glRotatef problem

Graphics and GPU Programming Programming

Started by Andrew1979 August 10, 2008 12:50 PM

11 comments, last by MARS_999 15 years, 8 months ago

Andrew1979

112

Author

August 10, 2008 12:50 PM

Hi All I'm wanting to rotate a quad around the Y-axis which I have got it to do, but it rotates the wrong side of the quad. Is there anyway I can get it to rotate the other side of my quad? Thank you in advance. Andrew

MARS_999

1,627

August 11, 2008 01:37 AM

What do you mean it rotates the wrong side? I need a bit more info on what is wrong or you want the rotation to do.

Andrew1979

112

Author

August 11, 2008 02:33 AM

Well the quad I have rotating rotates from the left side of the quad. It seems to do this with all the quads i've created by default. What I'm wanting to know is how would I rotate a quad from the right hand side? :)

MARS_999

1,627

August 11, 2008 02:46 AM

flip your sign value... if you are +100 try -100....

Andrew1979

112

Author

August 11, 2008 03:07 AM

what is a sign value? could u explain further please?

Thank you.

michaweyel

128

August 11, 2008 03:53 AM

uhmmm...mathematically speaking, a sign is what you use to distinguish between positive and negative values, e.g. +100 and -100, where + and - are signs.

Rotations can be either clockwise or counterclockwise (which I assume is what you mean by saying that your quad rotates from the left or from the right side).

So if you are using glRotate, try using a negative value for the rotation amount instead of a positive value to make your quad rotate in the opposite direction

Andrew1979

112

Author

August 11, 2008 04:00 AM

Ah I get you now, yes I'll try that, thank you.

so basically,

instead of glRotatef(angleValue, 0.0f, 1.0f, 0.0f)

it would be:

glRotatef(-angleValue, 0.0f, 1.0f, 0.0f)

Andrew

michaweyel

128

August 11, 2008 04:53 AM

Exactly :)

V-man

813

August 11, 2008 09:29 AM

Whoaaaaaaa

Sig: http://glhlib.sourceforge.net
an open source GLU replacement library. Much more modern than GLU.
float matrix[16], inverse_matrix[16];
glhLoadIdentityf2(matrix);
glhTranslatef2(matrix, 0.0, 0.0, 5.0);
glhRotateAboutXf2(matrix, angleInRadians);
glhScalef2(matrix, 1.0, 1.0, -1.0);
glhQuickInvertMatrixf2(matrix, inverse_matrix);
glUniformMatrix4fv(uniformLocation1, 1, FALSE, matrix);
glUniformMatrix4fv(uniformLocation2, 1, FALSE, inverse_matrix);

Andrew1979

112

Author

August 11, 2008 12:31 PM

Hi

I'm afraid it does not work by just changing the sign, any other ideas? :)

glRotatef problem

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