CODEWITHSUNDEEP
opengl
Kozaki
Kozaki

Reputation: 615

Procedure for alpha blending textured quads in OpenGL 3+?

I am attempting to put together a simple program that just displays two textured quads overlapping each other, and the textures have alpha channels.

I have successfully been able to get the quads themselves to display, and the textures look correct except for the transparency.

On this topic, the GL_BLEND flag does not seem to do anything at all, whether it is enabled or not. Is this particular flag inapplicable when shaders are being used?

I know for a fact that the alpha is being rendered in correctly, since if I set out_color = texture.aaaa, I get a nice pattern of blacks/whites/grays that match the original texture.

So, if GL_BLEND doesn't work, what are the usual methods for getting alpha blending working?

Upvotes: 1

Views: 2673

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473212

All that glEnable(GL_BLEND) does is turn blending on and off. It does not say what the blending will do; that is controlled by the various blending functions. If you want a typical alpha transparency blend, you do this:

glBlendEquationSeparate(GL_FUNC_ADD, GL_FUNC_ADD);
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ZERO);

If you use pre-multiplied alpha (and you probably should), you would do:

glBlendEquationSeparate(GL_FUNC_ADD, GL_FUNC_ADD);
glBlendFuncSeparate(GL_ONE, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ZERO);

Upvotes: 3

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