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Asked: 2026-05-23T12:01:21+00:00

I’ve implemented a scene using 3 quads, A, B, and C. Quad C is

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I’ve implemented a scene using 3 quads, A, B, and C. Quad C is behind quad B. Quad B sits on top of Quad A.

All quads are using the same texture with the exception of Quad B. Quad B makes use of an alpha map and the base texture all the other Quads are using.

The code sample below is how I’m drawing Quad B with fixed function multi-texturing.

GLuint alphaGLTexture; 
SDL_SurfaceToGLTexture(aTexture, &alphaGLTexture);

GLuint baseGLTexture;
SDL_SurfaceToGLTexture(baseTexture, &baseGLTexture);

glActiveTexture(GL_TEXTURE0);
glEnable(GL_TEXTURE_2D);
glBindTexture (GL_TEXTURE_2D, alphaGLTexture);

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_REPLACE);

glActiveTexture(GL_TEXTURE1);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, baseGLTexture); 

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_PREVIOUS);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB, GL_ALPHA);
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_MODULATE);

glTranslatef(0.0f, 0.0f, 0.0f);

double u_coord = 0.0f, v_coord = 0.0f;

// Renders Quad B
glActiveTexture(GL_TEXTURE0)
glBegin(GL_QUADS);

// remainder of code modifies u_coord, v_coord and draws the other glVertex3f's.
glMultiTexCoord2f(GL_TEXTURE0, u_coord, v_coord);
glMultiTexCoord2f(GL_TEXTURE1, u_coord, v_coord);
glVertex3f(posax, posay, posaz);

When I apply a multitextured alpha map to Quad B, the textured Quads A and C change color and end up darker than Quad B.

When I remove any mention to GL_TEXTURE1 everything renders nicely, no dimming, though I lose my alpha mapping on Quad B.

Any recommendations or tips? Is there something wrong with my glTexEnvi calls?

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1 Answer

  1. Editorial Team

    You need to disable all of the texture units so that none of the textures influences the rest of the scene. By the looks of your code you probably just forgot to keep track of your states.

    Add

    glActiveTexture(GL_TEXTURE0);
glDisable(GL_TEXTURE_2D); // or whatever texture target used

glActiveTexture(GL_TEXTURE1);
glDisable(GL_TEXTURE_2D); // or whatever texture target used

glActiveTexture(GL_TEXTURE…);
glDisable(GL_TEXTURE_2D); // or whatever texture target used
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