How to detect these points in red circle from point cloud. The attached data.txt is the point cloud data. Many thanks.

[darova]

Can you show your attempts? What have you tried?

[Yunfeng Ge]

Thanks for your replys. I want to extract the three groups of points (red circle) for the point cloud. I do not know how to do. Maybe, first, the all points can be clustered into lots of sets based on connectivity. If the points close to each other, and will be grouped into one set. Second, calculate the dimension of each set. If the size of sets is small and can be regarded as the dents (shown on the bottow of figure) can be removed. If the sets look like a line (line fitting) and can be regarded as the horizontal sets and be removed. I am not sure if it can work or not. Many thanks.

[darova]

• Maybe, first, the all points can be clustered into lots of sets

I like that idea. Maybe divide data using some y value: 1 group: data > 2.6 (for example) then try to select -0.3 < data < 0

[Yunfeng Ge]

It made sense. Thanks. But, I want to think out a commen solution. Actually, the background for this problem is how to extract cracks from building surface via 3D point cloud. Now, it can be completed to find out all edges (horizontal and verticle) from point cloud. The horizontal ones are the stairs boundaries, and vertial ones are the cracks. In some case, boundaries and cracks maybe on the same plane (for example road surface).

[Yunfeng Ge]

Also, for the lower region, it si difficult to select range to removed the undesired point. Thanks.

Answer 1

[darova]

Here is an idea:

load data.txt
x = data(:,1);
y = data(:,2);
z = data(:,3);

plot3(x,y,z,'.r')
axis equal
datacursormode on

Using daracursormode create two points (data tips) and export coordinates to workspace

And try this script (find distance from each points to line (two points))

p = cat(1,cursor_info.Position);% extract data

dist = 0.03;                    % minimum distance to line
v1 = p(1,:)-p(2,:);             % vector along line
ind = x*0;
j = 1;
hold on
for i = 1:length(x)
    v2 = p(1,:) - [x(i) y(i) z(i)];
    v3 = cross(v1,v2);
    v4 = cross(v1,v3);
    v4 = v4/norm(v4);
    D = abs(dot(v2,v4));       % distance to line
    if D < dist                % if distance is small
        ind(j) = i;
        j = j + 1;
    end
end
ind = ind(1:j-1);
plot3(x(ind),y(ind),z(ind),'ob')
hold off

[Yunfeng Ge]

Thanks for your replys.

[darova]

Please accept my answer

[Yunfeng Ge]

Have accepted, thanks!

Answer 2

[Image Analyst]

Try this:

% Initialization / clean-up code.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;

data = importdata('data.txt');
x = data(:, 1);
y = data(:, 2);
z = data(:, 3);
subplot(2, 2, 1);
plot3(x, y, z, '.');
grid on;
xlabel('x', 'FontSize', fontSize);
ylabel('y', 'FontSize', fontSize);
zlabel('z', 'FontSize', fontSize);
% Get the 3 sets based on the x values.  These are indexes.
set1 = x <= 2;
set2 = x > 2 & x <= 2.5;
set3 = x > 2.5;

% Get the (x, y, z) coordinates for set #1;
x1 = x(set1);
y1 = y(set1);
z1 = z(set1);
% Get the (x, y, z) coordinates for set #1;
x2 = x(set2);
y2 = y(set2);
z2 = z(set2);
% Get the (x, y, z) coordinates for set #1;
x3 = x(set3);
y3 = y(set3);
z3 = z(set3);
% Plot the 3 sets in different colors.
subplot(2, 2, 2);
plot3(x1, y1, z1, 'r.');
hold on;
plot3(x2, y2, z2, 'g.');
plot3(x3, y3, z3, 'b.');
grid on;
xlabel('x', 'FontSize', fontSize);
ylabel('y', 'FontSize', fontSize);
zlabel('z', 'FontSize', fontSize);
% Get the z thresholds for each set.
zMin1 = min(z1)
zMax1 = max(z1)
zMin2 = min(z2)
zMax2 = max(z2)
zMin3 = min(z3)
zMax3 = max(z3)

% Trim set #1.
margin = 0.05 * abs(zMax1 - zMin1);
goodZ = z1 < (zMax1 - margin) & z1 > (zMin1 + margin);
x1 = x1(goodZ);
y1 = y1(goodZ);
z1 = z1(goodZ);
goodY = y1 > -0.3 & y1 < -0.2;
x1 = x1(goodY);
y1 = y1(goodY);
z1 = z1(goodY);

% Trim set #2.
margin = 0.05 * abs(zMax2 - zMin2);
goodZ = z2 < (zMax2 - margin) & z2 > (zMin2 + margin);
x2 = x2(goodZ);
y2 = y2(goodZ);
z2 = z2(goodZ);
margin = 0.05 * abs(zMax3 - zMin3);

% Trim set #3.
goodZ = z3 < (zMax3 - margin) & z3 > (zMin3 + margin);
x3 = x3(goodZ);
y3 = y3(goodZ);
z3 = z3(goodZ);
% Plot the 3 sets in different colors.
subplot(2, 2, [3,4]);
plot3(x1, y1, z1, 'r.');
hold on;
plot3(x2, y2, z2, 'g.');
plot3(x3, y3, z3, 'b.');
grid on;
xlabel('x', 'FontSize', fontSize);
ylabel('y', 'FontSize', fontSize);
zlabel('z', 'FontSize', fontSize);

Adjust parameters as needed.

[Yunfeng Ge]

Thanks for your answer.