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computer-visioncameracamera-calibration

How will the Camera Intrinsics change if an image is cropped/resized?

I have a recorded camera ROS bag file from a Realsense camera. The camera intrinsics for the recorded setting in already know. The initial resolution of the image is 848*480. Because of some visual obstruction in the FOV of the camera I would like to crop out the top of the image so it doesn't gets detected with Visual SLAM Algorithm I am using.
Since SLAM is heavily dependent on the Camera Intrinsics, I would like to know how will the camera parameters f_x, f_y, c_x and c_y change for :

  1. Cropped Image
  2. Resized Image (Image Scaling only)

Image Size Changes

There is no skew involved in the original camera parameters.
Will the new pricipal point c_x also change as Cropped_image_width? I am bit confused as to how to calculate the new camera parameters ? Am I correct in assuming the following for the Case 1 - Cropped Case :

enter image description here

Solution

  • Cropping:

    • cx,cy reduced by the amount of pixels cropped off the left/top. Cropping off of right/bottom edges has no effect.

    Scaling:

    • fx,fy are multiplied by the scaling factor
    • cx,cy are multiplied by the scaling factor

    Remember, the principal point need not be in the center of the image.

    Assuming top left image origin. Off-by-one/half (pixel) errors to be checked carefully against the specific scaling algorithm used, or just ignored.