OpenGL (libgdx) - blending alpha map

Question

I am trying to blend a white texture with varying alpha values with a colored background. I am expecting the result to retain colors from the background, and have alpha values replaced by ones from the blended texture.

So, for background I use:

Gdx.gl.glEnable(GL20.GL_BLEND);
Gdx.gl.glBlendEquationSeparate(GL20.GL_FUNC_ADD, GL20.GL_FUNC_ADD);
Gdx.gl.glBlendFuncSeparate(GL20.GL_ONE, GL20.GL_ZERO, GL20.GL_ONE, GL20.GL_ZERO);

I expect the background triangles mesh to override the destination, both color and alpha.

Question 1 - why with those blendFunc parameters, alpha value is being ignored? If I set blendfunc to GL_ONE, GL_ONE, GL_ZERO, GL_ZERO then the filled mesh is rendered with proper alpha level - but both the source and dest alpha are supposed to be multiplied by zero - why does this work?

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Now to blend the alpha map I use:

Gdx.gl.glBlendEquationSeparate(GL20.GL_FUNC_ADD, GL20.GL_FUNC_ADD);
Gdx.gl.glBlendFuncSeparate(GL20.GL_ZERO, GL20.GL_ONE, GL20.GL_ONE, GL20.GL_ZERO);

Question 2 - This supposed to keep the destination color and replace alpha. However, when I render the texture with those blendfunc params, I get no change to the output at all...

I've been reading the opengl blending chapter over and over again to understand what I fail to understand, please, share your insight on how those parameters actually work

Daniel Funicello Daniel Funicello · Accepted Answer · 2016-03-14T13:23:40

I use this:

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

It works but only in the order of rendering. In order to account for depth you will need alpha testing which libGDX does not include.

OpenGL (libgdx) - blending alpha map

1 Answers