MATLAB: Does the rows and columns appear flipped while using ‘regionprops3’ function

columnsimagecoordinateflippedimageImage Processing Toolboxprocessingregionprops3reveresedrowssystemtoolbox

The 'regionprops3' function measures the properties of a 3-D volumetric image regions; however, it produces the rows and columns values which appear to be flipped. For instance, in the following lines of code the two output values of props.Centroid can be compared:
>>%% make 3d image
test_image = rand(3,15,9);
bin_image = test_image > 0.5;
idx = find(bin_image);
[row,col,dep] = ind2sub(size(test_image),idx);
props = regionprops3(bin_image);
%% Check the outputs from the following to see that things flipped
[max(row),max(col),max(dep)] >= props.Centroid
props.Centroid = props.Centroid(:,[2,1,3]);
[max(row),max(col),max(dep)] >= props.Centroid
 

Best Answer

  • The 'regionprops3' function uses the image coordinate system where the top-left section of the image is considered the origin(0,0) and hence the (x,y) coordinate appears reversed.
     
    For more information on image coordinate system, please refer to the following documentation:
    https://www.mathworks.com/help/releases/R2019a/images/image-coordinate-systems.html
    In addition, please refer to the following MATLAB answers post by Walter Roberson:
    https://www.mathworks.com/matlabcentral/answers/284040-reverse-input-image-coordinates#answer_221991