[A5] Bitmaps to OpenGL textures

no-reply@allegro.cc (OICW) — Fri, 16 Nov 2012 20:24:47 +0000

I've ran into some problems when creating OGL textures from Allegro bitmaps. For the classic 2D texture I'm currently using this code which runs fine:

ALLEGRO_BITMAP *bitmap = al_load_bitmap(path);
GLuint texName = al_get_opengl_texture(bitmap);

After that I can just use texName everywhere I want to use GL_TEXTURE2D. But, I also want to use cubemap textures, which are essentially 6 2D textures. Therefore to load it I have a function that loads single bitmap containing all six cube faces and creates a GL_TEXTURE_CUBE_MAP from it:

#SelectExpand
  1// Omitted sanity checks etc. the textures are +x, -x, +y, -y, +z, -z in one column
  2// of size w * w, mBitmap is the bitmap containing them (either loaded from disk
  3// or procedurally generated
  4unsigned int w = al_get_bitmap_width(mBitmap);
  5unsigned int h = al_get_bitmap_height(mBitmap);
  6
  7glGenTextures(1, &mName);
  8glBindTexture(GL_TEXTURE_CUBE_MAP, mName);
  9glActiveTexture(GL_TEXTURE0);
 10
 11glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
 12glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
 13glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE);
 14glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
 15glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
 16
 17glPixelStorei(GL_UNPACK_ALIGNMENT, al_get_pixel_size(mBitmap->format));
 18
 19GLenum target = GL_TEXTURE_CUBE_MAP_POSITIVE_X; // Followed by -x, y, -y, z, -z
 20for (unsigned int i = 0; i < TextureCubemap::FACE_COUNT; ++i)
 21{
 22  al_set_new_bitmap_format(ALLEGRO_PIXEL_FORMAT_ABGR_8888_LE);
 23  ALLEGRO_BITMAP *face = al_create_sub_bitmap(mBitmap, 0, i * w, w, w);
 24
 25  glTexImage2D(target, 0, GL_RGBA, w, w, 0, GL_RGBA, GL_UNSIGNED_BYTE, face->memory);
 26
 27  al_destroy_bitmap(face);
 28  target++;
 29}

This code works, but produces garbage. My guess is wrong data formats. I've been also taking a look at ogl_bitmap.c in allegro sources. So, essentially two questions are at hand:

1. What am I doing wrong.
2. Is there a better way of doing this, i.e., creating a cubemap texture from bitmap/bitmaps?

no-reply@allegro.cc (Elias) — Fri, 16 Nov 2012 20:40:59 +0000

There is no ->format or ->memory. You need to lock the bitmap with al_lock_bitmap to the format you want.

no-reply@allegro.cc (OICW) — Thu, 22 Nov 2012 16:19:51 +0000

Allright, seems I haven't understood bitmap locking. This time I think I have everything correct, but it still produces garbage:

#SelectExpand
  1unsigned int w = al_get_bitmap_width(mBitmap);
  2unsigned int h = al_get_bitmap_height(mBitmap);
  3
  4glGenTextures(1, &mName);
  5glBindTexture(GL_TEXTURE_CUBE_MAP, mName);
  6glActiveTexture(GL_TEXTURE0);
  7
  8glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
  9glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
 10glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE);
 11glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
 12glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
 13
 14GLenum target = GL_TEXTURE_CUBE_MAP_POSITIVE_X; // Followed by -x, y, -y, z, -z
 15for (unsigned int i = 0; i < TextureCubemap::FACE_COUNT; ++i)
 16{
 17  ALLEGRO_LOCKED_REGION *face = al_lock_bitmap_region(mBitmap, 0, i * w, w, w, ALLEGRO_PIXEL_FORMAT_ABGR_8888, ALLEGRO_LOCK_READONLY);
 18
 19  glPixelStorei(GL_UNPACK_ALIGNMENT, face->pixel_size);
 20  glTexImage2D(target, 0, GL_RGBA, w, w, 0, GL_RGBA, GL_UNSIGNED_BYTE, face->data);
 21
 22  al_unlock_bitmap(mBitmap);
 23
 24  target++;
 25}

The first cubemap face (positive x) is full of garbage (see attached image below). Inspecting mBitmap and face in debugger provides following interesting bits for the first iteration:

mBitmap:
 format: 9
 pitch: 512
 locked: true
 lock_x: 0
 lock_y: 0
 lock_w: 128
 lock_h: 128
 lock_flags: 1

face:
 format: 17
 pitch: -512
 pixel_size: 4

{"name":"606868","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/9\/1\/91c186a55a15f92dcbb50e7dd80df6c4.png","w":646,"h":507,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/9\/1\/91c186a55a15f92dcbb50e7dd80df6c4"}

no-reply@allegro.cc (Elias) — Thu, 22 Nov 2012 16:25:42 +0000

Try face>data + face->pitch * w.

no-reply@allegro.cc (OICW) — Thu, 22 Nov 2012 20:20:22 +0000

This did the trick:

(face->data + (int)w * face->pitch)

Thanks for help. One more question though, should I understand it that the actual pixel data are prepended by pitch * height of something else?

The only thing I need to solve now, is that apparently each face gets one line from the previous one accidentally. Anyway cookies

no-reply@allegro.cc (Elias) — Thu, 22 Nov 2012 20:34:33 +0000

No, face->data simply points to the beginning of the last row (because in Allegro bitmaps row 0 is the top-most row while in OpenGL textures row 0 is the bottom-most row).

The correct and only documented way of accessing the data is row-by-row, so face->data + row * face->pitch where row can go from 0 to (h-1). Since in your case you know that the data is an OpenGL texture it's not a horrible hack to just access the whole memory I'd say.

no-reply@allegro.cc (OICW) — Thu, 22 Nov 2012 21:19:33 +0000

Well, yeah. That also explains the pitch being negative.