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OpenGL Premultiplying and Fixed Rotation

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I'm currently learning OpenGL and i'm trying to rotate an object in a fixed coordinate system. I have been reading that I have to pre-multiply and that OpenGL post-multiplies. But I'm just having a little difficulty visualizing the concept. My code looks something like... glTranslatef( 0, 0, -6 ); glRotatef( rotateX, 1, 0, 0 ); glRotatef( rotateY, 0, 1, 0 ); DrawObject(); Basically I don't want the Y rotation to rotate on the new axis created by the X rotation(and vice versa). Can anyone help me out? Some psuedo-code would be appreciated. Thanks in advance. :)

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Guest Anonymous Poster
Rotation axis are not independant in 3d space, one will always affect the other, and the order they're applied matters.

Not sure what you're asking for...

*might* be that he's asking how to have a freely rotating object, who's rotation can be influenced from worldspace x and y torque forces...

In order to do That
You need to track the object's orientation yourself (usually via a matrix).
and for each succesive rotation you wish to apply, you need to 'hit' it's orientation matrix with a rotation transform then save the result...

Math will be required, opengl is for rendering the object afterwards, but it cannot make Actual cumulative changes to it's rotation in space...

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Here is some sample code from my project.

void OnDisplay()
glMatrixMode( GL_MODELVIEW );



glTranslatef (0, 0, -6);
glRotatef (0, 0, 0, 1);
glRotatef ( g_rotateX, 1, 0, 0 );
glRotatef ( g_rotateY, 0, 1, 0 );



void CheckInput()
if( g_keyboard['w'] ) // up
g_rotateX += 0.1;
if( g_keyboard['a'] ) // left
g_rotateY -= 0.1;
if( g_keyboard['s'] ) // down
g_rotateX -= 0.1;
if( g_keyboard['d'] ) // right
g_rotateY += 0.1;

if( g_keyboard['r'] )
g_rotateX = 0.0f;
g_rotateY = 0.0f;

if( g_keyboard[KEY_ESC] ) // Esc Key
if( g_FullScreen )

if( g_rightMouse )
g_rotateY += g_mousePos.x - g_mouseStartPos.x ;
g_rotateX += g_mousePos.y - g_mouseStartPos.y;
g_mouseStartPos.x = g_mousePos.x;
g_mouseStartPos.y = g_mousePos.y;
if( abs(g_rotateY) >= 360.0f )
g_rotateY = 0.0f;
if( abs(g_rotateX) >= 360.0f )
g_rotateX = 0.0f;

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Guest Anonymous Poster
Original post by w166um
Here is some sample code from my project.

*** Source Snippet Removed ***

....yeah? so?....

your original question is still unclear
and my previous advice still stands

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