how to open a depth map from a file and transform it into a 3D point cloud - wanna convert c++ code to MATLAB script.

The following c++ code snippet show how to open a depth map from a file and transform it into a 3D point cloud:

FILE* inFile;
  //read the file
  fopen_s(&inFile,"depth.txt", "rb");
  fread(d, sizeof(UINT16), nVertices, inFile);
  fclose(inFile);

  for(int i=0 ; i < nVertices ; i++)
  {
    int r_i = i / (int)n_c;
    int c_i = i % (int)n_c;

    //normalize depth
    float d_i = (float)d[i] / (float)MAX_DEPTH;
    //color
    vertices[i].a = 255.;
    //the color of the point is a shade of gray proportional to the depth
    vertices[i].b = vertices[i].g = vertices[i].r = d_i;

    //calculate x-coordinate
    float alpha_h = (M_PI - theta_h) / 2;
    float gamma_i_h = alpha_h + (float)c_i*(theta_h / n_c);
    vertices[i].x = d_i / tan(gamma_i_h);

    //calculate y-coordinate
    float alpha_v = 2 * M_PI - (theta_v / 2);
    float gamma_i_v = alpha_v + (float)r_i*(theta_v / n_r);
    vertices[i].y = d_i * tan(gamma_i_v)*-1;

    //z-coordinate
    vertices[i].z = d_i;

  }

So i wanna convert this to a MATLAB script.

Answer 1

[Vidhi Agarwal]

Hi Attaullah,

I understand that you have a depth map and a C++ code snippet that generates a 3D point cloud from it. You wish to convert this C++ code into a MATLAB script.

You can create the MATLAB script by following the below mentioned steps

1. Start by defining constants.
2. Read Depth data
3. Initialize vertices Array.
4. Process Each vertex.
5. And Convert struct to Matrix and plot it.

Following is the code that can help you in moving forward with it.

% Define constants
MAX_DEPTH = 65535; % Example maximum depth value, adjust according to your data
theta_h = pi / 3;  % Horizontal field of view in radians
theta_v = pi / 4;  % Vertical field of view in radians
nVertices = 10000; % Number of vertices, adjust according to your data
n_c = 100;         % Number of columns, adjust according to your data
n_r = nVertices / n_c; % Number of rows

% Read the depth data from the file
fileID = fopen('depth.txt', 'rb');
d = fread(fileID, nVertices, 'uint16');
fclose(fileID);

% Initialize the vertices array
vertices = struct('x', zeros(nVertices, 1), ...
                  'y', zeros(nVertices, 1), ...
                  'z', zeros(nVertices, 1), ...
                  'r', zeros(nVertices, 1), ...
                  'g', zeros(nVertices, 1), ...
                  'b', zeros(nVertices, 1), ...
                  'a', 255 * ones(nVertices, 1));

% Process each vertex
for i = 1:nVertices
    r_i = floor((i-1) / n_c);
    c_i = mod((i-1), n_c);
    
    % Normalize depth
    d_i = double(d(i)) / double(MAX_DEPTH);
    
    % Set color
    vertices(i).r = d_i;
    vertices(i).g = d_i;
    vertices(i).b = d_i;
    
    % Calculate x-coordinate
    alpha_h = (pi - theta_h) / 2;
    gamma_i_h = alpha_h + c_i * (theta_h / n_c);
    vertices(i).x = d_i / tan(gamma_i_h);
    
    % Calculate y-coordinate
    alpha_v = 2 * pi - (theta_v / 2);
    gamma_i_v = alpha_v + r_i * (theta_v / n_r);
    vertices(i).y = d_i * tan(gamma_i_v) * -1;
    
    % z-coordinate
    vertices(i).z = d_i;
end

% Optionally, convert the struct array to a matrix for easier plotting or processing
points = [[vertices.x]', [vertices.y]', [vertices.z]'];
colors = [[vertices.r]', [vertices.g]', [vertices.b]'];

% Plot the point cloud
scatter3(points(:,1), points(:,2), points(:,3), 1, colors, 'filled');
xlabel('X');
ylabel('Y');
zlabel('Z');
title('3D Point Cloud');

For detailed understanding of “fopen”, “fread”, “fclose” and “scatter3” please refer to the following documentation:

• https://www.mathworks.com/help/matlab/low-level-file-i-o.html#:~:text=fopen,from%20binary%20file
• https://www.mathworks.com/help/matlab/ref/scatter3.html

Hope that Helps!.