Optional: enter fx, fy, cx, cy in PIXELS at the uploaded video's resolution (for example: 600, 610, 320, 240 for a calibrated 640 x 480 video). Use calibration for the actual uploaded video, after any previous resizing/cropping. With calibration, we resample to equal pixel focal lengths, crop around the principal point, and use the corrected intrinsics for inference and rendering. Output shows the corrected crop, so edge content may be removed. Leave blank to predict focal lengths and assume a centered principal point; this does NOT correct an unknown off-center principal point. Input must already be lens-undistorted; fisheye distortion and time-varying calibration are not handled.