Color dodge in OpenGL

Question

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.

Answer 1

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.

Answer 2

There is a way to make color dodge in GL blend func. It's like the Photoshop version of that mixing mode, but only it's darker than photoshop's "Color Dodge". You have to use this type of function:

glBlendFunc(GL_DST_COLOR, GL_ONE);