title: How to get displacement field or control point vector when we have tfrom?
body: <p>I am using image registration toolbox to register two 3D images. I register the moving image to the fixed image. I use "imregtform" to save tform. </p>
  
  <pre><code>tform = imregtform(moving,fixed,transformType,optimizer,metric)
  </code></pre>
  
  <p>for example this is my 'tform':</p>
  
  <pre><code>    1.0235    0.0022   -0.0607         0
     -0.0276    1.0002    0.0089         0
     -0.0170   -0.0141    1.1685         0
     12.8777    5.0311  -70.0325    1.0000
  </code></pre>
  
  <p>then I use 'Imwarp' to transfer moving image to the fixed image. In my code it is,</p>
  
  <pre><code>nii=load_untouch_nii(['mypath image.nii' ]);
  I = nii.img; 
  dii=nii.hdr.dime.pixdim(2:4);
  Rfixed=imref3d(size(I),dii(2),dii(1),dii(3)); 
  timg= imwarp(I, Rfixed, tform, 'OutputView', Rfixed);
  </code></pre>
  
  <p>'timg' is transfered image of source image.I checked it and it is working fine, but I need control points or displacement field of this transformation now. In another word, I need to know each voxel(3d pixel) moved to which position. If I know this I can draw vector field. In deformable image registartion methods such as <a href="http://sourceforge.net/projects/niftyreg/" rel="nofollow">NiftyReg package</a>, control point command is provided to make it easy for users but I do not know how to do it in simple 3d affine in Matlab. 
  Any help will be appreciated too much </p>