light cast shadow only

[Guest]

I am trying to make a light cast shadow only, so I untick affect diffuse and effect specular/reflection but it also removes the shadow casting. I am trying to create a strong direct shadow using either VRaySun or a 3ds Max standard spot light whilst using VRay plane lights as the lighting source. Is there a correct way to do this without having to do any post processing and masking?

[Guest]

I managed to get near what I wanted by not making the VRay lights to powerful which meant the shadow of the spot light shows up.

[Laserschwert]

Generally I think this is not possible with normal V-Ray shadows, because "shadow" is just the absence of light. So a shadow in V-Ray isn't something that gets projected on the ground, but just an area that doesn't get illuminated. I don't know how the "VrayShadowMap" in a standard light works though.

[mcnamex]

are you trying to isolate | create shadow pass or more like creating effects such as gobo lights?

[Morne]

Generally I think this is not possible with normal V-Ray shadows, because "shadow" is just the absence of light. So a shadow in V-Ray isn't something that gets projected on the ground, but just an area that doesn't get illuminated. I don't know how the "VrayShadowMap" in a standard light works though.

Yes what he said
The light doesnt make a shadow, it makes light. Shadow is a result if things in the way of the light. Without the light, there will be no shadow.
You can try doing this in passes. Use a shadow pass and then in comp paclage use layer type and see what you get (divide, darken etc)

That being said, it would be nice to have some kind of "inverted light". That is it creates light where there normally is shadow, and shadow where there normally is light.

[Guest]

Understand how the shadows are created, but I was looking at some way of turning off the light but keeping the shadow. Tried various work arounds and comping is not one of them as I have 90 renders to do. Using a 3ds Max spot light with V-Ray area shadows seemed to work as I could leave the multiplier at 1 but still cast a nice strong shadow. Then used V-Ray plane lights that seemed to wipe out any light generated from the spot light anyway as they were much brighter. But luckily the shadow remained

[windowlicker]

I can't see why comping would be a problem... Just set up an action in Photoshop and run it on all your renders.. Or comp them like an animation in AfterEffects and save every frame as a single image. Comping is probably the only way of doing it "right".

But looking at it the way you describe it, I guess you simply want a darker shadow... and the shadow you already have isn't dark enough because of the light that bounces around in the scene, right? So, I guess you will have to eliminate the gi bounces, which can be done in various ways, but all of which are not physically correct. Which way is best depends on your scene and your materials.

[simmsimaging]

I have not done this with Vraylights, but rather Max lights, but I think it should work anyway:

Create a light that cast the shadow the way you want it to.

Clone it (but not instance) in exactly the same place.

Set the intensity of the second light to be the negative value of the first one. So if your main light is at 10 set the second one to -10 but set it to not cast shadows. Might need to play with visibility to reflections etc. too.

Then you just need to parent the one light to the other so you can move them a unit. It's a bit cumbersome but it's the only workaround I know.
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[Guest]

Thanks for the suggestions guys. Reasons for not doing it in post is the budget. Client has not paid a lot for this and I am trying to reduce labour costs as much as possible, otherwise I would go to town on compositing.