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Using 2x color modulation in FFP

Started by Plerion Jan 26, 2015 at 9:03 PM 0 replies 2.2k views

Original Post

Jan 26, 2015 at 9:03 PM

Hello guys

Im asking this question for a friend of mine. He has an old program thats still using the FFP for rendering in OpenGL. Right now it still all works but there is a new feature hed like to implement. Every vertex has a color value. But its modulated in a different way. In GLSL in the fragment shader it would be like:


gl_FragColor = input_color.rgb * 2 * texture_color;

So essentially a color value of 0x7F7F7F7F would just return the texture color whereas 0xFFFFFFFF would double all channels of the texture.

The color values are bound to a buffer and sent to the FFP using glColorPointer. In DirectX i remember there was some kind of texture sampler state that allowed to specify MODULATE_2X as color operation. Is there something similar in OpenGL?

Greetings

Plerion

21st Century Moose

Jan 26, 2015 at 10:04 PM

The good news is yes, there is.

It's part of the GL_ARB_texture_env_combine extension, or is core in IIRC GL1.4, so if one's hardware meets those requirements then one can do it.

The basic setup is:

glTexEnvi (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
glTexEnvi (GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_MODULATE);
glTexEnvi (GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_TEXTURE);
glTexEnvi (GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_PRIMARY_COLOR);
glTexEnvf (GL_TEXTURE_ENV, GL_RGB_SCALE, 2.0f);

Note that though the API appears otherwise, by specification the only legal values for GL_RGB_SCALE are 1, 2 or 4. If you come across hardware or drivers that accept other values, be aware that it's non-standard behaviour.

Also be aware that the GL_RGB_SCALE value seems to "stick" for other GL_TEXTURE_ENV_MODEs on some (most?/all?) drivers, so you should also reset it to 1.0f when done.

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