View Full Version : Calculating Positions based upon glRotatef

i have the following code:

{

DoTranslateRotate();

glBegin(GL_POINTS);

glVertex3f(x,y,z);

glEnd();

UndoTranslateRotate();

}

void DoTranslateRotate()

{

//Perform XYZ Translation

glTranslatef(COrigin.x,COrigin.y,COrigin.z);

//Perform XYZ rotations

glRotatef(COrigin.xAngle, 1.0f,0.0f,0.0f);

glRotatef(COrigin.yAngle, 0.0f,1.0f,0.0f);

glRotatef(COrigin.zAngle, 0.0f,0.0f,1.0f);

}

void UnDoTranslateRotate()

{

//Undo XYZ rotations

glRotatef(-COrigin.zAngle, 0.0f,0.0f,1.0f);

glRotatef(-COrigin.yAngle, 0.0f,1.0f,0.0f);

glRotatef(-COrigin.xAngle, 1.0f,0.0f,0.0f);

//Undo XYZ Translation

glTranslatef(-COrigin.x,-COrigin.y,-COrigin.z);

}

i want to know where the point (x,y,z) will be drawn, and i do know the Data in COrigin

is there some way i can use the OGL matrix and just multiply the matrix by my {X Y Z} values to obtain the new X, Y, Z that the point will appear at (relative to the world)

say if i had the point (1,0,0) and i rotated my y - axis 180 degrees, the point is now at (-1,0,0) correct? given the XYZ, and the rotations i want to find the new XYZ location.

M/\dm/\n

01-08-2003, 03:38 AM

When you call glrotatef(alpha,x,y,z) you give angle alpha, & amount to rotate on x,y&z axis. So everything else is pure math (sin,cos)

But you can use modelview matrix, as 1st row will include all x coord trans, y all y trans and so on (or maybe not, I forgot that as I had a lot of work to be done at uni) take a look at matrix multiplication defs.

im using the perspective matrix so that wont work exactly. i need openGLs calculation

unless a better way of figuring it out can be done

what im trying to do is to see if a user has clicked on a square which can be viewed from any angle. so this is affected by the matrix that openGL is using.

DFrey

01-08-2003, 10:12 PM

See gluUnproject and/or gluProject.

[This message has been edited by DFrey (edited 01-08-2003).]

nexusone

01-09-2003, 05:17 AM

You do understand the order in which you call the translate and rotate has a big effect on the location of the object.

Also the the last command before drawing the object is the one effecting the object first.

In the order you have it:

glRotate

glTranslate

Draw_object();

What happens is the object is translated from its origin to some location, then rotate about the openGL's axis of 0,0,0.

So if say you translate the object 5 units out on the X axis and then rotate 20 degrees. The object will be moved along the openGL origin in a 20 degree arc along the axis of rotation with a radius 5 units.

Now if you do the following:

glTranslate

glRotate

Draw_object()

The object is rotated on the openGL 0,0,0 axis and then moved out to the location.

{

DoTranslateRotate();

glBegin(GL_POINTS);

glVertex3f(x,y,z);

glEnd();

UndoTranslateRotate();

}

void DoTranslateRotate()

{

//Perform XYZ Translation

glTranslatef(COrigin.x,COrigin.y,COrigin.z);

//Perform XYZ rotations

glRotatef(COrigin.xAngle, 1.0f,0.0f,0.0f);

glRotatef(COrigin.yAngle, 0.0f,1.0f,0.0f);

glRotatef(COrigin.zAngle, 0.0f,0.0f,1.0f);

}

void UnDoTranslateRotate()

{

//Undo XYZ rotations

glRotatef(-COrigin.zAngle, 0.0f,0.0f,1.0f);

glRotatef(-COrigin.yAngle, 0.0f,1.0f,0.0f);

glRotatef(-COrigin.xAngle, 1.0f,0.0f,0.0f);

//Undo XYZ Translation

glTranslatef(-COrigin.x,-COrigin.y,-COrigin.z);

}

i want to know where the point (x,y,z) will be drawn, and i do know the Data in COrigin

is there some way i can use the OGL matrix and just multiply the matrix by my {X Y Z} values to obtain the new X, Y, Z that the point will appear at (relative to the world)

say if i had the point (1,0,0) and i rotated my y - axis 180 degrees, the point is now at (-1,0,0) correct? given the XYZ, and the rotations i want to find the new XYZ location.[/B]<HR></BLOCKQUOTE>

[This message has been edited by nexusone (edited 01-09-2003).]

Yeah, i get all that.

but i need the X,Y position in terms of pixels from the top left of the window given the 3d co-ord for my model.

my model code works fine, it draws it textures, but i cant obtain the xy position of a given xyz position after it has been drawn and rotated and viewport transformed.

JustHanging

01-10-2003, 05:04 AM

Like DFrey said, use gluProject. It does exactly what you need. For more advanced picking try gluPickMatrix and glRenderMode(GL_SELECT).

-Ilkka

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