[SOLVED] Color dodge in OpenGL

Color dodge in OpenGL

I need to render an image on top of a background in OpenGL and I'm trying to get the same result as the "Color Dodge" in Photoshop but I'm not able to do it.

Right now I'm doing:

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE);

// background
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, background);
glBegin(GL_TRIANGLE_STRIP);
glTexCoord2f(0.0, 0.0);
...
glEnd();
glDisable(GL_TEXTURE_2D);

// image
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, image);
glBegin(GL_TRIANGLE_STRIP);
glTexCoord2f(0.0, 0.0);
...
glEnd();
glDisable(GL_TEXTURE_2D);

The background is a tga with no alpha channel. The image is a tga with alpha channel.

This renders the image with alpha on the background but way too bright.

I read that it should be as easy as:

glBlendFunc(GL_ONE, GL_ONE);

But the image despite of having alpha channel gets rendered as a white square.

Clearly I'm doing something wrong.

Solution

You're not going to be able to use blending to get the equivalent of the Photoshop "Color Dodge" effect. It's a more complicated mathematical function than can be expressed using standard blending logic. So you're going to have to come up with some programmatic blending methodology to make it work.