Why Transform Matrix doesn't work in OpenGL

Question

Say I want to draw a Ball in the scene and here are two different ways to do them.

float SUN_TRANSLATION_MATRIX[] = {
  1.0f, 0.0f, 0.0f, 0.0f,
  0.0f, 1.0f, 0.0f, 0.0f,
  0.0f, 0.0f, 1.0f, -15.0f,
  0.0f, 0.0f, 0.0f, 1.0f
};

void displaySolarSystem1(){
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  glTranslatef(0.0f, 0.0f, -15.f);
  glColor3f(1.0f, 0.8f, 0.5f);
  glutSolidSphere(2.0, 50, 40);
  glutSwapBuffers();
}

void displaySolarSystem(){
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();         
  glMultMatrixf(SUN_TRANSLATION_MATRIX);
  glColor3f(1.0f, 0.8f, 0.5f);
  glutSolidSphere(2.0, 50, 40); 
  glutSwapBuffers();
}

displaySolarSystem1 applies glTranslatef where displaySolarSystem uses Matrix operation the problem is that displaySolarSystem1 works as expected but the matrix failed.

What went wrong with displaySolarSystem()?

See stackoverflow.com/questions/4360918/…. Basically your matrix is transposed. — unwind
– unwind, Commented Jun 13, 2012 at 11:17
I'm not sure, but i gues that you should simply get the translation information from the matrix, and use glTranslatef to translate it just like in the displaySolarSystem1() — Subtle Development Space
– Subtle Development Space, Commented Jun 13, 2012 at 11:18
Not an answer but a debugging hint, use glGetDoublev(GL_MODELVIEW, modelMatrix); to inspect the matrix in the case where it does work then compare to the other one. — stonemetal
– stonemetal, Commented Jun 13, 2012 at 11:20

Spo1ler · Accepted Answer · 2012-06-13 11:51:01Z

2

http://www.opengl.org/sdk/docs/man/xhtml/glMultMatrix.xml

Calling glMultMatrix with an argument of m = [...] replaces the current transformation with C × M × v

Which means that transformations are applied by multiplying matrix by vector, not vice versa. So, translation matrix would be something like this 1 0 0 X 0 1 0 Y 0 0 1 Z 0 0 0 1 But this is matrix, that is written in row-major order, and OpenGL takes column-major order matrices, so you need to transpose it. So, finally, you just need to use glMultTransposeMatrix which, if I remember right, is slightly slower, or transpose your matrix to look like this

float SUN_TRANSLATION_MATRIX[] = {
  1.0f, 0.0f, 0.0f, 0.0f,
  0.0f, 1.0f, 0.0f, 0.0f,
  0.0f, 0.0f, 1.0f, 0.0f,
  0.0f, 0.0f, -15.0f, 1.0f
};

answered Jun 13, 2012 at 11:51

Spo1ler

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Comments

Korben · Accepted Answer · 2012-06-13 11:46:44Z

0

Thanks Unwind. The problem was solved by changing glMultMatrixf(SUN_TRANSLATION_MATRIX); to glMultTransposeMatrixf(SUN_TRANSLATION_MATRIX); thanks for your hint of "this is a transposed matrix"

answered Jun 13, 2012 at 11:46

Korben

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