3 Replies Latest reply on Apr 9, 2018 3:18 AM by MartyG

    Undistorting 415 colour images

    SteveRealSense

      Hi,

       

      Although 415 image distortion coefficients do not seem to be calculated (seem to be all 0), I have been trying to calculate these myself. If these are known, is there a way to get an undistorted RGB image through librealsense? To explain my use case: I created a mesh from all my depth images. Now I want to texture it with the colour images. But I am getting a shift on my mesh. I presume this may be due to the RGB image being projected is not undistorted. I have tried to undistort the images using the left/right distortion coefficients in OpenCV (since these may be quite similar) but my code is just crashing on using those values. I remember reading somewhere that the coefficients were a 'modified' brown conrady model. Is this a model which is used just by RealSense? Or is it a common model.

        • 1. Re: Undistorting 415 colour images
          MartyG

          Yes, D415 uses Modified Brown-Conrady.

           

          Others have modified (or 'extended') the Brown-Conrady model, suggesting that whilst the Modified Brown Conrady model may be unique to RealSense, modified models in themselves are not unusual.

           

          Modeling of radial asymmetry in lens... (PDF Download Available)

          • 2. Re: Undistorting 415 colour images
            SteveRealSense

            Is there any info on what modifications are made for RealSense? So that I can translate this model into a more conventional one that I can undistort with OpenCV.

            • 3. Re: Undistorting 415 colour images
              MartyG

              A technical paper that had one of the 400 Series camera's engineers as an author says about the modified model: "The modification comes in the form of computing tangential terms with radially-corrected x, y.  The open-source library for accessing the Intel RealSense cameras details the distortion model exactly".

               

              Here is a link to the full paper:

              https://arxiv.org/pdf/1705.05548.pdf

               

              Digging through the Librealsense code for Modified Brown-Conrady formulas, the closest I found to such a thing was the code below that is applied when Modified Brown-Conrady is the distortion mode.

               

              const r2 = x * x + y * y;

                  const f = 1 + intrinsics.coeffs[0] * r2 + intrinsics.coeffs[1] * r2 * r2 +

                      intrinsics.coeffs[4] * r2 * r2 * r2;

                  x *= f;

                  y *= f;

                  const dx = x + 2 * intrinsics.coeffs[2] * x * y + intrinsics.coeffs[3] * (r2 + 2 * x * x);

                  const dy = y + 2 * intrinsics.coeffs[3] * x * y + intrinsics.coeffs[2] * (r2 + 2 * y * y);

                  x = dx;

                  y = dy;