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.