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jessestormer
01-26-2010, 03:39 AM
Hello, i'm trying to get some shadows and maybe some reflection off of a ground plane in an image based lighting scene. But, I don't know how to make the ground plane invisible while leaving the shadows or reflections that have been cast on it. Any ideas?

Tobian
01-26-2010, 05:06 AM
it's not easily possible, you probably need to render the object with the ground 'unseen by camera' and then the ground with the object unseen by camera, but export several channels, such as the diffuse and reflection channels.

If you download http://www.hdrlabs.com/sibl/loader.html SIBL it has a great 'trick' in the ground plane which basically feeds the diffuse shader through the transparency channel, but up's it's contrast a little, to remove the other elements. Doesn't solve your reflection issues though...

Otterman
01-26-2010, 05:37 AM
Yeah this is a problem i have all the time. The way i workaround it is set my ground plane surface to have a constant alpha of 1 and edit it in post. It would be nice if we could turn off hdr lighting specific to surface or object.

Mind another way round would be to use the PSD Export plugin in the Effects panel...that will give you a layered file containing spec, reflection, diffusion etc....and give you ultimate control. Great for single renders but i dont think it will work across a network for animation.

biliousfrog
01-26-2010, 05:40 AM
You can choose to include shadows into the alpha channel on a per surface basis...so create a shadow catcher object, set the surface's alpha channel to shadow and when it's rendered only the shadowed area will be visible.

If you render to passes or via the PSD export you can export various buffers including reflection and shading.

Nangleator
01-26-2010, 07:02 AM
I do the compositing in PS. I'll make a floor white, with alpha at 0, and all other objects with alpha at 255.

In PS, I make a layer based on the alpha, which is just the non-floor objects, and below it, the full RGB layer, I Multiply over whatever background.

Or I make the floor black and Screen that layer.

Depending on the background you're comping this on top of, it can look pretty cool.

Tobian
01-26-2010, 07:06 AM
Yeah the only real problem with that method is reflections.. If you're say making a car model, and it's got reflective glossy car paint, and you want to composite it on a background, only it's reflecting a white diffuse ground plane.... :)

Nangleator
01-26-2010, 07:14 AM
D'oh!

I had a PPT template I wanted to composite on top of. I used it as a front projection on my ground plane. That was an interesting effect, and it handled the reflections reasonably well.

I guess the multi-channel compositing trick should give you the basis of what you want, though.

Tobian
01-26-2010, 07:18 AM
The problems I had with front projection are: What happens if you're just using a panorama HDR image, such as with SIBL.. it would be good if you could get a node or something which took the output of one image or a background, and put that into a front projection or something... It's doable to render the image of the BG plate and apply it to the ground plane as a front projection, but then the problems start when you can see the edge of the ground plane in the reflection :D

Teruchan
01-26-2010, 10:40 AM
If you're trying to do this in camera, it may pose some problems. If you're compositing, it can be easy. You can make an object and have it render only shadows in the alpha channel. The object should be solid black. If I remember correctly, though, you no longer have to make a separate solid black version of the object, but can click some buttons in the object panel to turn any object solid black and make it render only shadows.

toby
01-29-2010, 08:35 PM
The problems I had with front projection are: What happens if you're just using a panorama HDR image, such as with SIBL.. it would be good if you could get a node or something which took the output of one image or a background, and put that into a front projection or something... It's doable to render the image of the BG plate and apply it to the ground plane as a front projection, but then the problems start when you can see the edge of the ground plane in the reflection :D
Maybe the cctv shader could pick up a camera image of the hdr bg which could then be projected onto the ground. For speed racer they wrote a camera shader that projected an HDR out in 360, that's what we really need. Not sure if the advanced camera can project, but if it does there may be an answer there.