Blending Modes in LWRP Particles

I created a particle effect using a material created from Particles/Simple Lit shader with an image of white circle tinted red through Start Color setting. On the left I have Alpha Blending Mode and on the right Premultiply Blending Mode.



Why do I get black lines on the left?

 

it is a white circle on a black background right?

alpha blending =(RGB * Alpha) + (destination * inverse_Alpha)
which means that white circle is getting it's edges multiplied darker by it's own alpha


trying making RGB a plain white square, only the alpha defines the circle shape
that is for alpha blending

a texture that is pre-mupltipled (on black) needs to us pre-mult blending to look as you expect
pre-mult blending = RGB + (destination * inverse_Alpha)

 

Even if it's not, the LWRP Lit and SimpleLit particle shaders are straight up broken, even the latest on the Github. They do not handle Alpha blending properly.

The bug is in the AlphaModulate() function in LWRP\ShaderLibrary\Particles.hlsl
It currently looks like this:

Code (csharp):

1. half3 AlphaModulate(half3 albedo, half alpha)
2. {
3. #if defined(_ALPHAMODULATE_ON)
4.     return lerp(half3(1.0h, 1.0h, 1.0h), albedo, alpha);
5. #else
6.     return albedo * alpha;
7. #endif
8. }

But should look like this:

Code (csharp):

1. half3 AlphaModulate(half3 albedo, half alpha)
2. {
3. #if defined(_ALPHAMODULATE_ON)
4.     return lerp(half3(1.0h, 1.0h, 1.0h), albedo, alpha);
5. #elif defined(_ALPHAPREMULTIPLY_ON)
6.     return albedo * alpha;
7. #else
8.     return albedo;
9. #endif
10. }

The short version, the albedo should not be getting multiplied by the alpha when using Alpha mode. Otherwise you get the black edge you see above even with a solid white particle texture.

Really, I kind of disagree with premultiplied multiplying the albedo by the alpha as well, at least with the Unlit shader, though it makes sense for the lit shaders due to the fact "premultiplied" means different things for lit and unlit shaders.