Direct light from a plane

at my job Im working with Vray, and if one create a plane-light you get a parameter thats called “directional”. So you are able to control the light if its more diffuse or more direct. I found that very handy and tried to archieve the same function in Cycles.

My thought was: isn`t it possible to get the light path from the light-path-node and do a math operation that leaves only the rays in normal direction through?

I tested different node and math operations but didn`t really understood what happend.
Is this not possible to get such a result with the existing nodes?
Or is my idea wrong how to solve the problem?!

Any response is welcome. Thanks in advance.

  • Vince

Is this what your thinking?

Actually, this may be better…


Thanks for the reply.
I made a quick test, but t doesn`t seem to make the light “directer”. I would expect that also the shadows would get sharper…

Maybe I have to explain a bit more: the aim is to get parallel light rays and diffuse light rays, so that one can control the “directness” :slight_smile:

Hello, Maybe this one helps. You can control the angle(its not accurate angle but it works) of spreading. But it works only for one side.I dont know how to invert normal vector. And last thing if you set it below 1 i think it will only shoot less rays and cause more noise.

wow Im impressed! Thanks a lot. I started to play around with that and it seems to point in the right direction, even if I dont really understand how it works with the “multiply” and “less than” nodes…
I put in another hold out to keep out the lighting from the backside, but it seems that now the light is emitted from the backside, but this way I had less diffuse light - but more noise…

I keep trying :slight_smile:

Well from what I know the layer weight node takes a vector and output value based on how much it differs from it, try putting that facing output into color and you will see. Multipy node is there only so you can input numbers from 0 to 180 instead of 0 to 1. And Less than only keeps values that are smaller than inserted angle so that basically cuts out the light itself(therefore more noise if set to small numbers because it cuts almost all rays). But i think this can be done better, with both sides and without noise but I am no expert in this.

Great, thanks for explaining things. Actually I don`t need to use both sides, a lamp points to one side only, so that is OK. But the noise… I have to test that in a scene with other lights. I just wanted to get more control over my light and the shadows. I guess I have to experiment a bit more.

No problem, I feel like there should be a oficial feature like this without the noise. This just feel like cheating it.

Personally, I would have stopped at this:

hmmm… yep, it seems it doesn`t work. A linear light source should have sharp shadows and at least enough light to light the areas in front of the light. But everything I tried I could only cut the ray count, but not the ray direction :frowning:
But thanks nevertheless!

Hi VinceSpäce.In blender you can not do that. You can increase the size of the plane to get diffuse shadows, or decrease it to get them sharp. The light node you said is for incoming rays of the objects, not for the emitter. In the emitter you can check if you want the light will be for diffuse/glossy/transparent/shadow/camera (Object button >> Ray Visibility panel). For example, you can set one emitter plane only for camera rays and another with different size for diffuse/glossy rays.

Another option is using a spot lamp. There you can tweak the sharpeness in its border.

Also, light reflected from the flat mirror surface can be more directed.
At least in the real world it happens.

I suspect that this will lead to a huge amount of noise in the unidirectional path tracer like Cycles.
Or maybe not…

Hi elbrujodelatribu,
thanks for clearing things about the light node in - and out coming rays. I know there are always different ways to get the wanted results. I just sat there in Vray and found this directional slider very handy. At home in Blender I tried to do the same. But the only way to get directional light in Blender is a Sunlight. But it has an infinite Light-plane (of course - thats what you expect by a sunlight:-) )
So that`s why I started this, to get nice control of the light without setting up too much…

But thanks to all, maybe one day we get a feature like that or new Nodes to build that. Happy blending!