Shader - Unormalized Light Vector

Ace-Angel · Jul 2012

Heya peeps, it's me again!

Question: I was wondering how I could create an Unormalized Light Vector from scratch?

I was taking a look at several engines out there for RT shaders, and most, if not, all of them, only had premade Light Vectors which were Normalized right off the bat.

So yeah, any helpsies here? I tried a google search, but I'm getting some non-so-standard-language which I'm unable to read, such as this:

/ compute the light vector in model space float3
vec = refPos - pos;

Which I have no idea on how to read it. Does RefPos = to ObjectSpace? And what does Pos refer to? Argh, my poor little mortal brain!

Anyhow, cheers and ciao!

PS: I'm using UDK if that matters, but any HLSL explanation is more then welcome for me.

LoTekK · Jul 2012

For the example code, I read that as
Light Vector = objectPos (World Space) - lightPos (World Space)

which would give you the unnormalised light vector in object space...

Curious, though, why do you need an unnormalised light vector?

Ace-Angel · Jul 2012

Official Story: Trying to get some kind of pseudo-attenuation for a backscatter SSS effect I'm trying to create on some plants (since from what I understood, to get the proper Attenuation, I need an Unormalized Light Vector which is has it's Length calculated instead).

Unofficial Story: I like learning about shaders ;P

Cheers and thanks.

Gestalt · Jul 2012

Unnormalized as in distance? A normalized light vector would give you a direction, unnormalized would deal with the length, so multiplied by a scalar. I guess x,y,z distance with the light as the origin would give you one with a scalar referring to the distance, which I guess could be pretty useful info (knowing that scalar distance for falloff or something).

LoTekK · Jul 2012

Ah, gotcha. Yeah, basically your unnormalised light vector in the case above would be the same as your normalised light vector multiplied by the scalar distance from light to object.

Ben Cloward · Jul 2012

A light vector is calculated by subtracting the object's position from the light's position. In order for the vector to be correct, both the light position value and the object position value must be in the same space (object space, world space, camera space etc.) So you'll probably need to transform one of the vectors into the space of the other before you do the subtraction. I generally prefer to work in world space. Most shader environments give you the light's position in world space, but the object's position in object space - but this totally depends on what the engine gives you. I'm not familiar enough with UDK to get more specific.

Ace-Angel · Jul 2012

Gestalt wrote: »

Unnormalized as in distance? A normalized light vector would give you a direction, unnormalized would deal with the length, so multiplied by a scalar. I guess x,y,z distance with the light as the origin would give you one with a scalar referring to the distance, which I guess could be pretty useful info (knowing that scalar distance for falloff or something).

Yep, that's the idea

LoTekK wrote: »

Ah, gotcha. Yeah, basically your unnormalised light vector in the case above would be the same as your normalised light vector multiplied by the scalar distance from light to object.

Cool to know, cheers!

Ben Cloward wrote: »

A light vector is calculated by subtracting the object's position from the light's position. In order for the vector to be correct, both the light position value and the object position value must be in the same space (object space, world space, camera space etc.) So you'll probably need to transform one of the vectors into the space of the other before you do the subtraction. I generally prefer to work in world space. Most shader environments give you the light's position in world space, but the object's position in object space - but this totally depends on what the engine gives you. I'm not familiar enough with UDK to get more specific.

Ah, got you, that makes sense. Cheers!

ambershee · Jul 2012

I did this (also an SSS backscatter effect) by passing in the light data from UnrealScript.=; it seems to work smoothly enough

Ace-Angel · Jul 2012

I was thinking of doing that, but frankly, US is kinda out of my league at this point, especially for materials.

Ace-Angel · Jul 2012

Hey guys, it's me again, some quick questions:

1- When transforming my Light Vector to World Space, does it need to be Local, or is there something more to it?

I currently have this LV -> Transform from Local to World -> Etc... (Object World Position - World Light Vector).

2- I was looking up the math for Length, which seems to be this V, is it the correct one, or is there more to it?

Vector3Lenght = (R^2 + G^2 + B^2) SquareRoot'ed.

3- Pretty daft question, but how exactly would I know if my attenuation is working in the 'correct' way? Currently, due to Post-Processing, the results looking like a glowing ball of fire, in UDK atleast.

Cheers!

JordanW · Jul 2012

1. Light vectors in UE3 are provided in tangent space, so you need to transform tangent space to world space.

2. UE3 has a node called distance: http://udn.epicgames.com/Three/MaterialsCompendium.html#Distance

Ace-Angel · Jul 2012

Cheers Jordan! Should prove most useful in my case.

EDIT: OK, to UDK pro's out there, let me ask this, is it even possible to over-ride UDK's shadows when it comes to the materials, in a friendly way, to achieve a controllable SSS?

The reason I'm asking this, is because I noticed the SSS only appears in the non-shadow-culled parts of the mesh, so you're limited in what you can do, unless you very specifically get rid of ALL shadows, which is what I would like to avoid.

The reason I'm asking all this, is because my attenuation doesn't seem to be working at all, while on the other hand, controlling the radius of my light does what I'm trying to do with the material...if that makes sense.

Basically, controlling the Light Radius = Light Propagation on Mesh vs. Attenuation Material = Nothing.

Here is the material I currently have, not sure if anything is wrong with it. Kudos to Ben for his examples, from which I copied to test out the results.

ambershee · Jul 2012

I'm not sure if you can get around shadows in custom lighting. These days, I've been tending towards avoiding custom lighting and using the emissive channel for custom lighting effects instead.

Ace-Angel · Jul 2012

That explains it then, it was vexing me on why the shadows aren't playing nice with the Attenuation, yet manually changing the site of the Lights Radius did.

Also, you can control the Emissive Channel? Mind telling me how, that would be most welcome in my case. I tried playing around with the Emissive as a cheap hack, but it doesn't respect the values I put in it.

For example, putting in an Inverted Lambert term, with my SSS mask will make ALL of my mesh Glow, even if I 'cut' the Lambert term in Half with a Power Node, the Emissive will make all of my mesh Glow, it doesn't respect any Terms I put in it.

LoTekK · Jul 2012

Ace-Angel:
I just spent some time trying to get that working, and it appears that the emissive input, for whatever reason, doesn't like the Light Vector node. If I use world up, I can get the emissive playing nicely, but the moment I switch the dot from world up to Light Vector, I get a constant value over the entire surface of the mesh...

ambershee · Jul 2012

That's annoying - I have used the light vector with emissive before and not had any real problems, but not with this kind of effect. If I get some time on Sundya I'll have a tinker.

Ace-Angel · Jul 2012

@LotekK: Cheers, I was hoping you would say that, or else I think I would be going insane, haha.

@Ambershee: Fingers crossed, maybe the old versions of UDK didn't have this issue? Argh, silly UDK.

Shader - Unormalized Light Vector

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