Confusion about dual-source blending

[shaofent]

I recently know dual-soruce blending technique from some computer graphics materials. It is fresh to me although it is not a new technique. And after some researching, I got that, to use this, besides setting proper blend state parameters, I need to write multiple outputs for one attachment/buffer in a fragment shader. For example in GLSL, I need to speicify an additional parameter called index in fragment shader’s output layout qualifier, like:

layout (location = 0, index = 0) out vec4 outputColor0;
layout (location = 0, index = 1) out vec4 outputColor1;

Then, I write value to them respectively.

But I am still confused about this technique. My questions are:

1. Does this index parameter in layout qualifier only exists for dual-source blending? Or, is there any other scenerios in whch I can use index parameter?
2. If I use multiple outputs for one attachment/buffer in a fragment shader, but I do not enable duel-source blending when setting blend state, or even disable blending at all. Do additional outpus (I mean ouputs specified with index which is greater than 0) contribute to final value in a attachment/buffer?
3. Is there any recommanded scenerio that I should use dual-source blending?

Thanks a lot!

[GClements]

@shaofent

1. Does this index parameter in layout qualifier only exists for dual-source blending? Or, is there any other scenerios in whch I can use index parameter?

It’s only used for blending.

@shaofent

2. If I use multiple outputs for one attachment/buffer in a fragment shader, but I do not enable duel-source blending when setting blend state, or even disable blending at all. Do additional outpus (I mean ouputs specified with index which is greater than 0) contribute to final value in a attachment/buffer?

No. If blending is disabled or the blending function is one which doesn’t use the second source colour, then it has no effect. The “source colour” used for blending is always the first source colour (index 0). The second source colour (index 1) is only used as a (source or destination) blending factor.

@shaofent

• Is there any recommanded scenerio that I should use dual-source blending?

The extension which introduced it (ARB_blend_func_extended) gives the following examples:

Example Use Cases

There are several potential uses for this functionality. A first example is in the implementation of sub-pixel accurate font rendering algorithms. Given a known layout of pixel elements (red, green and blue components), coverage may be calculated independently for each element and passed to the blender in the second source color as a per-channel opacity. To use this mode, use the following blend functions:

    glBlendFunc(GL_SRC1_COLOR, GL_ONE_MINUS_SRC1_COLOR);

As a second example, consider a partially reflective colored glass window. It will attenuate light passing through it, and reflect some of the light that strikes it. Using an appropriate combination of functions, this effect may be simulated in a single pass using only fixed-function blending hardware. In this case, the following blend functions may be used:

    glBlendFunc(GL_SRC_ALPHA, GL_SRC1_COLOR);

Logically, there doesn’t seem to be much use for it unless you are using the second source colour as a destination factor. As a source factor, you could just perform the multiplication in the shader.

[shaofent]

It’s nice. Thanks for your answers!