Question

glAlphaFunc(GL_GEQUAL, 0.5) can display the image where alpha >= 0.5.

Can opengl display the accumulation of alpha?

Example:

2 images, they will not display some part separately, because alpha < 0.5.

Now some of them overlap, their alpha sum to 0.6, how to display this overlap part?

I try to make a metaball example use opengl, if you have any idea, please please give me some hint.

Thank you so much

Answer 1

I would render the images to separate buffer (with Alpha Test off, and adding their alpha). Then, render the buffer onto the screen with Alpha Test.

1. First, create an empty buffer and set it’s alpha to 0 for every pixel.
2. Then, render all of your objects on said buffer, using your blend function on colors and adding alpha.
3. Then, re-render the buffen on screen with Alpha Test turned on.

And the answer to the “Can opengl display the accumulation of alpha ?” – yes. You can render alpha as grayscale, for example.