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problem with coordinates

Started by AndreaG Jun 4, 2003 at 11:14 AM 3 replies 350+ views

Original Post

Jun 4, 2003 at 11:14 AM

hi all! First of all Congratulations for the site, i''ve found it very useufl! I have a little problem, i''m not very experienced with opengl, so maybe its stupid. I am drawing a cube as descripted in nehe tutorials, i do the easy steps: glLoadIdentity(); glTranslatef(..); glRotatef(..); and its ok. I apply a translation and a rotation to the cube, so that the face that is at right comes to the front. It works, the cube is drawed correctly. Now, having defined the transformation matrix, I draw the cube with glVertex, but i need to know the TARGET coordinates of such points. In other words, i want to know witch face is IN FRONT in the cube, so if right face comes in front after a rotation of 90 degrees, i simply need to check Z coords of the vertexes. Is there a way to obtain target coords of the cube AFTER it has been rotated and translated? I tried to have them manually by multiplying the starting coords of each vertex for the GL_MODELVIEW_MATRIX matrix, but the result isnt what i expected (the Z coords of the right face arent equal, but by the drawing i see the righ face in front!) Can someone help me? WHat am i missing? Thanx! Bye! Andrea

lopatsch

Jun 4, 2003 at 12:13 PM

hmm i can''t actually said what went wrong or what you could have forgotten. but here is the way i would do it:

first of all do your translations, rotations and so on
then extract the modelview matrix
and then multiply your verts by hand

just take a look at the opengl documentation it''s something with glGetMatrix or so i think (i never had to that and so i am not sure)

hope that helps, otherwise post a piece of code how you do your calculations

AndreaG

Author

AndreaG

Jun 4, 2003 at 2:12 PM

uhm so I think my calculations are wrong.
Thats what i do:
glGetFloatv(GL_MODELVIEW_MATRIX, matrix);
so i have the transformation matrix which moves the face of hte cube from right to front, then i get the starting coords of every vertex of right face (x, y, z)
and i do the following:

|a b c d| |x|
|e f g h| x |y|
|i l m n| |z|
|0 0 0 1| |1|

resulting in my x'', y'', z'' that i obtain with:
x'' = ax + by + cz + d1
y'' = ex + fy + gz + h1
and so on
What do you think? is this correct?
Thanx a lot!!
bye!
Andrea

AndreaG

Author

AndreaG

Jun 4, 2003 at 3:59 PM

uoops solved the problem! I just was wrong in handling arrays!
Thanx 4 all!
AndreaG

5010

Jun 4, 2003 at 4:06 PM

Seems to me if you want to know if the face is facing you, apply the modelview matrix to the normal of the face. Then check which way the Z is. You'd only have to do this once per face instead of once per vertex.

But then again, you might have other uses in mind for those vertices...

No wait.... I think you have to do that with just the rotations, not the translations... hmmmm...

[edited by - 5010 on June 4, 2003 5:08:38 PM]

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