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Houdini Trace in camera space

A very handy tool which I tend to use a lot, do you often find your self in a situation where you need a 3d object to match a live plate, whether you wanna use it as a:

- Collision mesh.

- shadow casting / matte shadow.

- 3D masking (particles goring around a character and you don't wanna render the front and back as a separate passes).

It will be time consuming to accurately take the measurements and model it, and wont be pixel accurate.

Okay then a nice trick that goes:

- draw a mask of your desired model (photoshop), also it can be an animated mask.

- trace it in Houdini.

- move it to camera space.

- push it in depth to the desired location to match the angle of your object, (I used "Ray sop" all the points have normals pointing towards the camera).

- add some extrusion if it might help in your case.

- done.

Technical notes:

This tool remap uv attributes (applied to points) to camera space, so:

- Trace sop in Houdini apply a uv attribute to the mesh when "add point Texture" is ticked.

- Or you need to add a "uv texture sop" and pay attention to the projection plane.

In the file you'll find another variation where I used the Bounding box to give the points a value of 0 to 1 to be remapped.

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