OpenGL ES 2.0 blending

Question

I set glBlendFunc to

glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

and for testing purposes I set the color in my fragment shader like this

gl_FragColor = vec4(1.0,0.0,0.0,0.0);

Shouldn't the object be fully transparent? What could the reason be if it's not?

Answer 1

The first argument of glBlendFunc() is the source factor, the second is the destination factor. In your case:

sfactor = 1.0;
dfactor = 1.0 - src.alpha;

being src.alpha = 0.0, from your gl_FragColor:

sfactor = 1.0;
dfactor = 1.0;

So the color put to the buffer will be:

buffer = sfactor * src + dfactor * dst;

Substituting...

buffer = (1.0,0.0,0.0,0.0) + dst;

So, putting it simple, you are adding 1 to the red channel of the existing buffer.

If you want to make the output fully transparent, the usual function is:

glBlendFunc(GL_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

The one you wrote is usually used for pre-multiplied alpha in the source. But (1, 0, 0, 0) is obviously not a premultiplied alpha value!