casting shadows on invisible objects

[FromHell]

Hey guys, any ideas of how to cast shadows on an object which is invisible on rendering?

I have tried the check box in render menu of an object, but it makes the object black.

I also tried shadow matte shader, it creates a shadow but also a black object during the render.

but what i want is a transparent object with a shadow on top of it.

[anim]

just connect alpha from Shadow Matte VOP to Of output and black constant to Cf

[FromHell]

Doublepost

shadows.hipnc

Edited December 8, 2012 by FromHell

[FromHell]

Thanks for your help, for some reason i cant edit the shader network of the matte shadow vop, it just doesn't allow me to dive into it.

I've attached the scene file that im working with.

Edited December 8, 2012 by FromHell

[anim]

what I meant was Shadow Matte VOP not Shadow Matte SHOP

look at the file

shadows_fix.hipnc

[FromHell]

thanks, this is almost the result i need, however the matte object is still visible in the reflection of the object.

I have created an environment light with a white color, to make it easier to see, that the grid is still visible as a black reflection on

the other object. Is it possible to fix that? I have tried to exlude the grid in the reflection mask, but doesn't seem to work.

shadows_issue.hipnc

[anim]

it seems that mantra has some bugs regarding reflection of the lights and handling objects opacities and reflection masks, I've noticed this a few times

because even if it doesn't reflect object it is still using it for occluding lights

use projected sphere as an reflection environment for now, if you exclude it from shadowcasting to don't block envlight it will be used just for reflection/refraction and if you don't want to see it in render turn Phantom on

another thing is that if you wish to have different shadow color by setting Cf to non black (as you have right now), you need to multiply it with alpha from shadowmatte prior to connecting to Cf to get just shadow portion

shadows_issue_fix.hipnc

[FromHell]

It works if i have just 1 color as my background, but the problem comes back if i apply an HDRI map to the background.

I have tried to apply HDRI to the sphere without the environment light and sometimes it works, but as soon as the size of the HDRi map exceeds 4,5 MB or so

Houdini crashes with a memory error message.

Does side FX actually know of this problem with the lights??

[anim]

it shouldn't be crashing when you apply map to sphere (you can try rat file, but it shouldn't matter anyway)

on the other note, when I want just shadow I am usually using Direct Shadow and Indirect Shadow image planes with specifying object visibilities and matte/phantom stages in mantra node to get shadows only on objects I want, I noticed much better sampling than using matte shadow directly, on the other hand you need to do some comp afterwards

[FromHell]

Thanks Tomas for your help, I'm going try this out!