background problem for matte surface

[jasonhuang1115]

Hi,

I have a setup that contains the following:

1. a hemispherical dome geo with spherically projected HDR (via maya’s project node in spherical mode to self-illumination slot) that only has the Primary Visibility on. I use this for rendering the background because I need to rotate the camera around.

2. a ground geo that set as matte surface to serve as the shadow catcher.

The question is: for the matte surface ground, Vray renders what is plugged in the Background texture (Override Environment) for matte surfaces. In this scenario, my options are:
a) nothing plugged in, so Vray renders a chunk of black.
b) If I plug in a screen space projected backplate, I then can't rotate the camera around for rendering from other angles.
c) If I plugin the HDR (that is used to spherically projected on the hemispherical dome), Vray force to use the VRayPlaceEnvTex node. In renders, the ground won't match the rest of the background that is projected via Maya's default projection node.

Is there a workaround for this situation or did I miss something?

Thanks.

[Zdravko.Keremidchiev]

Try using VRayDomeLight (spherical) with that same HDRI instead and you should get the effect your are after. Also make sure you keep the HDRI in the override environment background slot.

[jasonhuang1115]

Hi Zdravko,

I have tried the idea you mentioned. Domelight and override environment background with same HDR won't work for this because it will provide a different perspective, when rendered as background, from using an hemi-spherical dome geo with same HDR projected.

I assume in this situation you can say the background is completely CG with an HDR plugged into self-illumination as texture.
Can a shadow catcher (matter surface or VrayMtlWrapper or any trick) be catching the shadow for cg characters while let the camera render the CG environment behind/underneath?

Thanks.

[Zdravko.Keremidchiev]

Could you please specify what do you mean by "different perspective"? It will be best if you could send us an example scene to support@chaosgroup.com so we could get a better idea of the issue. Just make sure to mention this thread in your e-mail.
Generally the workflow with the dome light and the matte surface should do the trick.

[sharktacos]

If the goal is to have the ground have the HDR mapped to it, couldn't you just change the VRayPlaceEnvTex from screen to sphere, and use that same VRayPlaceEnvTex for your sphere?

[jasonhuang1115]

Zdravko, by "different perspective" I mean when you plug the same HDR into a dome light and to a dome geo (that has a finite size) and have Primary Visibility on for both, you get different render results, right? I assume it is because the spherical projection in controlled by VRayPlaceEnvTex for the dome light and Maya's spherical project function respectively. Also, the dome light has "infinite" size whereas the dome geo has a finite size the user defined. I will send you a mock-up scene to play with.

sharktacos, that's a great idea. I tried using VRayPlaceEnvTex to replace the Maya's Project node (with Spherical Proj mode). It sync the primary visibility result of the ground and dome geometry but I couldn't quite figure out how to transform/rotate the projection center of VRayPlaceEnvTex intuitively. For example, if I want to project from the 150 cm ground up. Also, it doesn't solve the issue of shadow catching because the HDR map is plugged into Self-Illumination slot.

[sharktacos]

The way I use the matte properties in the VrayObjectProperties for shadow catching is to render it against black so I get a shadow in the alpha. I then comp that over my background. The ground plane would be used for bouncing GI and reflection onto the object on it, but it would itself not render. If you want instead to have it all in your render (i.e. not comping), just turn the primary visibility on your sphere on (with cast shadows off).

The VRayPlaceEnvTex has a thingy that lets you rotate it around the Y axis. I prefer to use a domeLight for this purpose with the "affect diffuse" turned off (this makes is super fast).

[jasonhuang1115]

The way I use the matte properties in the VrayObjectProperties for shadow catching is to render it against black so I get a shadow in the alpha. I then comp that over my background. The ground plane would be used for bouncing GI and reflection onto the object on it, but it would itself not render. If you want instead to have it all in your render (i.e. not comping), just turn the primary visibility on your sphere on (with cast shadows off).

I am trying to achieve this in render. The primary visibility on for the hemi-sphere is already on but when the HDR is spherically projected to the Self-Illumination slot of a VRayMtl, how would you get the shadow for CG character?

The VRayPlaceEnvTex has a thingy that lets you rotate it around the Y axis. I prefer to use a domeLight for this purpose with the "affect diffuse" turned off (this makes is super fast).

I guess you are referring to the Horizontal rotation attribute. Still, I am looking for a viewport manipulator for the VRayPlaceEnvTex so you can visually control how you spherically project the HDR (like you get a manipulator for the dome light when you enable "Use transform").

[sharktacos]

I am trying to achieve this in render. The primary visibility on for the hemi-sphere is already on but when the HDR is spherically projected to the Self-Illumination slot of a VRayMtl, how would you get the shadow for CG character?

What I would do is this:
1. Have the HDR in the self-illumination of the ground plane's vrayMtl (this is for GI and reflections from the ground on your object)
2. A DomeLight with the same HDR, with affect diffuse and invisible ON (for environment reflections).
3. The same HDR in the Render Settings environment override background texture (this makes the HDR show up in the ground with the shadow

[jasonhuang1115]

Hi sharktacos,

Thanks for the suggestions. If I understand correctly, the same VRayPlaceEnvTex is used to control the spherical projection for the ground, the hemi-spherical dome geo, and Environment background texture, so when the ground is set to a matte surface, everything just line up in terms of shadow and background perspective. A big issue is I couldn't find an intuitive control for the VRayPlaceEnvTex node to achieve rendered background as if the HDR is projected onto a 10-meter radius hemi-spherical dome geometry.

With Maya's Projection node (spherical mode), it's pretty easy to transform/rotate/scale the Projection node and/or hemi-spherical dome geo with RT to get a desired background with acceptable distortion.

I have attached a sample archived scene file with HDR that I also had sent to the support team. Feel free to give it a try and I look forward to seeing what you have come up with.
background problem for matte surface.ma.zip

Edit: to clarify the different background perspective between dome light and dome geometry with spherically projected HDR.

[sharktacos]

When you say that the HDR does is distorted, can you explain what you mean?

If you want to use the Maya projection node in all of these, you could.

[jasonhuang1115]

Zdravko, I have sent support an email with archive Maya project and link to this thread.

sharktacos, I have edited my last reply to make it clearer.
You get different primary visibility results between projecting an HDR to a dome geometry and plugging that HDR in a dome light. That's what I was referring.

[sharktacos]

That's a scale issue with the geo. The domeLight is infinitely distant.