGRO_PIXEL_FORMAT_ARGB_8888);
 37
 38   if(!lr) {
 39      abort_example("Could not lock bitmap.\n");
 40      return 1;
 41   }
 42
 43   c = al_get_pixel(bmp, 0, 0);
 44   al_unmap_rgba(c, &r, &g, &b, &a);
 45
 46
 47   pixel = *(uint32_t *)lr->data;
 48
 49   printf("a(al_get_pixel)  == %d", a);
 50   printf("\na(locked region) == %d", (pixel & 0xFF000000) >> 24);
 51
 52   al_unlock_bitmap(bmp);
 53   al_destroy_bitmap(bmp);
 54
 55
 56   return 0;
 57
 58}

no-reply@allegro.cc (Arthur Kalliokoski) — Fri, 05 Aug 2011 03:33:50 +0000

Wouldn't you have to set the color depth before creating the display?

   al_set_new_display_flags(ALLEGRO_DIRECT3D);
   al_set_new_display_option(ALLEGRO_DEPTH_SIZE, 32, ALLEGRO_SUGGEST);
   display = al_create_display(640, 480);

no-reply@allegro.cc (Trent Gamblin) — Fri, 05 Aug 2011 04:56:23 +0000

Verified, but here it's not a Direct3D issue: here I get the same results using OpenGL.

EDIT: Just noticed you have the last two parameters of al_lock_bitmap reversed.

no-reply@allegro.cc (Arthur Kalliokoski) — Fri, 05 Aug 2011 04:59:56 +0000

So what color depth does A5 choose by default? 8 bit like A4?

no-reply@allegro.cc (Trent Gamblin) — Fri, 05 Aug 2011 05:04:50 +0000

32 bit.

It indeed does work with the parameters in the right order..

no-reply@allegro.cc (Polybios) — Fri, 05 Aug 2011 12:24:35 +0000

Ah! Thank you!

al_get_pixel not working with Direct3D and locking?