3d Point Cloud forming like a prism

Dear All I am trying to plot a point cloud in Matlab 2016. The point cloud is forming something like this instead of a regular point cloud. Can anyone help me with why this is happening?

Thank you

My code

clear all 
close all
%%Load the Parameters of the Stereo Camera
% Load the |stereoParameters| object, which is the result of calibrating
% the camera using either the |stereoCameraCalibrator| app or the
% |estimateCameraParameters| function.

% Load the stereoParameters object.
load('webcam.mat');

% Visualize camera extrinsics.
showExtrinsics(stereoParams);

% %%Create Video File Readers and the Video Player
% % Create System Objects for reading and displaying the video
% videoFileLeft = 'second_1.mp4';
% videoFileRight = 'first_1.mp4';
% 
% readerLeft = vision.VideoFileReader(videoFileLeft, 'VideoOutputDataType', 'uint8');
% readerRight = vision.VideoFileReader(videoFileRight, 'VideoOutputDataType', 'uint8');
% player = vision.DeployableVideoPlayer('Location', [20, 400]);

%%Read and Rectify Video Frames
% The frames from the left and the right cameras must be rectified in order
% to compute disparity and reconstruct the 3-D scene. Rectified images 
% have horizontal epipolar lines, and are row-aligned. This simplifies 
% the computation of disparity by reducing the search space for matching
% points to one dimension.  Rectified images can also be combined into an
% anaglyph, which can be viewed using the stereo red-cyan glasses to see
% the 3-D effect.
frameLeft = imread('first.jpg');
frameRight = imread('second.jpg');

[frameLeftRect, frameRightRect] = ...
    rectifyStereoImages(frameLeft, frameRight, stereoParams);

figure;
imshow(stereoAnaglyph(frameLeftRect, frameRightRect));
title('Rectified Video Frames');

%%Compute Disparity
% In rectified stereo images any pair of corresponding points are located 
% on the same pixel row. For each pixel in the left image compute the
% distance to the corresponding pixel in the right image. This distance is
% called the disparity, and it is proportional to the distance of the
% corresponding world point from the camera.
frameLeftGray  = rgb2gray(frameLeftRect);
frameRightGray = rgb2gray(frameRightRect);

disparityMap = disparity(frameLeftGray, frameRightGray);
figure;
imshow(disparityMap, [0, 64]);
title('Disparity Map');
colormap jet
colorbar

%%Reconstruct the 3-D Scene
% Reconstruct the 3-D world coordinates of points corresponding to each
% pixel from the disparity map.
points3D = reconstructScene(disparityMap, stereoParams);

% Convert to meters and create a pointCloud object
points3D = points3D ./ 1000;

ptCloud = pointCloud(points3D, 'Color',frameLeftRect);
% 
% % Create a streaming point cloud viewer
% player3D = pcplayer([-3, 3], [-3, 3], [0, 8], 'VerticalAxis', 'y', ...
%     'VerticalAxisDir', 'down');
% 
% % Visualize the point cloud
% % view(player3D, ptCloud);
% pcshow(ptCloud.Location,ptCloud.Color);

figure

pcshow(ptCloud, 'VerticalAxis', 'y', 'VerticalAxisDir', 'down', ...
    'MarkerSize', 45);

% filename='xxx.ply';
% pcwrite(ptCloud,filename,'PLYFormat','binary');

Answer 1

[Qiuheng Zhou]

Hi, i think, i have the same problem as yours. Do you have already solved your problem?

I guess the problem should be hier.

ptCloud = pointCloud(points3D, 'Color',frameLeftRect);

The algorithm of KNN should be changed. I am not sure.

[Munin]

@Walter Roberson @Qiuheng Zhou @Pooshpanja Roy I've got a bulletproof workaround, and I've contacted the Mathworks team:

stereo2stl/script.m at v1.5.0 · SNDST00M/stereo2stl (github.com)

Answer 2

[Munin]

I contacted the MATLAB support team about this issue and told them I wrote a few dozen lines of code to constrain the giant prism point cloud to the world scene at its z-apex:

https://github.com/SNDST00M/stereo2stl/blob/v1.5.0/script.m#L216-L380

They didn't accept the code and suggested a simple > 3m = 0 filter. I.e. it will never be patched. The world scene is the millionth of the point cloud surface that is closest to the origin