Question about Intrinsic Adjustment Formula in Center-Crop Operation

[Rashfu]

Hi,

Thank you for sharing your work! I have a question regarding the adjustment of intrinsic parameters after the center-crop operation in the code.

# Center-crop the image.
images = images[..., :, row : row + h_out, col : col + w_out]

# Adjust the intrinsics to account for the cropping.
intrinsics = intrinsics.clone()
intrinsics[..., 0, 0] *= w_in / w_out  # fx
intrinsics[..., 1, 1] *= h_in / h_out  # fy

From my understanding, when performing a center crop, the intrinsic focal lengths (fx and fy) should scale with the crop ratio. For example, if w_out < w_in (the cropped image is smaller than the original), the focal lengths should decrease accordingly.

Wouldn't the adjustment need to scale by the factor w_out / w_in and h_out / h_in instead of w_in / w_out and h_in / h_out? And if the image is cropped, the new cx and cy should shift relative to the new origin of the cropped image, which is not implemented in the code.

[botaoye]

Hi, thank you for your insightful question. Our used intrinsic matric is normalized, therefore the principal point should stay the same and the focal length changes as in the code. We share the same intrinsic format as pixelSplat, a more detailed explanation of the focal length can be found here.