Problem with Brightness / contrast of texture

Hi, I need fast image processing in my application. I find example of code on some site:

static void brightness(V2fT2f *quad, float t)	// t [0..2]
{
	// One pass using one unit:
	// brightness < 1.0 biases towards black
	// brightness > 1.0 biases towards white
	//
	// Note: this additive definition of brightness is
	// different than what matrix-based adjustment produces,
	// where the brightness factor is a scalar multiply.
	//
	// A +/-1 bias will produce the full range from black to white,
	// whereas the scalar multiply can never reach full white.

	glVertexPointer  (2, GL_FLOAT, sizeof(V2fT2f), &quad[0].x);
	glTexCoordPointer(2, GL_FLOAT, sizeof(V2fT2f), &quad[0].s);
	glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
	if (t > 1.0f)
	{
		glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB,      GL_ADD);
		glColor4f(t-1, t-1, t-1, t-1);
	}
	else
	{
		glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB,      GL_SUBTRACT);
		glColor4f(1-t, 1-t, 1-t, 1-t);
	}
	glTexEnvi(GL_TEXTURE_ENV, GL_SRC0_RGB,         GL_TEXTURE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SRC1_RGB,         GL_PRIMARY_COLOR);
	glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA,    GL_REPLACE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SRC0_ALPHA,       GL_TEXTURE);

	validateTexEnv();
	glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

i tried to use this code in my app,
but the result for t>1 was absolutely white image and black for t<1. I tried many variations of code changes, but still hadn’t anyone differed from white or black color. What may be the problem? Could anyone help me?

The code looks fine to me (quite clever actually).
What values are you trying? You say > 1 but can you be specific?
I’d have thought you could try 1.3 or 1.5 and see what that produces. I assume this code is to be used during some sort of blending operation as the alpha is set to the texture alpha during the RGB_COMBINE operation.
Otherways to acheive this could be to use a shader to reproduce the same effect. Is that an option which is available to you?

I never used shaders before. But i think, it is time to start using them. But i really don’t know, why this code doesn’t work. I spent about a day to make it works.
Topic may be closed. I’m starting learn shaders.