openGL fixed function pipeline - scale texture intensity beyond 1.0

is there a way other than multitexturing or multipass rendering to scale a texture's intensity above factor 1.0 using the fixed function pipeline?

it can be achived with multipass rendering. for example factor 1.5 can be achieved by rendering the texture at full开发者_如何转开发 intensity once and then with color(0.5,0.5,0.5,1.0) a second time using additive blending.

but is there also a way to achieve this with a single pass and without multitexturing?

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I'm assuming you're using either an OpenGL 3.0 or OpenGL ES library. I think you could do something like this, though I'm not sure how supported it is.

This should be a bit more supported than my original answer, but you're stuck with using 1.5 as the modulation value.

// Setup...
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, tex1);

// Combine: Result.rgb = Texture + Texture - 0.5
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_ADD_SIGNED);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB, GL_SRC_COLOR);

// Combine: Result.a = Texture
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_REPLACE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_ALPHA, GL_SRC_ALPHA);

If you can do it using shaders, you'd be much better off:

//Fragment shader
uniform sampler2D texture;
uniform float blend;
varying vec2 texcoord;
void main()
{
   vec4 texel0 = texture2D(texture, texcoord);
   texel0 *= blend;
   gl_FragColor = texel;
}

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You can use the GL_RGB_SCALE and GL_ALPHA_SCALE properties of the texture environment to apply a 2× or 4× scale factor, but this requires you to use the GL_COMBINE texture environment mode instead of the predefined modes. To get your 1.5 scale factor, you’d set GL_RGB_SCALE to 2.0 and set a color of {0.75, 0.75, 0.75, 1.0}. Note that the texture environment for each texture unit still clamps to [0, 1], so if you need this type of scaling, you should apply the scale on the last texture unit to avoid any clipping.