matlab rigid body animation using patch and hgtransform - matlab

Question

I am trying to perform human body animation using translation and orientation data I am given. I have a set of rigid body segments made using patch all centered at (0,0,0) to represent the human body and translated accordingly. I have set up a hierarchy for each of them and performed a transformation matrix for each rigid body segment. The limb segments begin to offset one another and give problems. For example, the rigid body of the arm moves as if it does not have a relative point of origin even though it follows the proper motion. The motion is akin to moving the rigid body from the patch center of gravity? Whereas it is supposed to move with one end being fixed while the other end follows translation data. Can someone let me know what it is that I am doing wrong? Layout of my code is:
% Body segment lengths
xlength = somevalue
ylength = somevalue
zlength = somevalue
% Translation data
Xdata
Ydata
Zdata
% Orientation data
Yaw = rotation about z axis
Pitch = rotation about x axis
Roll = rotation about y axis
Vertices = [xlength*ones(8,1),ylength*ones(8,1),zlength*ones(8,1)]...
.*[-0.5,-0.5,-0.5;
0.5,-0.5,-0.5;
-0.5,0.5,-0.5;
-0.5,-0.5,0.5;
0.5,0.5,-0.5;
-0.5,0.5,0.5;
0.5,-0.5,0.5;
0.5,0.5,0.5];
% Create patches
for i = 1:6
% create faces for patches
end
% create axes
ax = axes(...)
% draw patches
bodysegmentPatch = patch(patchxdata,patchydata,patchzdata)
% create hierarchy using hgtransform
pelvis = hgtransform('Parent',ax);
trunk = hgtransform('Parent',pelvis);
head = hgtransform('Parent',trunk);
leftupperarm = hgtransform('Parent',trunk);
leftforearm = hgtransform('Parent',leftupperarm);
rightupperarm = hgtransform('Parent',trunk);
rightforearm = hgtransform('Parent',rightupperarm);
leftthigh = hgtransform('Parent',pelvis);
leftcalf = hgtransform('Parent',leftthigh);
rightthigh = hgtransform('Parent',pelvis);
rightcalf = hgtransform('Parent',rightthigh);
% set patches to hierarchy
set(pelvisPatch,'Parent',pelvis)
% Animation loop
for i = 1:n
% translation of body segment
bodysegmentT = makehgtform('translate',[x(i) y(i) z(i)]);
% rotation of body segment
bodysegmentR = makehgtform('yrotate',Roll(i),'xrotate',Pitch(i),'zrotate',Yaw(i));
% Create transform matrices
set(pelvis,'Matrix',pelvisR);
set(trunk,'Matrix',trunkR*pelvisR);
set(leftupperarm,'Matrix',leftupperarmT*leftupperarmR*trunkR*pelvisR);
drawnow
end

Answer 1

I'm not sure what your problem is exactly, but without looking at your code too closely my guess is it is likely one of two common mistakes while doing these skeleton transformations.
Your matrix transformations are in the wrong order. Remember that A = A*B is not the same thing as A = B*A. When you do this kind of transformation stack this order is very important.
The objects are rotating around the wrong point. Usually they rotate around the origin. So if you want something to rotate around the center of the object you will have to translate the image to the origin, rotate the image, translate the image back to the original location.
Don't give up! These transformations can be tricky, and often times it may look totally chaotic while in reality the code is really close to being correct.