Showing only shadows on a plane in Mental Ray

[william.84.48]

Hello all!

I’ve seen this question posted many times ino ther places, but there never seems to be a confirmation that the solutions work fully or anything.  I have a sphere in my scene which is casting shadows onto a plane, acting as the ground.  However, I want to make the ground plane invisible, but I want the shadows to still be visible.

This is easily achieved in Scanline by applying the matte/shadow material to the plane.  However, I’m using mental ray, and it does not support this material.  The matte/shadow/reflections material that mr has also does not seem to work properly.

Just for record, I’m using a spherical environment map (a 360 panorama) for reflections, but showing the image I’m compositing this sphere onto in the end for the viewport background (so that I can align my camera).  When I render, I want the end result to be the sphere on a transparent background with the shadow(s) so that I may composite this onto my original image in After Effects or Photoshop.  I’m okay with the shadows being a different render.

I’ve thought about rendering the sphere without shadows in mr, and then the shadows by themselves with scanline (so that I can use the matte/shadow material), but if there is a better way to do this, I would love to know about it.

Thanks in advance.

Beginner 3ds Max User

[PROH]

Hi. When using mr you need to use the special mr Matte/ Shadow/ reflection material (isn't near Max right now, and can't remember the exact name). Overall it acts like standard Matte/Shadow material, but one important thing differs: you need to actively insert the color or bitmap you want to use as background into the slot named "camera mapped background" (or something like that).

Hop it helps

[william.84.48]

Thank you for the quick reply PROH.

I did as you suggested, and I used the matte/shadow/reflections material for the ground plane.  I had to pass the bitmap of my background through an environment/Background camera map node in the slate material editor for it to work, however.

I also had to go into the plane’s object properties and uncheck visible to reflection / refraction.  I did the same with the sphere.  I had to do this, because the ground plane was reflecting on the sphere, but the plane was set to blend into the background image, not into what the sphere was reflecting.  Hence, you could see something wasn’t right.

This had one side effect though.  Now, the chrome ball does not reflect the shadows that the plane catches.  Is this just me, or is there a way to fix this?  If you look at the attachment image “sphere_forum_shadow.jpg”, you will notice that the shadow is being reflected, but the chrome ball is also reflecting the plane, making it look weird (this is when using the matte/shadows/reflection material with the background image passed through the Environment/Background node).  The attachment image “sphere_forum.jpg” however, does not reflect shadows, but does not reflect the plane either (this is when I uncheck visible to reflection/refraction in the object properties for both the sphere and plane).

Thanks.

[PROH · Accepted solution]

Hi. If I understand it correct, you want the sphere to cast shadow and to reflect a spherical background image + its own shadow. I just tried it out, and this works:

1) use a mr Environment/Background Switcher, with the spherical background image in the "Environment/Reflection" slot, and the Background color set to white (white is only for testing - color should be black in final render for composit). Put (instanced) the switcher in the environment map slot.

2) create a plane and apply a mr Matte/Shadow/Reflection material to it. Activate "Receive Shadows" and "Use AO", but NOT "Recieve Reflections". In the "Camera Mapped Background" slot you put (instance) the switcher.

3) you don't need to alter any object properties, just render and things should work as wanted.

4) for final render, set the Background color in the switcher to black, and save the image as a TIF with alpha. Now you can use the alpha channel as a mask in PS (or other software), when compositing your image.

Hope it helps

[william.84.48]

Thanks again for the response. 

Unfortunately, this still does not seem to solve my problem.  I'm probably doing something wrong.  I've attached what the render (when using the color white for background in the switcher), as well as what my material slate editor looks like.  The render, using mr photographic exposure control, looks basically all black (render1.jpg).  The render using logarithmic control however, is brighter (render1_log.jpg).  But, you can see that the plane is being reflected in a very weird way.  What's going on? 

Thanks.

[PROH · Accepted solution]

Well, it's some time ago, but if I remember correct I used IBL as light-source without exposure-control (IBL = standard skylight with a spherical HDRI in the color slot + FG set to IBL).

Regarding your reflections of the background, then yes, it does look strange. It's hard to say what's wrong, since your material picture doesn't show any setting, but there are a couple of things to check:

1) Is you spherical bitmap set to "Spherical Environment"?
2) is the Background/Reflection map put into the Environment slot?
3) How is you mirror material on the ball set up? Try using mr A&D Chrome material (if it isn't that already).

If nothing works, then maybe you should try to start from scratch in a new file.

Hope it helps

[william.84.48]

Thank you!!!  

It turns out that I had my spherical bitmap set as a texture (as you pointed out in your first point).  I changed it to a spherical environment, and bam, it worked.

For anyone wondering out there in the future who might see this post, I used a daylight system.  Using the Photographic exposure control however, everything appeared black.  I changed this by pressing '8' and going into the environment and effects dialog.  Here, I changed the Physical Scale to Unitless, and changed the value to something like 90,000.  It was better, but still slightly dark, so I changed the EV (exposure value) to 14 and enabled "Process Background and Environment Maps".  This got me what I wanted.  I also was using Arch & Design materials.

This may be too much to ask from you, you've really been a great help to me...but I can't resist the urge to ask.  Could you explain briefly what the Environment/Background Switcher is doing?

Thanks again so much!

[PROH]

No problem. The Environment/Background switcher simply makes its possible to render with another background than the one used for scene reflection and lighting (environment). It's typically used to render out images for compositing and postprocessing (black background), or when using a spherical environment for light, reflections etc. with a "backplate" background image (set to screen).

In the "Background" slot you input the image/color that will be used as background in the render. This slot will NOT affect reflections, light or anything else in the scene.

In the "Environment/reflections" slot you input the image/color that will be used for reflections, light etc. (Environment)

To make it work, the Switcher must be placed in Max's "Environment Map" slot (press 8 to acces this). When used in combinations with the mr "Matte/Shadow/Reflections" material, it must also be placed in this materials "Camera Mapped Background" slot.

If the answers solved your problem, then please mark it as accepted solutions. This will make it easier for others with the same problem to find a solution.

[Anonymous]

I want the answer to this question also but.

NOT FOR MENTAL RAY!!!

Using ray trace and have not found a ray trace tutorial or solution.

I know the process but the exact naming and nodes for ray trace.

Thanks.

[maxxeric]

Can try to select Environment/Background Camera Map for Camera Mapped Background and select backing mapping in the Map.

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@william.84.48 wrote:

Hello all!

I’ve seen this question posted many times ino ther places, but there never seems to be a confirmation that the solutions work fully or anything.  I have a sphere in my scene which is casting shadows onto a plane, acting as the ground.  However, I want to make the ground plane invisible, but I want the shadows to still be visible.

This is easily achieved in Scanline by applying the matte/shadow material to the plane.  However, I’m using mental ray, and it does not support this material.  The matte/shadow/reflections material that mr has also does not seem to work properly.

Just for record, I’m using a spherical environment map (a 360 panorama) for reflections, but showing the image I’m compositing this sphere onto in the end for the viewport background (so that I can align my camera).  When I render, I want the end result to be the sphere on a transparent background with the shadow(s) so that I may composite this onto my original image in After Effects or Photoshop.  I’m okay with the shadows being a different render.

I’ve thought about rendering the sphere without shadows in mr, and then the shadows by themselves with scanline (so that I can use the matte/shadow material), but if there is a better way to do this, I would love to know about it.

Thanks in advance.

Beginner 3ds Max User

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