# Calculate relative 3D position from points in stereo images?

Hi everybody,

I have a (DIY) stereo camera readily calibrated so that it can be used with stereo_image_proc which successfully and beautifully creates a point cloud. That's nice so far... but it's not the question.

In addition to the image data (RGB) I also have several key points in image coordinates (x, y) that are generated from the pictures of both, the left and the right eye of the stereo camera. I would like to match these points so that I can calculate the Z-component for each point. That is, I have x1, y1, x2, y2 for each corresponding point and I'd like to have a full blown relative (and later on through tf absolute) x, y, z position for each point.

Thanks a lot!

//edit: oh, just to add to it: It is known which key point in the left corresponds to which one in the right image. That is, there is no feature matching necessary.

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I'm slightly confused: you mention you already have pointclouds, so that implies that dense-stereo matching / processing is already working and disparity can be (successfully?) calculated. Are you now asking to do the same thing again, but for some specific features in both images?

( 2017-09-28 10:46:51 -0600 )edit

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You've got all the information from the stereo calibration to construct an image_geometry::StereoCameraModel. You could use the projectDisparityTo3d function from the StereoCameraModel to get the 3D coordinates. Your input is a pixel location in rectified image (u, v) and the corresponding disparity (x2 - x1).

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Works like a charm. Thanks a lot!

( 2017-10-05 10:04:04 -0600 )edit

You`ll want to use OpenCV's triangulatepoints function, which is doing exactly what you're asking for (unless I misinterpreted your question :) ).

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