Matlab: Set shading of single surface - matlab

Question

I'm making a function which generalizes the cylinder function so that the cylinder has caps, can be any size and orientation. However on the look of the cylinder I am running into a jam. To get the caps to look right the curved part needs one set of shading and the caps need another. (And before you ask making 3 surfaces is not an option)
Here is the relevant code:
surface(xSurf,ySurf,zSurf,c,'EdgeColor','none','FaceLighting','phong');
and in case you want to see the whole code.
Thank you for your help,
John
function varargout = DrawCylinder(x,y,z,r,h,aVec,bVec,cVec,ccolor, npts)
% DrawCylinder Generate a three-dimensional cylinder
%
% DrawCylinder(x,y,z,a,b,c,aVec,bVec,CVec,ccolor, npts)
% creates a surface plot of a cylinder whose center is at (x,y,z), has
% semiaxes of length a, b, and c. The unit vectors associated with each
% semixis are aVec, bVec, and cVec and must be size 3 x 1 (column vector)
% with size of npts + 1.
%
% H = DrawCylinder(...) creates the surface plot and returns the handle H to each
% graphical object created.
%
% [X Y Z] = DrawCylinder(...) does not generate the surface plot and returns
% the data necessary to create the surface using:
% SURF(X,Y,Z);
%
% [X Y Z C] = DrawCylinder(...) does not generate the surface plot and returns
% the data necessary to create the surface using:
% SURF(X,Y,Z,C,'cdataMapping','direct');
%CREATE SURFACE FOR CYLINDER
[xCyl,yCyl,zCyl]=cylinder(1,npts);
xSurf=[zeros(1,max(size(xCyl)));xCyl;zeros(1,max(size(xCyl)))];
ySurf=[zeros(1,max(size(yCyl)));yCyl;zeros(1,max(size(yCyl)))];
zSurf=[zeros(1,max(size(zCyl)));zCyl;ones(1,max(size(zCyl)))] - 0.5;
xSurf = xSurf*r;
ySurf = ySurf*r;
zSurf = zSurf*h;
%ROTATE CYLINDER
%Make sure aVec,bVec, and cVec are column unit vectors:
if all(size(aVec)==[1,3])
aVec=aVec';
end
if all(size(bVec)==[1,3])
bVec=bVec';
end
if all(size(cVec)==[1,3])
cVec=cVec';
end
aVec=aVec/norm(aVec); %Make unit vectors
bVec=bVec/norm(bVec);
cVec=cVec/norm(cVec);
rot = [aVec,bVec,cVec]; %The rotation matrix
[iMax, jMax] = size(xSurf);
for i=1:iMax
for j=1:jMax
rotatedPt = rot*[xSurf(i,j);ySurf(i,j);zSurf(i,j)];
xSurf(i,j) = rotatedPt(1);
ySurf(i,j) = rotatedPt(2);
zSurf(i,j) = rotatedPt(3);
end
end
%TRANSLATE CYLINDER
xSurf = xSurf + x;
ySurf = ySurf + y;
zSurf = zSurf + z;
c = ccolor*ones(size(xSurf));
if nargout == 0
surface(xSurf,ySurf,zSurf,c,'EdgeColor','none','FaceLighting','phong');
elseif nargout == 1
varargout = {surface(xSurf,ySurf,zSurf,c,'EdgeColor','none','FaceLighting','phong');};
elseif nargout == 3
varargout = {xSurf ySurf zSurf};
elseif nargout == 4
varargout = {xSurf ySurf zSurf c};
end
end
Edit 8/18/12:
Just so you can see.
This is what I am getting ...
And this is what I want ...

Answer 1

I'm pretty sure that by "shade" you simply mean the colors of the end caps and not a more complicated effect. If so that's pretty straightforward, I'll give an example in gray scale
Change
c = ccolor*ones(size(xSurf));
to
c = (ccolor/255)*ones(size(xSurf));
c([1 3],:)=max(0,(ccolor-10))/255;
the first line initializes the c matrix with a normalized ccolor (expecting an 8 bit greyscale ccolor input, it normalizes to 0..1). The second line changes the caps (rows 1 and 3) to be a slightly darker color bottoming out at 0 and leaves the cylinder surfaces (rows 2 and 4) alone.
In order to make sure you see the results correctly, you need to alter the nargout==0 condition so that it looks like this
surface(xSurf,ySurf,zSurf,c,'EdgeColor','none','FaceLighting','phong');
colormap(gray(256));
caxis([0 1]);
The colormap just sets the colormap, similar to an 8 bit gray scale. The caxis command is fairly critical. According to Matlab's surface documentation
MATLAB performs a linear transformation on this data to obtain colors from the current colormap
For our purposes that is bad. Since we only have two values the lowest would be changed to 0 and the highest to 1. That effectively ignores our ccolor input and gives a white cylinder with two black caps. Using caxis([0 1]) preserves the full scale and ccolor's position within it.
Update:
Sounds like I misunderstood what you wanted, the easiest way to achieve something very close to the effect you want is to set 'MeshStyle' to 'row', like this:
surface(xSurf,ySurf,zSurf,c,'EdgeColor','k','FaceLighting','phong','MeshStyle','row');
This will give you the following result :
There's still a central point, but it's by far the simplest way to produce that effect.