I'm not very skilled with math, but i'm wondering what does glRotatef do internally?
I have some legacy code which does some math i don't really understand and sends values to glRotatef. I am implementing a physics engine to go with this legacy code and i need a 4x4 matrix instead of that output, i don't really understand how the rotation along a vector glrotatef thing works, but i just need a resulting matrix - which if i multiplied with glMultMatrixf instead of glrotatef would result in same working output.
Thanks.
instead of glRotatef build a matrix
I'm not very skilled with math, but i'm wondering what does glRotatef do internally?
I have some legacy code which does some math i don't really understand and sends values to glRotatef. I am implementing a physics engine to go with this legacy code and i need a 4x4 matrix instead of that output, i don't really understand how the rotation along a vector glrotatef thing works, but i just need a resulting matrix - which if i multiplied with glMultMatrixf instead of glrotatef would result in same working output.
Thanks.
Try the Euler matrices for x,y,and z axes; glMultMatrixf(euler_y_mat_with_desired_angle); will get you spin around the y axis, and you can also multiply them together for multi-axis transforms taking care to avoid locks.
If you need just a matrix, here it is:
[x, y, z] - a vector about which the rotation should be done
c - cos(alpha)
s - sin(alpha)
alpha - rotation angle
| x^2*(1-c)+c x*y*(1-c)-z*s x*z*(1-c)+y*s 0 |
R = | y*x*(1-c)+z*s y^2*(1-c)+c y*z*(1-c)-x*s 0 |
| x*z*(1-c)-y*s y*z*(1-c)+x*s z^2*(1-c)+c 0 |
| 0 0 0 1 |
[x, y, z] - a vector about which the rotation should be done
c - cos(alpha)
s - sin(alpha)
alpha - rotation angle
May be of some use to you:
http://en.wikipedia.org/wiki/Rotation_matrix
Most are 3x3, just add 0, 0, 0, 1 column/row vectors in the 4th colum/row to get it 4x4.
The http://en.wikipedia.org/wiki/Rotation_matrix#General_rotations section shows you euler angles
http://en.wikipedia.org/wiki/Rotation_matrix
Most are 3x3, just add 0, 0, 0, 1 column/row vectors in the 4th colum/row to get it 4x4.
The http://en.wikipedia.org/wiki/Rotation_matrix#General_rotations section shows you euler angles
Author
Ok thanks, based on that i wrote this function (sorry for pascal syntax):
The results aren't what it's supposed to be:
I replaced: glRotatef(Angle * 180 / Pi, Axis[0], Axis[1], Axis[2]);
With:
rotang:= CreateGlRotateMatrix( Angle * 180 / Pi, Axis[0], Axis[1], Axis[2]);
glMultMatrixf(@rotang[0,0]);
The results make a total mess when objects are rendered, none of rotations look correctly, did i get the matrix array addressing right? in delphi the matrix order are row-col based.
function CreateGlRotateMatrix(angle, x, y, z: single) : TMatrix;
var
axis: TVector3f;
b, c, ac, s: single;
begin
result:= IdentityHmgMatrix;
c:= cos(angle);
s:= sin(angle);
result[0,0] := (x*x) * (1-c)+c;
result[1,0] := x*y * (1-c)-z*s;
result[2,0] := x*z * (1-c)+y*s;
result[0,1] := y*x * (1-c)+z*s;
result[1,1] := (y*y) * (1-c)+c;
result[2,1] := y*z * (1-c)-x*s;
result[0,2] := x*z * (1-c)-y*s;
result[1,2] := y*z * (1-c)+x*s;
result[2,2] := (z*z) * (1-c)+c;
end;
The results aren't what it's supposed to be:
I replaced: glRotatef(Angle * 180 / Pi, Axis[0], Axis[1], Axis[2]);
With:
rotang:= CreateGlRotateMatrix( Angle * 180 / Pi, Axis[0], Axis[1], Axis[2]);
glMultMatrixf(@rotang[0,0]);
The results make a total mess when objects are rendered, none of rotations look correctly, did i get the matrix array addressing right? in delphi the matrix order are row-col based.
[x, y, z] - a vector about which the rotation should be done
A normalized vector. I guess you knew that already, but just making sure everyone knows it as well
.
A normalized vector. I guess you knew that already, but just making sure everyone knows it as well
Yes, I've forgotten to say that. glRotatef auto-normalizes that vector. In our code, we should take care of normalization.
Also, be aware that GL assumes column-major order of matrices. If normalization doesn't help alone, transpose your matrix.
Author
Ok, i tried normalizing the vector & transposing / not transposing but still no luck, here's new function i have:
I can show some screenshots of the working (glrotatef) & non-working results (CreateGlRotateMatrix) if that would help in any way clear the situation on what's wrong with it.
function CreateGlRotateMatrix(angle, x, y, z: single) : TMatrix;
var
axis: TVector3f;
b, c, ac, s: single;
invLen : Single;
begin
invLen:= RSqrt(x * x + y * y + z * z);
x:= x * invLen;
y:= y * invLen;
z:= z * invLen;
result:= IdentityHmgMatrix;
c:= cos(angle);
s:= sin(angle);
result[0,0] := (x*x) * (1-c)+c;
result[1,0] := x*y * (1-c)-z*s;
result[2,0] := x*z * (1-c)+y*s;
result[0,1] := y*x * (1-c)+z*s;
result[1,1] := (y*y) * (1-c)+c;
result[2,1] := y*z * (1-c)-x*s;
result[0,2] := x*z * (1-c)-y*s;
result[1,2] := y*z * (1-c)+x*s;
result[2,2] := (z*z) * (1-c)+c;
// vectorgeometry.TransposeMatrix(result);
end;
I can show some screenshots of the working (glrotatef) & non-working results (CreateGlRotateMatrix) if that would help in any way clear the situation on what's wrong with it.
c:= cos(angle);
s:= sin(angle);
The results aren't what it's supposed to be:
I replaced: glRotatef(Angle * 180 / Pi, Axis[0], Axis[1], Axis[2]);
With:
rotang:= CreateGlRotateMatrix( Angle * 180 / Pi, Axis[0], Axis[1], Axis[2]);
glMultMatrixf(@rotang[0,0]);
The results make a total mess when objects are rendered, none of rotations look correctly, did i get the matrix array addressing right? in delphi the matrix order are row-col based.
cos/sin take radians while glRotatef takes degrees. So you don't need the radians to degrees conversion (* 180 / PI) when calling your own function.
The results aren't what it's supposed to be:
I replaced: glRotatef(Angle * 180 / Pi, Axis[0], Axis[1], Axis[2]);
With:
rotang:= CreateGlRotateMatrix( Angle * 180 / Pi, Axis[0], Axis[1], Axis[2]);
Hey, I just noticed the way you pass angles... glRotatef() takes angle in degrees not in radians!!!
Previous two lines are not identical, because the angles are not the same.
This topic is closed to new replies.
Advertisement
Popular Topics
Advertisement
