Hi,
I Checked the “Matte Objects” box in the render settings but still, everything expect for the particles, are not visible when I try to render.
Matte (a.k.a. Holdout) objects do not render as such, they occlude particles and leave a gap in the particle cloud to composite another image of the objects from a different renderer. Basically Krakatoa renders a Z-Depth pass using the Matte objects and then checks each particle against it to decide whether to render the particle, or whether to skip it (or alternatively send it to a different layer for compositing as background).
There are two conditions for an object to render as a Matte - the checkbox you checked, and a Krakatoa Mesh Tag which has two checkboxes - the one controls the occlusion of particles relative to the camera, the other controls the occlusion relative to lights. When both are checked (default), matte objects will leave a hole in the rendered image AND cast shadows on particles (as long as the light’s shadow casting is not set to None). When the camera checkbox is unchecked, the object will not occlude particles in the final image, but will still cast shadows. When the light checkbox is unchecked, no shadows will be cast onto particles. When both are unchecked, the object will be completely ignored by Krakatoa (same as removing the Tag).
Once you have your Krakatoa rendering, you have to render the objects in any other available renderer and composite the results together in a compositing application like After Effects, Fusion, Nuke etc.
My object is in the middle of the particles area (some particles are in front of the object). Is there a way to render only the visible part of the object? (so it could be composed later with the particles)
You would render your object normally with whatever renderenging you use and then render the krakatoa pass with the hold out matte Bobo described and comp the particles on top of the object.
Also, if the object is semi-transparent, you are supposed to enable the option to render the Occluded Particles background pass, and then composite the foreground (beauty RGBA) OVER the matte object rendering from the other renderer, OVER the background pass. This also helps with Anti-aliasing at the edges.
The following page was written for the Maya version of Krakatoa, but applies to the C4D version, too:
thinkboxsoftware.com/kmy-com … nd-mattes/