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TallGuy123
07-31-2007, 02:45 PM
I have a problem with trying to render a glucylinder between two 3d points.

Based on the code from here
http://home.neo.rr.com/jparris/OpenGL%20-%20draw%20your%20own%20cylinder%20between%202%20pt s.htm

This seems to work for all cases except when the cylinder is aligned to the Y axis. ie from (0,-5,0) to (0,+5,0). In that case the Y axis aligned cylinder either flickers (like its rotation is being flipped around each frame) or it renders from the first point out in space the opposite direction to the second point.

Can anyone help with the right "if then" code to fix the Y axis problem? I have done a lot of searching on these forums and the web trying to find working code. It will definitely help me and I am sure others who come here looking for the solution in the future.

Thanks,
Jason.

procedure CylinderGL(p1,p2:point3d;r1,r2:double;precision:in teger);
height,dx,dy,dz:double;
v,vx,vy,vz,rx,ry,rz,zero,ax,r2d:double;
begin
r2d:=180/pi;
//length of cylinder
dx:=p1.x-p2.x;
dy:=p1.y-p2.y;
dz:=p1.z-p2.z;
height:=sqrt(dx*dx+dy*dy+dz*dz);

glpushmatrix;
gltranslatef(p1.x,p1.y,p1.z);

//now rotate the matrix into position

// orientation vectors
vx:=p2.x-p1.x; // component in x-direction
vy:=p2.y-p1.y; // component in y-direction
vz:=p2.z-p1.z; // component in z-direction

v:=sqrt(vx*vx+vy*vy+vz*vz); // cylinder length

// rotation vector, z x r
rx:=-vy*vz;
ry:=+vx*vz;
ax:=0.0;
if vz=0 then
begin
ax:=r2d*arccos(vx/v); // rotation angle in x-y plane
if vx<=0 then ax:=-ax;
end
else
begin
ax:=r2d*arccos(vz/v); // rotation angle
if vz<=0 then ax:=-ax;
end;

if vz=0 then
begin
glRotated(90.0, 0, 1, 0.0); // Rotate & align with x axis
glRotated(ax, -1.0, 0.0, 0.0); // Rotate to point 2 in x-y plane

end
else
begin
glRotated(ax, rx, ry, 0.0); // Rotate about rotation vector
end;

gluCylinder(quadratic,r1,r2,height,precision,preci sion); // Draw A cylinder
glpopmatrix;
end;

Brolingstanz
07-31-2007, 05:20 PM
Draw a cylinder of radius R between the points A and B:

Let the unit vector B - A be the z-axis of a Frenet frame located at A. With this frame on top of the modelview stack,
call gluCylinder with top = base = R and height = length(B - A), OR render a unit cylinder (base = top = height = 1) but prescale the frame's xy-axes by the radius and use B - A as the z-axis. For extra credit, do all this using a geometry shader instead ;-)

TallGuy123
07-31-2007, 06:31 PM
Same question, can anyone see where in the code it fails for Y axis aligned cylinders?

Problem is calculating the right rotation angles. After the gltranslatef and before the glnewquadric.

This is so close to working. If anyone has a working snippet of code or can understand the math please help.

Once it works I will post the full working procedure for people to find in the future (so all you gurus can concentrate on better problems).

Thanks,
Jason.

TallGuy123
07-31-2007, 07:47 PM
An update on this...

The flickering of the cylinder is fixed. Looks like it was due to me not calling gludeletequadric(quadratic) at the end of the procedure. But still strange that it only effected cylinders aligned with the Y axis.

If I reverse the P1 and P2 points when calling the procedure then the cylinders on the Y axis do correctly join between the two points. So adding a small check at the start of the procedure to swap the points if required gets it all working.

But, if anyone can see the error in the math so I can omit this swap check it would be very helpful.

Thanks,
Jason.

Here is the working code if anyone needs this sort of thing in the future.

procedure CylinderGL(p1,p2:point3d;r1,r2:double;precision:in teger);
height,dx,dy,dz:double;
v,vx,vy,vz,rx,ry,ax,r2d:double;
tmp:point3d;
begin
if (p1.x=p2.x) and (p1.z=p2.z) and (p1.y<p2.y) then
begin
tmp:=p1;
p1:=p2;
p2:=tmp;
end;

r2d:=180/pi;
//length of cylinder
dx:=p1.x-p2.x;
dy:=p1.y-p2.y;
dz:=p1.z-p2.z;
height:=sqrt(dx*dx+dy*dy+dz*dz);

glpushmatrix;
gltranslatef(p1.x,p1.y,p1.z);

// orientation vectors
vx:=p2.x-p1.x; // component in x-direction
vy:=p2.y-p1.y; // component in y-direction
vz:=p2.z-p1.z; // component in z-direction

v:=sqrt(vx*vx+vy*vy+vz*vz); // cylinder length

// rotation vector, z x r
rx:=-vy*vz;
ry:=+vx*vz;
ax:=0.0;
if vz=0 then
begin
ax:=r2d*arccos(vx/v); // rotation angle in x-y plane
if vx<=0 then ax:=-ax;
end
else
begin
ax:=r2d*arccos(vz/v); // rotation angle
if vz<=0 then ax:=-ax;
end;

if vz=0 then
begin
glRotated(90.0, 0, 1, 0.0); // Rotate & align with x axis
glRotated(ax, -1.0, 0.0, 0.0); // Rotate to point 2 in x-y plane
end
else glRotated(ax, rx, ry, 0); // Rotate about rotation vector

//create a pointer to the quadric object
//tell it to smooth normals