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Rotating in Perspective Projection

Posted on 2004-04-29
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Last Modified: 2013-12-06
Hi everyone,

I currently have an OpenGL program which draws objects in the center of my display area.  I rotate my objects by setting up x and y rotating variables on the arrows key press command such as:
                                       case WM_KEYDOWN:
                  {
                  if(wParam == VK_UP)
                        xRot-= 5.0f;
      
                  if(wParam == VK_DOWN)
                        xRot += 5.0f;

                  if(wParam == VK_LEFT)
                        yRot -= 5.0f;

                  if(wParam == VK_RIGHT)
                        yRot += 5.0f;

                  if(xRot > 356.0f)
                        xRot = 0.0f;

                  if(xRot < -1.0f)
                        xRot = 355.0f;

                  if(yRot > 356.0f)
                        yRot = 0.0f;

                  if(yRot < -1.0f)
                        yRot = 355.0f;

and then in my RenderScene function I simply call:
glRotatef(xRot, 1.0f, 0.0f, 0.0f);
glRotatef(yRot, 0.0f, 1.0f, 0.0f);

The object rotates in place in orthographic rotation mode and this is what I want. But when I switch to perspective projection mode, my object rotates in and out of view in both the x and y directions. Suggestions to fix this?
Thanks.
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Question by:fsalcedo
4 Comments
 
LVL 2

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SWortham earned 250 total points
ID: 10969872
It sounds like you're also using glTranslatef() after you make the glRotatef() calls.  If this is the case, then try moving your call to glTranslatef() so that it's before the rotation calls.  

So it should be something like this:
glLoadIdentity();
glTranslatef(0.0f, 0.0f, -5.0f);
glRotatef(xRot, 1.0f, 0.0f, 0.0f);
glRotatef(yRot, 0.0f, 1.0f, 0.0f);

The order in which you translate and rotate is important in perspective mode.  Think of it this way.  If you call glTranslatef(0,0,-5) first to position the object in front of your viewport, and then rotate the object it will rotate around the point or axis of 0, 0, -5.  But if you rotate the object first, it will rotate around 0,0,0 and then when you call glTranslatef(0,0,-5) afterwards it will basically move the object in the opposite direction in which it was rotated (causing it to move all over the place in & out of your viewport).
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LVL 2

Assisted Solution

by:enkimute
enkimute earned 250 total points
ID: 10970678
Here in belgium, they teach childeren to read matrix multiplication not as 'multiply by' but as 'multiply after'. I still use the same trick for
opengl ..

You should make it a reflex .. whenever you see glRotate, glTranslate or glScale, just immeadiately think 'after' .. so the piece
of code

gltranslate(...)
glrotate(..,1,0,0)
glrotate(..,0,1,0)

should be read as 'a translate AFTER a rotate around x AFTER a rotate around y) ..

this way its a lot less confusing .. first you rotate the object then you translate it.

also, it would be somewhat more common to order your rotate y,x,z or heading, pitching, banking. This is
important because of the same reason .. doing these multiplications in a different order yields a different result.
So, in other words .. if one writes a rotation vector as (30,20,10) .. he means :

  glRotatef(30,0,1,0);
  glRotatef(20,1,0,0);
  glRotatef(10,0,0,1);

and not

  glRotatef(30,1,0,0);
  glRotatef(20,0,1,0);
  glRotatef(10,0,0,1);

both will give different results !




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