Blend mode (lighten/screen/add) issue

[onyxlee]

Hi guys, I am trying to create this effect but couldn't figure out the blend mode. I found some very old posts from Vlado about using vraylightmtl and turning on "multiply color by opacity", but it doesn't help in my test (image 3). I understand that this is probably against "energy conservation" for a physically accurate rendering. What's the best way to achieve this, please?



The overlaying parts don't get brighter here.

[Joelaff]

Perhaps if you made them all use a single VRayBlendMtl with the Additive (shellac) mode on that might do what you want. Though I am not sure if particles of the same color would add, it looks like they do in your test, but not sure if that will exceed 1.0 or not. That might just be a normal composite rather than an add.

Basically you want all the particles to add. Eager to hear other solutions. I bet there is a simple trick.

Perhaps using a single LightMtl and mapping the colors would work, rather than multiple LightMtls. Still not certain this will add though.

[fixeighted]

Your two example images are showing the bokeh effect from shallow depth of field, which does exhibit this multiplying.
Is that what you need to do or have I misunderstood?

[onyxlee]

Perhaps if you made them all use a single VRayBlendMtl with the Additive (shellac) mode on that might do what you want.

I tried and got no luck, no matter I put the light material in the base material slot or the coat material slot.

Perhaps using a single LightMtl and mapping the colors would work, rather than multiple LightMtls.

Originally I had one light material for the particle system, it didn't show the "add" effect. Then I tried to simplify my test scene and got this.

[onyxlee]

Your two example images are showing the bokeh effect from shallow depth of field, which does exhibit this multiplying.
Is that what you need to do or have I misunderstood?

Yeah, very simple in after effects, but very difficult in VRay. I try to do everything in 3D. If it's really impossible, I will have to do it in post, which involves more work.

[Joelaff]

Yeah, I ran some quick tests. The closest I got was using VRayMtls (not LightMtls) and setting Self illumination, and the diffuse to 0, and the refraction amount to pure white, and the IOR to 1.0

Still doesn't seem to be quite right.

[fixeighted]

This example took only minutes to set up and is very quick to render.
I reckon if there was a way to do this somehow with just a shader,
some clever person would have done it already....doesn't mean it
doesn't exist though, so fun to try

[onyxlee]

This example took only minutes to set up and is very quick to render.
I reckon if there was a way to do this somehow with just a shader,
some clever person would have done it already....doesn't mean it
doesn't exist though, so fun to try

How did you do it? This looks quite close to the reference.

Yeah, I ran some quick tests. The closest I got was using VRayMtls (not LightMtls) and setting Self illumination, and the diffuse to 0, and the refraction amount to pure white, and the IOR to 1.0

I tried your way just now. It does blend r+g+b into white. But somehow when I assign a single mat to a particle billboard, it doesn't work anymore. So strange.

[fixeighted]

My example is very simple; a blizzard particle system with a tiny sphere per particle, with a nice intense light material.
Camera with both dof enabled and bokeh enabled and a suitable F#/focus distance

[onyxlee]

My example is very simple; a blizzard particle system with a tiny sphere per particle, with a nice intense light material.
Camera with both dof enabled and bokeh enabled and a suitable F#/focus distance

I see. Seems like bokeh did all the job, not really the blend mode. Thanks for the tips.

[fixeighted]

Honestly I don't understand the 'blend mode' thing.
Blend modes are 2D operations on layers, so how that could work in this situation is
not something I can visualise.

[Joelaff]

It’s more that lights are additive. So they blend in add mode.

In camera DOF looks great, but when you get those particles really close to the camera, where the bokeh of the foreground particles becomes 1/4 the frame, it gets very slow to get rid of all the noise and prevent sampling errors from too few rays. (Especially if the foreground particles have actual shapes/geometry too them) So cheating it with billboard shapes on particles can help— but only if you can get them to bend right.

Also, a lot of people just need creative cool looking imagery that isn’t necessarily photographically accurate. Even in VFX we often need weird sci-fi effects that are non-photographic. Personally, my first instinct is to keep things as photographic as possible, but that is not always the ask.

[fixeighted]

I must be missing something I think....I just can't imagine a way it'd work within a shader, given all the possible parameters a scene might present, so I'll leave it to a willing dev to contribute more than I can at this point

I'm looking forward to any and all results.