Translucent material?

Blender banner image contains red tarp over alleyway. Light is coming from top and transmitted through red fabric. Shadow falling on the tarp can be seen from back of the tarp. How do you light through a material with shadow pattern to show on other side?

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Sorry misread your question!

Increase the Translucent property for the “tarp” object.

That was rendered with Luxrender, a unbiased rendering engine, not BI

I tested translucent material using Ray Transparency; a desk lamp with shade on it.

I kind of made it work but I had problem with shading. Lamp lighting with shade will produce visible shade line in the room as in image “A”. But this is not quite right. The ball and wavy desktop sculpture is made out of glass. Wavy one is same material as lampshade. To get the proper shadow I needed to add Transparent Shadow on the wall and table material. But that will wipe out lampshade line on the wall image “B”. Lastly, transparent material setting really produce frosted glass look so first two render had Emit on it to make it glow. But, that is not quite right for the tabletop sculpture, it should not be glowing. So for the last image “C” I set the Emit to zero. And the lampshade went dark.

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There are several thing you can try.

Firstly get out of the thinking that what you see must be a direct result of what is ‘there’ in the scene. You are trying to make an image, not a real room, so the characteristics of the objects do not have to match those of real life.

Translucency is a difficult thing to imitate correctly, so here are a few ideas:

  1. Do not assume that the sculpture and the lampshade are the same material, even though they may look similar in real life.
  2. Make two copies of the lampshade. Copy 1 is set to partially transparent, and to shadow only. This throws the shadow on the wall. Copy 2 is set to translucent, and allows the light from the lamp to produce the modelling that you want.
  3. Perhaps use SSS rather than translucent for the shade.
  4. You could produce an inside out single sided mesh of the shade, meaning that you will only see the inside of the back half. Composite this onto the original image to get the look of an internally lit outside.
  5. The sculpture could have it’s own layer and layer light, specifically to throw a shadow of the correct density and blurriness, independently of the shading of the rest of the scene.

Matt

Hummm, many workarounds, I need to try them out. Thanks Matt.