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m1j
05-14-2008, 10:07 AM
I want to use an object to cast shadows onto so the shadows look like they are on an object in the background.

Background image with table. My 3D object on a plane that matches table. I want the shadows that cast on the plane but I do not want to see the plane. I want to see the background image instead. I also want to do this with a 180x360 image on a sphere which I am using for the HRDI lighting.

Any one know how to do this with out multi passes? A node shader or plug-in perhaps?

Thanks!

archijam
05-14-2008, 10:32 AM
Not sure what the constraints of your scene and setup are, but PSD exporter (image filter, works with F10) has a shadow layer .. is that useful?

Surrealist.
05-14-2008, 10:50 AM
I want to use an object to cast shadows onto so the shadows look like they are on an object in the background.

Background image with table. My 3D object on a plane that matches table. I want the shadows that cast on the plane but I do not want to see the plane. I want to see the background image instead. I also want to do this with a 180x360 image on a sphere which I am using for the HRDI lighting.

Any one know how to do this with out multi passes? A node shader or plug-in perhaps?

Thanks!

The sphere and HDRI I will defer so someone else. But I can tell you about the table. What you want to do is map the plane with front projection mapping using the same image as the BG. This will project the image from the camera directly down the Z axis at the BG image as if it was a light parented to the camera. So this will effectively render the plane as transparent but you can cast a shadow on it. From there use your light exclusions so you can control the lighting on the plane so it matches the BG.

pixelranger
05-15-2008, 08:15 AM
just make a black plane with 100 % diffuse and render out a pass with that (main object is unseen by camera). In the Surface Editor -> advanced tab change alpha channel parameter from surface opacity to shadow density. Composite in an app like AE or Fusion. Make sure you render the shadow with a clean black background so you can premultiply it.

You can also just project the background on the plane and find a nice balance between luminosity and diffuse so that the background looks the same on and outside the plane (probably needs some light tweaking) and render it together with the main object and background.
But it is allways better to comp them in post. Then you can tweak the shadows transfer mode, opacity, color, softness etc...

Skonk
05-15-2008, 08:55 AM
Apply the same image as the background to the objects color channel and set its projection to "Front". Then render so the shadows land on it.

m1j
05-15-2008, 11:19 AM
So I am gathering there has not been a third method found that simplifies the process. I am using a spherical perfection onto a background so the front projection will not work unless I render the background sphere alone then use that as the image for the projection.

I was guessing the multi pass post production method would be the choice but I had hopes that Lightwave had finally found a simple way to do this very common task. Perhaps someone could write a shader node for this?

Thanks everyone.

Sekhar
05-15-2008, 08:01 PM
Are you saying you want the shadow to follow the contour of the sphere? If I understand you correctly, you have the picture of a sphere on the background, and you want your main 3D object to cast a shadow that follows the shape of the sphere - except, you only have the sphere as a 2D image on the background.

If yes, you'll need to create a proxy 3D sphere and generate your shadow like say:

(1) Set that 3D sphere surface to generate alpha based on shadow density (like pixelranger suggested) and use that alpha for compositing
(2) Give the sphere a white surface and use the inverted version of that render (or use a black light with white shadow and use the render straight) for your alpha - you will get a different look than from (1)

If you're trying to make a 2D object in an image behave like a 3D object for catching a shadow, I don't see how LightWave can simplify anyway... Or may be I just completely missed what you're trying to do.