How to take object shadow for a Camera Match

[DANIELE_CASAROLA]

Hi!

Really sorry if yet discussed this argument, but I can't really find a working explanation and solution.

I have to put a 3d object (a chair) inside a picture. The picture rapresents an interior. Quite easy to set, I've modeled all the room and recreated the same illumination. The only thing I need is to take the little shadow (not shadow of direct light, but of indirect light) all around the base of the chair on the floor, really soft shadow, but only the chair soft shadow, not other shadow.

Which way to take it and composite in photoshop?!

Read a lot about Render Elements and Matte feature (both on properties and material), but with no a working workflow.

Thank you very much for your support.

[svetlozar.draganov]

Hello DANIELE,

Could you please give us a few image-examples of what you want to achieve, a sample scene would be even better.
Shadows from Direct and Indirect Illumination could be captured with Matte Objects , for contact shadows you could use VrayDirt map.

[Morbid Angel]

set your chair visibility to off and select your objects (room and other stuff) and go to vray properties check matte object and alpha -1, this should give you what you need.

[DANIELE_CASAROLA]

Hello DANIELE,

Could you please give us a few image-examples of what you want to achieve, a sample scene would be even better.
Shadows from Direct and Indirect Illumination could be captured with Matte Objects , for contact shadows you could use VrayDirt map.

Thank you svetloza for answering me! you're right, the matte parameter allow object to catch both direct and indirect shadow.. but... in my example (that I will let you see when I have a moment to breath...) I have to insert a chair inside the picture of a room, so an entirior environment. To better simulate the 3d model insert I have re-build in 3d the room (more or less). In this way I can simulate the exact light bounces on the 3d model. So... if I activate the matte option for the floor... well.. it catch all shadows.. not only the chair indirect shadow on it, but also of the walls and soffit... in this way it appears completely black...
This is the real problem.. how to put direct or indirect shadow on the floor for only the 3d chair object... and on for any other.

I suppose the the "matte" properties is intended to use just for a plane and the 3d model in the scene, but you know... to re-build the structure of the room give more more realism.

Anyway, your trick to use the dirt map could be a solution.. even if you know.. if indirect light enter from a window.. the shadow on the floor is "shift" in the opposite side of the chair.

Thank you!

[svetlozar.draganov]

If you want to catch only the shadow from the particular objects - you can select all other objects and turn off "cast shadows" option from Objects Properties.
But if you decide to use this trick and you want to render anything else than Matte Objects and the Chair - you have to do a second render with Cast Shadows ON, and use image editing software to compose them.
This will be also necessary if you want to catch shadows on chair objects casted by another objects in the scene.