Making movie: position of camera & multiple scenes

Question

This is an example code that represents what I need to do:

ang=0:0.01:2*pi; 
r = 1;
xp=r*cos(ang);
yp=r*sin(ang);

frequency  = 1/0.016;         % real frequency
datalength = length(ang);     % n° of points
scale      = 10;              % resampling
nFrames    = floor(datalength/scale);

% figure
figure()
set(gcf, 'Color','white')
hold on 
axis equal

set(gca, 'nextplot','replacechildren', 'Visible','off'); % ?

% preallocate
mov(1:nFrames) = struct('cdata',[], 'colormap',[]);

% create movie
for k = 1:nFrames
    axes(gca); cla;
    hold on 

    plot3(xp,yp,zeros(size(xp)));
    plot3(1,0,0,'kx');

    % Update Target
    for j = 1:scale
        id = (k-1)*scale + j;
        CurTar = ang(id);
        PsiTar = CurTar + pi*0.5;
    end    

    % Plot obj
    plot3(2*xp(id),2*yp(id),0,'k.','LineWidth',4)

    % Plot Target
    plot3(xp(id),yp(id),0,'r.','LineWidth',3)

    a = -PsiTar - 0.75*pi;
    % Set Camera
    xc = xp(id) - sin(a);
    yc = yp(id) - cos(a);
    zc = 0.5;

    campos([xc,yc,zc])
    camtarget([xp(id),yp(id),0])

    mov(k) = getframe(gcf);
end

close(gcf)

movie2avi(mov, 'mymovie', 'fps', round(frequency/scale));

I have the following problems/questions with it:

• I would like to plot the circle once, and not every loop. I tried to place it before the for loop, but at that point is not maintained at each iteration, unless I comment out the axes(gca); cla; line, but this has a significant other drawback: everything is maintained, not only the circle.

• I would like to have the same scene seen from multiple angles. I know how to do subplots and I would know how to plot the same thing multiple times and simply set the camera differently, is there a way of plotting once and having multiple projections?

• I have a problem with the position of the camera. You see that I have that sin and cos in the definition of xc ynd yc. In this example everything is fine, but the same code in my full script leads to the error :

  ??? Error using ==> set
  Value must be a 3 element numeric vector
  
  Error in ==> campos at 50
  set(ax,'cameraposition',val);
  

  how may I get rid of it? I have printed to screen all values and they are numerical, I have no idea what might be happening.

EDIT:

additional question

If I plot the following

for k = 1:nFrames
    axes(gca); cla;
    hold on 

    plot3(zeros(length(ang)),ang,zeros(length(ang)),'b')

    % Update Target
    for j = 1:scale
        id = (k-1)*scale + j;
    end    

    % Plot obj
    plot3(0,2*yp(id),0,'k.','LineWidth',4)

    if k==1
        view(0,0)
        camzoom(50)
    end
    camtarget([0,yp(id),0])

end

close(gcf)

you can see that the camera is not aligned with the y axis, otherwise you would not see the blue line. Why is that? how is it possible to avoid it?