View Full Version : Rotation issue in OpenGL Window

04-08-2001, 10:49 PM
Dear Advance OpenGL Users:

I post a question in the beginner section,
but can not get answers. Either the question
is too simple or too difficult (should not be).
I am hoping to get some feelback from any of you.

My question is regarding the rotation command
in OpenGL. Surely, the command is "glRotate".
Generally, in the plot routine, I have something
like the following:
glRotated(m_xRotate, 1.0, 0.0, 0.0);
glRotated(m_yRotate, 0.0, 1.0, 0.0);
glRotated(m_zRotate, 0.0, 0.0, 1.0);


My first question is: does the sequence (order) of using
glRotate makes different orientation of the Objects drawn
on the OpenGL window? (ie, if I put the
glRotated(m_xRotate,1.0,0.0,0.0) after glRoated(m_zRoate,0.0,0.0,1.0),
will it make the final objects (if they are just simple
3D axis X-Y-Z) display differently? It should not be
different, right? But, let say, if
m_xRoate = m_yRoate= m_zRoate = 80; and if ONLY m_xRoate
changes value by adding 1 when F1 key is press (the m_yRoate
and m_zRoate are still 80), and the plot routine called
when F1 is press, somehow the order of glRotate commands
make the different visual effect -- ie, if only m_xRotate
changes, the original sequence shows a rotational 3D axis,
but the X axis is moving too! (note, only the m_xRotate changing
the value, the m_yRotate and m_zRotate keep the constant values,
like 80), if I change the glRotate sequence, say put the
glRotated(m_xRoate,1.0,0.0,0.0) after glRotated(m_zRoate,0.0,0.0,1.0),
then, the axis is rotating, but the X axis stand still!
Will anyone explain this situation? (ie, does the sequence
of glRotate make different visual effect?)

My second question: if I want my objects be rotated with
particular direction (say, press F1, rotate only with X axis,
F2 for Y axis, and F3 for Z axis), how can I do it? I certainly
can make a variable to track which Function key (F1, F2 or F3)
is press, and according to this variable to put the glRotated
command (say, put the glRotated(m_xRoate,1.0,0.0,0.0) to the
last when F1 key is press), However, when I change the rotational
direction (say, press F2 after a sequence of F1 operations),
the orientation of the objects change dramatically when F2 is
just press. Why this happens and How to overcome?


04-09-2001, 12:21 AM
prolly no answers cause it + its kin gets asked to often.
using eular angles like you are will cause "gimbal lock" type this in with google + u should find a heap of info + solutions

04-09-2001, 03:24 AM
you should note that when you call glRotate, the local coordinate system gets rotated, i.ie originally the x-axis points right, y-axis up and z-axis out of screen. if you call glRotate around the x-axis, the y and z axes are now pointing in different directions respectively. perhaps pushing and popping the matrix before your rotations will get you the desired results you seek. also, look into zed's solution.


04-09-2001, 08:22 AM
Originally posted by zed:
prolly no answers cause it + its kin gets asked to often.
using eular angles like you are will cause "gimbal lock" type this in with google + u should find a heap of info + solutions

I hardly catch what you try to say. Can
you please write a little bit more with plain English? Thanks.

04-09-2001, 08:42 AM
Answer to Question 1 is No.
What you did in your code is multiplying a rotation matrix at each glRotated call.
Since A*B != B*A, you result will be different if the order is different.