[Bf-vfx] Distortion and you

[Keir Mierle]

One important remaining item for the blender motion tracker is supporting
camera distortion end-to-end. That means supporting distortion in the camera
solver (which means solving for the distortion model during the 3D solve);
this is in progress. The other thing that has to be supported is both of the
following:

   1. Correcting distorted footage to make it perfectly rectilinear, and
   compositing it with rendered footage, which is by default perfectly
   rectilinear.
   2. Rendering footage with distortion, either by distorting the rendered
   frames (which means cropping the final video) or somehow integrating
   distortion into the rendering, which I believe is possible with cycles.

Matthias is going to start enhancing the distortion code in libmv and
building warping nodes in blender's compositor which use the camera models
from libmv. It's critical that there is a distortion node that uses the same
distortion code as libmv, for future sanity and quality of solutions. This
does not preclude other distortion nodes, just that there needs to be one
that matches libmv.

The distortion node should be able to take a "camera intrinsics" data block
from the solver. Currently the solver doesn't recover the distortion terms,
but this will change.

The other issue is that we will probably support a couple of different
distortion models, but not at the start. We'll have to keep this in mind
when desiging things.

The immediate plan is

   1. Write a blender compositior node that runs forward distortion; this
   will involve getting the machinery in place to pass tiles of the image to
   libmv and back
   2. Make undistortion in libmv fast
   3. Extend blender's compositor node to also do undistortion with libmv
   using the faster code
   4. (maybe) add more camera models

That's it.

Keir
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