glRotatef(), Setting the center of rotation...

Hi everybody,

I'd like to rotate this triangle :

glPushMatrix();
glRotatef(10.0,0,0.0, 1.0);

glBegin(GL_TRIANGLES);
      glVertex3f( x1 ,0.2,0.0);
      glVertex3f( x2 ,0.25,0.0);
      glVertex3f( x3 ,0.15,0.0);      
glEnd();

glPopMatrix();

but the rotation is made around the origin (0,0,0), how can I rotate around this point : [x1 ,0.2,0.0] ?

Thank you!

Vincent
vinceTraceAsked:
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Jose ParrotConnect With a Mentor Graphics ExpertCommented:
Hi,

OpenGL uses matrix approach to transformations: rotation, translation and scaling.

The matrix for rotation is:
            |  cos(A)  0   sin(A)   0 |
            |  0         1   0          0 |
            | -sin(A)  0   cos(A)   0 |
            |  0         0   0          1 |
for a given angle A. This rotate the point around the origin. Actually when you call glRotatef(10.0,0,0.0, 1.0) that matrix is solved.

An important feature of matrix calculations for transforms is that you can do BOTH at same time:

[1,0,0,0]              [cos(a),0,-sin(a),0]    [1,0,0,0]
[0,1,0,0]              [0,1,0,0]                   [0,1, 0,0]
[0,0,1,0]              [sin(a),0,cos(a),0]     [0,0,1,0]
[-X1,-0.2,-Z,1]      [0,0,0,1]                  [X1,0.2,Z,1]

The above matrix executes, in one single pass, the three calls in the sequence translate->rotate->translate. As each call solves a matrix, the single pass has the advantage of speed the execution by a factor of 3.

Jose
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putaNerdConnect With a Mentor Commented:
Vincent,
Off the top of my head I suggest you try two things, both should work but they are good to know.

this might distort things a little in some cases
glPushMatrix();
glRotatef(10.0,x1 ,0.2,0.0, 1.0);

/**********************/
//this way moves the rotation point back to the origin. then rotates.
glTranslate(-x1,-0.2f,0.0f);
glPushMatrix();
glRotatef(10.0,x1 ,0.2,0.0, 1.0);

//at the end you should translate back to put your triangle back at the point it was. before popmatrix
glTranslate(x1,0.2f,0.0f);

hope this helps
j.
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Jose ParrotGraphics ExpertCommented:
Complementing:
You may want take a look at Dustin Juliano's excellent comment at

http://www.experts-exchange.com/Programming/Game_Development/3D_Programming/Q_20627768.html?query=glrotate+matrix&clearTAFilter=true

The approach on quaternions is very interesting if you look for speed.

Jose
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