RC3 matte issue

[jacksc02]

Hi Guys,
Im just trying to render out a matte pass of an interior and im getting the shadows from my vraysun coming through the matted out walls floors and ceiling.
The model is all solid poly modelled with normals all facing the right way.

I can post up an image if needed

Any ideas why?

[Erik de Graaf]

Cant really help, all i know is i had the same problem with a parking garage and i thought it was just me doing something stupid.

I also would like to know about the cause of this.

[jacksc02]

Erik, Thanks for the reply. Candelero has confirmed that its a bug.

[vlado]

This is not a bug, and the result is correct (although obviously not what you need).

There is an option in the V-Ray object settings called "No GI on other mattes" that is ON by default, which means that matte objects will not cast shadows on matte objects, thus letting the light pass through them. Switch this option OFF to make matte objects to cast shadows on other mattes and you will get correct shadows.

However, if you want to use GI with this, then you will get another problem - you will lose all GI on the matte surfaces. The reason for this would be that the GI is too weak compared to the sunlight, and gets practically lost. This is because the matte shadows are computed according to the formula 1-(shadowed light)/(unshadowed light). In this case, the unshadowed light is the sunlight which is so bright that practically reduces the effect of any other lighting on the surface.

In general, my advice has always been to avoid matte objects as much as possible and use more reliable methods for compositing (e.g. the render elements). Matte objects are often unpredictable, as the relation of the lights is not intuitive. For example, if you have a single very bright light, like in this case the sun, it will affect the matte shadows, even though the light may not actually affect the particular scene much in any way (f.e. if everything is in shadow relative to this light). So, turning the light on/off may have little visual effect on the regular image, but may produce a drastic difference in the matte shadows - which is certainly not what you'd expect.

Best regards,
Vlado

[Erik de Graaf]

Ok, i see. Thanks for clearing this up.

[jacksc02]

thanks for the reply vlado, but how come there when using LC/QMC everything works as it should?

[vlado]

Because it doesn't... the light cache does not support matte shadows for GI, so they are simply ignored.

Best regards,
Vlado

[Druski]

Vlado,

Do you know of a way to make objects not self-shadow for mattes the same way vray does? Would be really handy for our scanline hair passes.

Cheers