OpenGL / Allegro 5.1 MipMaps

With thanks to Tomasu on #allegro IRC for the help with this...

Here's how to enable mipmaps for OpenGL programs using Allegro 5.1.

In the previous post, I applied a texture to a "board" on OpenGL utilizing the allegro function al_load_bitmap(), followed by al_get_opengl_texture(). However, that texture was simply an applied bitmap. If I tried using one of the mipmap options for glTexParameterf(), such as;

glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_NEAREST);

...the texture would not be mapped at all onto the board (it would be plain white). The solution is in making the bitmap a mipmap before using it as an OpenGL texture using al_set_new_bitmap_flags(). This must be called prior to loading the bitmap, in order for Allegro to generate the mipmaps. I opted for this as an experiment...

al_set_new_bitmap_flags(ALLEGRO_MIPMAP | ALLEGRO_MIN_LINEAR);
bmp = al_load_bitmap("texture.jpg"); // 256 x 256 pixels
ogl_tex = al_get_opengl_texture(bmp);

Then, when I called glTextParameterf() for setting the the GL_TEXTURE_MIN_FILTER and GL_TEXTURE_MAG_FILTER, I did it this way...

glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

The result is now a mipmap texture on the board. Pretty cool. When done loading bitmaps as mipmaps, the bitmap loading can be reset to default by resetting the flags, as such...

al_set_new_bitmap_flags(ALLEGRO_CONVERT_BITMAP);

That's the default, and any loading of subsequent bitmaps will no longer be mipmaps. The complete listing that can be used to test this function is in my previous post on this blog.