Can someone give me a hint how to create something like a clipmap in houdini. Something like a tranparency map, but which cuts real holes into geometry ( eg. visible in depth buffer .. )
I try to recreate a technique from old Lightwave days to get animated booleans without geometry artefacts ( in lw it was quite simple with clipmaps and procedural boolean texture as input to this map ( this map is just white for inside and black for outside of 2nd booled object ))
I do not want to alter geometry because it will alway yield artefacts while rendering ..

The default Mantra Surface material from the gallery can do this. Just drive your existence Opacity Map in to the Opacity Map parameter. Pretty intuitive. 0 = transparent, 1 = opaque and anything in between gives you varying opacity/transparency.

Does an opacity value of zero make the textured polygon invisible in the depth buffer ?

Yes in the case of Mantra Surface. Of (final Opacity out) of 0 means no surface, therefore the renderable geometry behind it must be kept and not tossed. Any opacity level below the Opacity threshold (anything less than 1 really) will force Mantra to keep the visible geometry around.

Invariably this is depends on how the material shader is authored and how it sets Of. I know that the Mantra Surface gallery material takes an opacity texture and drives it directly in to Of or Output opacity so yes in this case specifically.

All well written shaders will/should give you direct access to Of via a texture map and/or some procedural grunge noise.

In my own surface shaders, I like to create a transmit parameter along with an exist parameter. When setting Of (final Opacity) in a regular geometry surface shader I will:

Of = 1;
Of -= transmit;
Of *= exist;
Cf *= exist; # pre-multiply Color by Opacity or premult alpha
Af = alpha * exist;

where
Of = final Opacity out
Cf = final Color out
Af = final Alpha out
the standard output variables in VEX/VOPs.

The transmit parameter is set by a Transmission color vector and multiplied by a Transmission Map parameter. Same for exist where I create an Existence parameter and then multiplied by a Transmission Map. Makes it nice and easy for the artist using the shader.

Thank you very much for the detailed explanation.
Houdini often has so many degrees of freedom that it can be quite confusion. Often so many roads lead to your desired effect that it is hard to decide which one to take and find proper documentation for the chosen direction.

thanks

dulo