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Jernej.L

instead of glRotatef build a matrix

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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.

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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.

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If you need just a matrix, here it is:


| 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

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Ok thanks, based on that i wrote this function (sorry for pascal syntax):


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.

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[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 :cool:.

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A normalized vector. I guess you knew that already, but just making sure everyone knows it as well :cool:.


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.

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Ok, i tried normalizing the vector & transposing / not transposing but still no luck, here's new function i have:


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.

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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.

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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.


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