how to render a shadow

[ronbubl]

hi,
my problem is this: if i want to put my model in any kind of background and make it as if its really there, i usually use a plane that i set to only shadow
+ztransp+ alpha channel to 0.5 - my object can project a shadow on this plane with out seeing the plane, only the shadow - that way my object seems to be as part of the background. i know how to render it in blender but how can i get the same result using luxrender

(i only need to know how to get this effect - i use premultiplyalpha to get a transparent background so i can paste my object in the background using photoshop - but i also want to be able to paste it with the correct shadow - and only shadow without a plane).

tnx

[jeanphi]

Hi,

That's currently not possible with LuxRender. It's actually a rather difficult problem with a global illumination renderer since a shadow is still influenced by the surrounding objects: it's not just an attenuated color, the color can be modified by indirect lighting.

Jeanphi

[ronbubl]

http://www.weirdhat.com/blender/compositing2/

this is an example to what i mean - instead this background and monkey i got my own set of items - how do i render it in luxrender

[acrionx]

Hi,

That's currently not possible with LuxRender. It's actually a rather difficult problem with a global illumination renderer since a shadow is still influenced by the surrounding objects: it's not just an attenuated color, the color can be modified by indirect lighting.

Jeanphi

So how was this render done? It was uploaded to the User Upload gallery several weeks ago:
"mercedes hdri 8 composicion"
gallery2.php?g2_itemId=22206

Somehow, he was able to render a shadow against a photo background (backplate) using HDRI lighting. I PMed the owner about how it was done but it has been a week now and I still got no reply. I don't think the image was rendered in LuxRender.

[zeealpal]

Well one can easily render the shadow only from the car onto a plane that recieves shadows oly, then compose tat on as wished? Photoshop/Gimp also works.

[B.Y.O.B.]

You can render just the shadow in Blender, for example, and then composit with the Lux-output onto the backplate.