Question

I am using OpenGL (the fixed pipeline, I need to support GL1.1) in a futile attempt to turn images into a sepia-like color.

So far I’ve been able to (somewhat, it ends up too dark) grayscale the image, but when I try to modulate or add another color to it to give it the sepia appearance all that appears is a constant color.

So far the code is

float weights_vector[] = { 0.2126f, 0.7152f, 0.0722f, 1.0f };
float color_vector[] = {0.1f, 0.2f, 0.05f, 0.0f};

gl.glActiveTexture(GL11.GL_TEXTURE0); // Select texture unit 0
gl.glBindTexture(GL11.GL_TEXTURE_2D, _texturePointer); // Bind the texture
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_TEXTURE_ENV_MODE, GL11.GL_COMBINE); // Set to combine
if (grayscale) { // Gives better results, but can't understand yet why
    gl.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
}
else {
    gl.glBlendFunc(GL11.GL_ONE, GL11.GL_ONE_MINUS_SRC_ALPHA);
}

/* Grayscale */
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_COMBINE_RGB, GL11.GL_DOT3_RGB);
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_SRC0_RGB, GL11.GL_TEXTURE);
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_OPERAND0_RGB, GL11.GL_ONE_MINUS_SRC_COLOR);
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_SRC1_RGB, GL11.GL_CONSTANT);
gl.glTexEnvfv(GL11.GL_TEXTURE_ENV, GL11.GL_TEXTURE_ENV_COLOR, weights_vector, 0);

/* Give color */
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_COMBINE_RGB, GL11.GL_ADD);
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_SRC0_RGB, GL11.GL_PREVIOUS);
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_SRC1_RGB, GL11.GL_CONSTANT);
gl.glTexEnvfv(GL11.GL_TEXTURE_ENV, GL11.GL_TEXTURE_ENV_COLOR, color_vector, 0);

EDIT 1:

I finally got it working thanks to @Tim, who categorically pointed out my code would never work. I leave here the working code in case anyone ever needs it.

The final code looks like this and works like a charm, albeit a bit slow on my mobile device with lots of sprites on screen.

gl.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);

// STAGE 0
gl.glActiveTexture(GL11.GL_TEXTURE0);
gl.glEnable(GL11.GL_TEXTURE_2D);
gl.glBindTexture(GL11.GL_TEXTURE_2D, _texturePointer);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_TEXTURE_ENV_MODE, GL11.GL_COMBINE);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_COMBINE_RGB, GL11.GL_DOT3_RGB);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_SRC0_RGB, GL11.GL_TEXTURE);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_SRC1_RGB, GL11.GL_CONSTANT);
//gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_OPERAND0_RGB, GL11.GL_SRC_COLOR);
gl.glTexEnvf(GL11.GL_TEXTURE_ENV, GL11.GL_OPERAND0_RGB, GL11.GL_ONE_MINUS_SRC_COLOR);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_OPERAND1_RGB, GL11.GL_SRC_COLOR);
gl.glTexEnvfv(GL11.GL_TEXTURE_ENV, GL11.GL_TEXTURE_ENV_COLOR, weights_vector, 0);

// STAGE 2
gl.glActiveTexture(GL11.GL_TEXTURE1);
gl.glEnable(GL11.GL_TEXTURE_2D);
gl.glBindTexture(GL11.GL_TEXTURE_2D, 100); // Dummy texture - use any that's not your current
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_TEXTURE_ENV_MODE, GL11.GL_COMBINE);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_COMBINE_RGB, GL11.GL_ADD);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_SRC0_RGB, GL11.GL_PREVIOUS);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_SRC1_RGB, GL11.GL_CONSTANT);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_OPERAND0_RGB, GL11.GL_SRC_COLOR);
gl.glTexEnvi(GL11.GL_TEXTURE_ENV, GL11.GL_OPERAND1_RGB, GL11.GL_SRC_COLOR);

Answer 1

I think you have a misconception on how the state machine works.

If you’re expecting that code to convert the color to grayscale, and then convert the grayscale to sepia, it doesn’t work that way.

The code under Give color isn’t being appended to the end of the Grayscale stage, it’s just overwriting all of it’s settings. So you’re going directly from the texture sample to the GL_ADD part, because you’ve overwritten all of the grayscale settings. You need to switch to another texture unit to setup the grayscale to sepia transform.

There’s lots of examples here if you want an example:

http://www.opengl.org/wiki/Texture_Combiners