the size of image shrinks

[derrick-xwp]

hi,

I use colmap to reconstruct,
the size of input image is (376, 541)
But the size of image in the output of colmap is (355, 512)
Do you why does this happen?

Thanks

[tsattler]

Is this the image size after dense reconstruction, i.e., the size of the undistorted images?

[derrick-xwp]

Hi Torsten The size of colmap depth maps also shrunk. Is that due to the dense reconstruction?

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------------------&nbsp;Original message&nbsp;------------------ From: "Torsten Sattler"; Sendtime: Sunday, Jan 24, 2021 10:42 PM To: "colmap/colmap"; Cc: "Derryck"<843553050@qq.com>; "Author"; Subject: Re: [colmap/colmap] the size of image shrinks (#1100) Is this the image size after dense reconstruction, i.e., the size of the undistorted images? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or unsubscribe.

[tsattler]

As part of the dense reconstruction process, Colmap removes radial (and potentially tangential) distortion from the images. The size of the undistorted images (and the depth maps since they are computed using the undistorted images) typically differs from the size of the original images.

[ahojnnes]

The size of images can change in the image undistortion stage before dense reconstruction. This is because COLMAP automatically crops unobserved regions in the undistorted images, which can occur if you use a camera model which models non-linear distortion (radial/tangential, etc.).

[tom-bu]

The undistortion seems to cut out the majority of each image, leaving only a small part for dense reconstruction. Is that because there's strong radial distortion in my camera? Is there a work around? Is there no way for it to create an undistorted image that is similar to the original image, rather than removing parts of the image that have some distortion?

[hummat]

Hi @tom-bu you could have a look at OpenCVs getOptimalNewCameraMatrix with alpha=1 to obtain a new camera matrix which preserves pixels.

For reference:

1. https://stackoverflow.com/questions/39432322/what-does-the-getoptimalnewcameramatrix-do-in-opencv
2. https://docs.opencv.org/3.3.0/dc/dbb/tutorial_py_calibration.html (under "Undistortion")