MATLAB: How can exchange “ginput” image data units to world units(mm)

Question

Hi everyone;

I want to measure the distance in mm so i use tform and invtform matrix to exchange the image and world data units. However, i have this error after running my code. Here is the code:

PS: First i calibrate my camera and then i run these codes…

a=imread('calib8.jpg');
>> im = undistortImage(a, cameraParameters);
>> figure; imshow(im);
>> [x,y] = ginput(2)
>> [imagePoints, boardSize] = detectCheckerboardPoints(im); 
>> Ainv=inv(cameraParameters.IntrinsicMatrix); 
>> squareSize=9;
>> worldPoints = generateCheckerboardPoints(boardSize, squareSize);
>> H=fitgeotrans(worldPoints, imagePoints, 'projective');
>> H=H.T;
>> h1=H(1,:);
>> h2=H(2,:);
>> h3=H(3,:);
>> lambda=1/norm(h1*Ainv);
>> r1=lambda*h1*Ainv;
>> r2=lambda*h2*Ainv;
>> r3=cross(r1,r2);
>> R=[r1; r2; r3];
>> [U,~,V]=svd(R);
>> R=U*V';
>> t= lambda*h3*Ainv
t =
  -18.7109   11.7857  150.6906
>> T = [R(1, :); R(2, :); t] * cameraParameters.IntrinsicMatrix;
>> tform = projective2d(T);
>> imagePoints1=[x;y]
imagePoints1 =
  106.0000
  104.0000
  132.0000
   55.0000
>> worldPoints1 = transformPointsInverse(tform, imagePoints1);
Error using projective2d/transformPointsInverse (line 189)
transformPointsInverse requires that X is an Nx2 matrix.

Answer 1

Instead of a semicolon in

imagePoints1=[x;y]

use a comma:

imagePoints1=[x, y]

This will create an two-column matrix instead of a one-column vector.