Blending semi-transparent images while not stacking their alphas

Question

In libgdx/OpenGL ES I want to render multiple semitransparent textures while keeping the alpha still semi transparent. For example If I render a 0.5 alpha circle on top of another 0.5 alpha circle, alpha should remain 0.5, output alpha should be maximum value of input alpha and destination alpha.

I looked a bit, it looks like glBlendEquationSeparate(GL_FUNC_ADD, GL_MAX) should do the trick, but it is not GL_MAX operator is not available for OpenGL ES. Weirdly, it works when I use integer value for (0x8008) but I guess I can't trust that :D

How can I achieve such an effect?

Answer 1

I believe GL_ONE_MINUS_SRC_ALPHA is what you are looking for.

If you are using a SpriteBatch you can enable blending like this:

batch.enableBlending();
batch.setBlendFunction(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);

Otherwise you can call the OpenGL functions directly:

Gdx.gl.glEnable(GL20.GL_BLEND);
Gdx.gl.glBlendFunc(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);

Answer 2

Depending on the device(s) you are targeting, you may be able to use GL_MAX. It is defined in the gl2ext.h header file. You should check that the device supports the extension called GL_EXT_blend_minmax (it sounds like the device you are using does support it).