CODEWITHSUNDEEP
opencvcomputer-visioncamerahomographyimage-stitching
nadhir
nadhir

Reputation: 21

Homography when camera translation (for stitching)

I have a camera which I take with it 2 captures. I want do make a reconstitution with the 2 images in one image.

I only do a translation with the camera an take images of a plane TV screen. I heard homography only works when the camera does a rotation.

What should i do when I only have a translation?

Upvotes: 2

Views: 2425

Answers (1)

Toby Collins
Toby Collins

Reputation: 823

Because you are imaging a planar surface (in your case a TV screen), all images of it with a perspective camera will be related by homographies. This is the same if your camera is translating and/or rotating. Therefore to stitch different images of the surface, you don't need to do any 3D geometry processing (essential matrix computation/triangulation etc.).

To solve your problem you need to do the following:

  1. You determine the homographies between your images. Because you only have two images you can select the first one as the 'source' and the second one as the 'target', and compute the homography from target to source. This is classically done with feature detection and robust homography fitting. Let's denote this homography by the 3x3 matrix H.
  2. You warp your target image to your source using H. You can do this in openCV with the warpPerspective method.
  3. Merge your source and warped target using a blending function.

An open source project for doing exactly these steps is here.

If your TV lacks distinct features or there is lots of background clutter, the method for estimating H might not be highly robust. If this is the case you could manually click four or more correspondences on the TV in the target and source images, and compute H using OpenCV's findHomography method. Note that your correspondences cannot be completely arbitrary. Specifically, there should not be three correspondences that are colinear (in which case H cannot be computed). They should also be clicked as accurately as possible because errors will affect the final stitch and cause ghosting artefacts.

An important caveat is if your camera has significant lens distortion. In this case your images will not be related by homographies. You can deal with this by performing a calibration of your camera using OpenCV, and then you need to pre-process your images to undo the lens distortion (using OpenCV's undistort method).

Upvotes: 3

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