Shadow without object

pipogianni · 3 Jun 2012, 17:36

Hi there,

I am wondering if it is possible to show shadows without showing the objects which cause them.

I need the shadow of a tree which I make by using a tree texture on a plane (in Vray I switch on 'use color texture as transparancy'). The problem is to get the shadow right, the plane needs to face the sun as much as possible. This position facing the sun is in conflict with the view of the camera so in the render I look at an oblique angle to the tree which makes it look strange. I thought to solve this by using 2 planes with the tree. One invisble facing the sun to create the right shadow and one facing the camera without creating a shadow. But I can't get the shadow to render without the object which is causing it.

Anyone knows how to solve this?

Thanks!

brookefox · 3 Jun 2012, 18:17

I'm unclear as to whether this is a render issue rather than a SU issue. As for SU, you might try placing the shadow casting object back out of the view in a position which casts the shadow where you want it and scaled so the shadow is correctly sized.

Aerilius · 3 Jun 2012, 18:32

In SketchUp, hidden objects don't cast shadows. Also shadows disappear when the opacity of a material approaches 0.

But: SketchUp does not recognize the opacity in transparent texture images (because it's separate from the opacity slider), so you could apply a material with a completely transparent png image to all entities of which you want to see only shadow, like in this example.

brookefox · 3 Jun 2012, 18:52

Very cool. I will struggle to remember that one.

To clarify, as far as shadows go, aren't they either there or not, no degrees of shadow shown as would be the case in reality as for semi-transparent, somewhat opaque materials?

Gaieus · 3 Jun 2012, 22:19

@aerilius said:

But: SketchUp does not recognize the opacity in transparent texture images (because it's separate from the opacity slider), so you could apply a material with a completely transparent png image to all entities of which you want to see only shadow, like in this example.

I also have something like that as an example in the WH: http://sketchup.google.com/3dwarehouse/details?mid=779b98e58f0afd42cb6ac4bba1570c24

brookefox · 3 Jun 2012, 22:42

That's helpful. Rather than 'dummy', I'm calling the material "shadow casting transparent png."

Gaieus · 3 Jun 2012, 23:04

This trick is also often used with buildings where there are walls but you remove the roof/ceiling so you can have a look at the room arrangement. The problem is that now the sun shines into the room from above, too. So put a single face ceiling back, paint it with this material and only the windows will let light through now.

Unfortunately not every rendering application support this (as this is in fact a "limitation" in SU - this is why you need to trace around 2D plant components instead of allowing the alpha transparency to cast that shadow).

pipogianni · 4 Jun 2012, 10:08

Thanks for the answers. The trick with a transparant .png is useful but not for my problem. I cannot use a transparant .png texture because the plane already has a texture on it (the tree). So I cannot make the object transparant.

Any other trick which could help?

Gaieus · 4 Jun 2012, 10:14

This is more of a V-ray issue then. I do not use V-ray but I can move the whole topic to our dedicated V-ray subforum if you wish.

brookefox · 4 Jun 2012, 17:10

@unknownuser said:

As for SU, you might try placing the shadow casting object back out of the view in a position which casts the shadow where you want it and scaled so the shadow is correctly sized.

MonoYpekfygesstaforum · 5 Aug 2013, 21:20

@aerilius said:

But: SketchUp does not recognize the opacity in transparent texture images (because it's separate from the opacity slider), so you could apply a material with a completely transparent png image to all entities of which you want to see only shadow, like in this example.

I have the exact same problem with pipogianni,
I am trying to get some tree shadows from png tree images (with transparent backrounds) in my model, without rendering the png tree plane itself.
I only want the shadows, so I can add the trees later with photoshop.

The example that you posted here seems to manage exactly that,
but i can't figure out how. !
Can anyone please explain what am I looking at in this model?
How are the two lateral shadows created without the smiley faces being displayed?