Pipeline color blend attachment - src & dst alpha blend factor is simply being ignored

[wheat thicket]

Hi all!

I've noticed that setting the src and dst alpha blending factor in VkPipelineColorBlendAttachmentState is being ignored, and alpha is passed to the framebuffer unmodified (color blending is however working as expected).

I had thought it was my error, but upon setting both alpha factors to VK_BLEND_FACTOR_ZERO (which should result in zero alpha - so fully transparent) in https://github.com/PAMinerva/LearnVulkan/ (specifically 02A-VkAlphaBlending chapter) and other tutorials, the result was the same - everything was visible as if alpha wasn't modified. The swapchain format is sRGB with an alpha channel and the fragment shader is outputting a vec4 color.

Software rendering, renderdoc and a different machine (both running linux) confirm this behavior. Therefore should I assume this is valid behavior, or am I missing a flag?

Thank you very much for any responses and your time!

[modern walrus]

did you remember to enable blending?

[wheat thicket]

Thanks for the reply! Yeah, color blending wouldn't work without it in the tutorial.

[modern walrus]

also, just noticed it's specifically alpha

have you confirmed with renderdoc?

[wheat thicket]

Yep... color blending works okay, but just alpha does not

[wheat thicket]

Yup, lemme post it here

[modern walrus]

and confirmed that alpha's not 0 in the texture?

[wheat thicket]

Wouldn't the indication that it's rendering mean that it's not zero (you mean the swapchain texture?)

[modern walrus]

hmm, you're doing additive blending... that might make a difference

[modern walrus]

no, it wouldn't, because the color pixels will be written regardless of what the alpha is

[wheat thicket]

The alpha blend equation should be A' := srcA * 0 + dstA * 0

[modern walrus]

so check the output texture

this is what happens when alpha 0 gets written to the framebuffer

(note the sky)

it rendered fine in-game, but screenshots broke

oh, and the background to the parts-picker

[wheat thicket]

Okay that makes sense - I can try setting the src alpha to ZERO & dst alpha to ONE and see if that does anything

[modern walrus]

right-click on the pixels in the output image

[wheat thicket]

Thanks :)

Yeah alpha is zero

[modern walrus]

oh, right, forgot. my transparent pass uses an OIT buffer

yeah, thought so, so set dst to 1 to confirm whether it works

(I expect it does, but...)

why -1?... oh, right

[wheat thicket]

Ah right it is ONE (and drawing)!

[modern walrus]

normally, alpha 0 doesn't prevent a framebuffer being visible

it's only when that framebuffer is composited over something else that alpha kicks in

and then only if the compositing supports alpha blending

[wheat thicket]

Gotcha, had it been like a tiny bit larger than 0, it would have applied it?

[modern walrus]

no, just no affect

ie, alpha is ignored

the reson for the alpha blend factors is for doing multi-pass blending

there was a cool article on it, I'll see if I can find it

[wheat thicket]

Interesting... but why is it that when I do src=SRCALPHA, dst=ZERO, it returns the same effect as src=ONE,dst=ZERO? (And in this case the alpha of the say, gray plane shouldn't be one, right?)

[modern walrus]

because you're not using the alpha later on

[wheat thicket]

Not sure what you mean by that, my intention was to scale the current source alpha by itself

[modern walrus]

meaning why you saw things no matter what the output alpha was

and yeah, your blend settings don't use dst alpha for the color blend

[wheat thicket]

Ahhh I see, so the alpha doesn't actually change the color at all (only with color blending) - if it is scaled, it'll only ever matter for compositing (or other elements with alpha in the scene, which will be used for color blending)

[modern walrus]

if your settings did use dst alpha for color, you might have seen something

[wheat thicket]

No destination alpha

Okay I understand now, thanks so much @modern walrus :))) much appreciated!

You've cleared up the misunderstanding! Thanks again :)

[modern walrus]

glad I could help

looking for that pdf on compositing

[wheat thicket]

Could be interesting, please do! :)

[modern walrus]

I think I found it: https://graphics.pixar.com/library/Compositing/paper.pdf

(hmm, and is that Duff as in Duff's device?)

@wheat thicket yeah, that's the one I was thinking of

[wheat thicket]

This is just what I needed, thanks so much!

[modern walrus]

I'm glad I found it: I'd lost the pdf and coudln't remember how I found it the first time round

there happened to be a link on a wikipedia page 🙂

https://en.wikipedia.org/wiki/Alpha_compositing

[wheat thicket]

Ohhh I see. It's very helpful. Thanks for going through the trouble :)