transform a patch using a transform object

Hi,

I created a square face at the origin using the "patch" function. Then I defined the square as a child of a transform object. I started to move the transform object to several locations and changed its orientation. The square object followed well the transform object. The problem is that when I asked to get the current vertices of the square (after it was moved), I got the original vertices around the origin. I need the current vertices in order to calculate the current normal-vector of that square-face.

Thanks

Answer 1

[Walter Roberson]

Let p be the patch handle. Let hgtrans be the hgtransform object. Then,

M = get(hgtrans, 'Matrix');

xd = get(p, 'XData');
xd = xd(:);
yd = get(p, 'YData');
try
  zd = get(p, 'ZData');
  if isempty(zd); zd = zeros(size(xd)); end
catch
  zd = zeros(size(xd));
end

XYZ1 = [xd, yd(:), zd(:), ones(size(xd))];

XYZ1_trans = XYZ1 * M';
x_trans = XYZ1_trans(:,1);
y_trans = XYZ1_trans(:,2);
z_trans = XYZ1_trans(:,3);

p does not need to be a patch, but it does need to have XData and YData; if it does not have ZData then the code will compensate. For example, image objects do not have ZData but can validly be translated to a non-zero Z coordinate. Some convenient graphics objects such as rectangle() objects do not have XData or YData; the code here does not account for those.

[Ehud]

Hi Walter Thanks for the quick reply. The problem is that the transform object is part of a long hierarchy chain of other transform objects. So its matrix is relative to its parent and not to the general axes system. Is there no way to retrieve directly the modifyed vertices of the patch? Thanks

[Walter Roberson]

No, there is no direct way. You need to multiply out all of the transform matrices.

For clarity: when you get the XData, YData, ZData of a graphics object, those are always the values that you passed to the graphics routines, not the transformed values.

[Ehud]

Hi Walter

thanks!