Solved

Rotating in Perspective Projection

Posted on 2004-04-29
4
1,308 Views
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.
0
Comment
Question by:fsalcedo
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
4 Comments
 
LVL 2

Accepted Solution

by:
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).
0
 
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 !




0

Featured Post

Announcing the Most Valuable Experts of 2016

MVEs are more concerned with the satisfaction of those they help than with the considerable points they can earn. They are the types of people you feel privileged to call colleagues. Join us in honoring this amazing group of Experts.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

What is RenderMan: RenderMan is a not any particular piece of software. RenderMan is an industry standard, defining set of rules that any rendering software should use, to be RenderMan-compliant. Pixar's RenderMan is a flagship implementation of …
Artificial Intelligence comes in many forms, and for game developers, Path-Finding is an important ability for making an NPC (Non-Playable Character) maneuver through terrain.  A* is a particularly easy way to approach it.  I’ll start with the algor…
Come and listen to Percona CEO Peter Zaitsev discuss what’s new in Percona open source software, including Percona Server for MySQL (https://www.percona.com/software/mysql-database/percona-server) and MongoDB (https://www.percona.com/software/mongo-…
NetCrunch network monitor is a highly extensive platform for network monitoring and alert generation. In this video you'll see a live demo of NetCrunch with most notable features explained in a walk-through manner. You'll also get to know the philos…

691 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question