@kerrj @jb-ye Would you mind reviewing this PR?

This is incorrect. #3612 is correct.
colmap's k4, k5 and k6 are still unsupported for the perspective model.
k4 should be supported for the fisheye model.

Hi @f-dy, for the perspective models—"SIMPLE_PINHOLE", "PINHOLE", "SIMPLE_RADIAL", and "RADIAL"—their k4 parameter is correctly set to 0 when reading the parameters. I'm not sure what the purpose of this error check is; please let me know if I've misunderstood anything.

It’s possible that k4 doesn’t support fisheye either; a new PR is needed here.

@Yubel426
You shouldn't set k4 to zero when you read camera parameters if it has k4 non zero.

Imagine you have some random function over [a;b] and you have a polynom which approximates it: y = a * x^2 + bx + c
You can't "approximate" it further to linear model just by dropping a and having something like y = bx + c.

Instead you need to build a proper new approximation, not just dropping one coefficient from the reacher one.