I''ve got totally stuck with multitexturing. What i want to do is blending two textures, and using one texture''s alpha channel as the blending map. For a test i use a bmp-picture as texture[0]''s RGB, and one channel of another bmp as alpha. The bmp for the alpha blending map is hand-drawn, only using black (0,0,0) and white (255,255,255) as colours. Texture[1] has the same configuration, but no alpha image specified. Using OpenGL''s GL_COMBINE_EXT in the glTexEnvf() function, i want to set up texturing unit 0 with texture[0], and texturing unit 1 with texture[1]: Code: glActiveTextureARB(GL_TEXTURE1_ARB); glBindTexture(GL_TEXTURE_2D,my_textures[1]); glEnable(GL_TEXTURE_2D); // setup for texture combining glTexEnvf (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT); // use GL_MODULATE as combining function. Params: Arg0*Arg1 glTexEnvf (GL_TEXTURE_ENV, GL_COMBINE_ALPHA_EXT, GL_MODULATE); // Arg0: previous texture unit glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_PREVIOUS_EXT); // Arg0: use previous texture''s alpha channel glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_ARB, GL_SRC_ALPHA); // Arg1: current texture glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB_ARB, GL_TEXTURE); // Arg1: use current texture''s RGB channel glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB_ARB, GL_SRC_COLOR); // the following lines set the parameters for texture[0], // which is used as the first parameter in the code above glActiveTextureARB(GL_TEXTURE0_ARB); glBindTexture(GL_TEXTURE_2D,my_textures[0]); glEnable(GL_TEXTURE_2D); glTexEnvf (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT); glTexEnvf (GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_REPLACE); glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_TEXTURE); glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_ARB, GL_SRC_COLOR); // draw the geometry doCube(); As the result of the operation only texture[1] appears on the testing cube, with no modulation at all. Any ideas?

yes. thats because you are blending the alpha channel. not blending with the alpha channel.

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RipTorn''s right.
You may use GL_INTERPOLATE combining environment instead of GL_MODULATE.

Also, I''dd add a few things :
- Use glTexEnvi instead of glTexEnvf.
- Simply call glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE) instead of :
glTexEnvf (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT);
glTexEnvf (GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_REPLACE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_ARB, GL_SRC_COLOR);
The state change will be faster, and the fillrate could be faster too.

no. what I meant was that the modulation was taking place within the alpha channel, not within the colour channels.



glActiveTextureARB(GL_TEXTURE1_ARB);
glBindTexture(GL_TEXTURE_2D,my_textures[1]);
glEnable(GL_TEXTURE_2D);

glTexEnvf (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT);
glTexEnvf (GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_INTERPOLATE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_PREVIOUS_EXT);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_ARB, GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB_ARB, GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB_ARB, GL_SRC_ALPHA);

//no change
glActiveTextureARB(GL_TEXTURE0_ARB);
glBindTexture(GL_TEXTURE_2D,my_textures[0]);
glEnable(GL_TEXTURE_2D);
glTexEnvf (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT);
glTexEnvf (GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_REPLACE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_TEXTURE);



doCube();


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[edited by - RipTorn on March 19, 2003 9:45:00 PM]

quote:
glTexEnvf (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT);
glTexEnvf (GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_INTERPOLATE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_PREVIOUS_EXT);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_ARB, GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB_ARB, GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB_ARB, GL_SRC_ALPHA);

Since texture1 has no alpha channel but texture0 has one, I''d rather replace :
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB_ARB, GL_SRC_ALPHA);
with :
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB_ARB, GL_PREVIOUS);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB_ARB, GL_SRC_ALPHA);

And for this to work, the texture combiner 0 needs to pass the alpha channel.
Which means that :
glTexEnvf (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT);
glTexEnvf (GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_REPLACE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_ARB, GL_SRC_COLOR);
is not enough. The following lines should be added otherwise the result of the alpha stage is undefined:
glTexEnvf (GL_TEXTURE_ENV, GL_COMBINE_ALPHA_EXT, GL_REPLACE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_ALPHA_ARB, GL_SRC_ALPHA);
In the end, this texture combiner mimics the glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE) behaviour, so why not calling it ?

First of all, thanx for your help!

That''s how my code looks now:

// TEXTURE-UNIT #0
glActiveTextureARB(GL_TEXTURE0_ARB);
glBindTexture(GL_TEXTURE_2D,my_textures[0]);
glEnable(GL_TEXTURE_2D);
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT);
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_REPLACE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_ARB, GL_SRC_COLOR);

glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA_EXT, GL_REPLACE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_ALPHA_ARB, GL_SRC_ALPHA);

// TEXTURE-UNIT #1:
glActiveTextureARB(GL_TEXTURE1_ARB);
glBindTexture(GL_TEXTURE_2D,my_textures[1]);
glEnable(GL_TEXTURE_2D);

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_EXT);
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB_EXT, GL_INTERPOLATE_EXT);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB_ARB, GL_PREVIOUS_EXT);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB_ARB, GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB_ARB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB_ARB, GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB_ARB, GL_PREVIOUS_EXT);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB_ARB, GL_SRC_ALPHA);

Result: Only Texture0 is visible, with no modulation at all. With other parameters both textures at least are mixed together, so it doesn''t seem to be a problem of initializing the multitexturing the wrong way.

Another thing i discovered is that GL_PREVIOUS has to be changed to GL_PREVIOUS_EXT, and GL_INTERPOLATE to GL_INTERPOLATE_EXT. Could it be that i used a outdated header file, and the definitions don''t match any more?

Are you sure that the alpha channel of texture 0 is filled the the right data ? Didn''t you fill the alpha channel of texture 1 instead ?

You''re right about the names of the tokens, though I''d rather call them GL_PREVIOUS_ARB and GL_INTERPOLATE_ARB but that''s just a matter of preference.
No it can not change the result. The value of GL_PREVIOUS, GL_PREVIOUS_EXT and GL_PREVIOUS_ARB are the same. Unless you downloaded a corrupted header, there is no chance that you experience any difference.

You were right, vincoof! Both textures have alpha channels, but there was an error in the initialisation call of texture[0] - it was initialized as RGB8 instead of RGBA8. That''s all! It now looks fine.

Thanx once again!

StormbringerCSP
http://csp.sourceforge.net