Clip Window

I had posted an issue about a month back relating to 3D models disappearing for a few frames and then reappearing. I was working on a new composite shot and ran into the same problem. I think I may have solved the issue but I don't really understand it.

In the new composite shot at the point where the model disappeared, I found that the 'clip window' in the second model's settings was the culprit. The model that disappeared was in 3D Unrolled mode but the second model is set to 3D Plane. When I adjusted the size of the Clip Window in the 2nd 3D plane model, I found the 1st model appeared and stayed visible.

So what is the purpose of the Clip Window and how does it work exactly?


  • Triem23Triem23 Moderator

    So this has to do with Hitfilm's order of operations, and also dovetails into why you can't put effects on a model in 3D Unrolled.

    As far as I can determine (And I've asked @SimonKJones to verify with a dev, but he's not gotten back to me on this topic) Hitfilm has to complete rendering a model into a 2D image before it can add effects to it. Obviously in a 2D model composite it's rendering the model layer to a 2D layer right away. in 3D Unrolled Hitfilm is looking at that entire "3D world" (until it hits a 2D layer, like an adjustment layer) then collapsing all those layers into a a single 2D image (in this case the Adjustment layer, when created, is becoming the 2D image of all layer below, then adding effects to that, right?).

    So putting a model on a 3D plane is this weird, halfway-between render. Hitfilm is calculating and lighting the model, then rendering it to the 2D image, THEN the 2D image can be moved around like a 3D layer.

    So, obviously the clipping window is just the edge of the layer. You can think of it like a "window" looking at the model, and, when you hit the edge of the clipping plane, you've hit the edge of the window and can't see through it anymore.

    Now, a Model can be moved in 3D space, and, if composited in 3D mode the clipping window can be moved independently of the model. Effects can be added to a model composited in 3D mode (but things like glows and flares will be cut off by the edge of the clipping window), so this kind of gives you a way of moving a model in 3D space while it continues to (mostly) properly occlude other 3D layers and particle systems (particles in 3D unrolled) while still applying effects to the model. Of course, by reducing the model to a 3D plane it's infinitely thin and won't quite occlude properly if flying through, say, a 3D unrolled particle clouds...)

    So, if you wanted to take the extra steps, you could animate a 3D model in a 3D space while adding effects directly to the model layer--but you'll also want to animate the position of the clipping plane in 3D space, and either increase the size of the plane, or keyframe it so that it's always facing directly towards the camera. It's a bit of a strange one, and I suspect FxHome more or less intends users to work with 3D models in either 2D mode or in 3D unrolled. That said, having a "Billboard to Camera" toggle for a 3D Model in 3D mode (like how a 3D plane can be set to billboard) would be very useful indeed.

  • Agreed. Thanks Triem23 - you da man! That explanation makes total sense to what I was suspecting. I went back to the original clip where I first experienced the problem and increased the size of the Clip Window and sure enough, the model didn't disappear. At least it's not the model or the way I was compositing or worse, another problem with my new computer.....whew.

    I'll have to play around with the Clip Window to learn how to best control it so it doesn't cause this problem again.

  • Triem23Triem23 Moderator

    Parent a point for the clip window to the move point for the model, then just animate the rotation of the clip window point so it's always facing as dead-on to the camera a possible. It's an extra step, but shouldn't take too long. :-)

  • edited June 2015

    Worked like a charm! Thanks again, Triem23!!!

    I parented the clip window to the camera 3D point and repositioned it to stay perpendicular to the camera.

  • Triem is correct on all points, I believe. The '3D plane' mode for particles and 3D models is partly for legacy support, as 3D models had a clipping window mode in HitFilm 2 Ultimate. That said, there some interesting niche use cases as Triem suggested.

    Most of the time, though, you're either going to be in 2D or full-on 3D unrolled modes.

  • Triem23Triem23 Moderator

    In Ultimate, it's actually very useful--@SimonKJones if you remember my HF2 Planet Rings shot, the planet in the center of the particle ring was a model on a clipping plane. That let the ring wrap correctly.

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