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Am I Doing this Matrix Multiplication Correctly?

Posted on 2008-06-16
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Last Modified: 2013-12-26
Basically I have written a class to draw a cube centered on a point (XYZ). I perform various tranlations on my scene using glTranslate/glRotate. Now I want to calculate the new position of the cube ("world co-ordinates" if that is the right term.)

It has been suggested that I should multiple the Point by the Modelviewmatrix I have achieved this by doing the following (see code).

Is this code correct and will it do what I want it to?


//function to do multiplication
 
#include <gl/gl.h>
 
void MultiplyVMatrix(GLfloat* Vertex, GLfloat* Matrix, GLfloat* ResultMatrix)
{
	for(int i=0;i<16;i++)
		for(int j=0;j<4;j++)
		ResultMatrix[j]+=Matrix[i]*Vertex[j];
 
}        
 
**************************************************************
//modelview matrix and multiply co-ordinate of cube
 
GLfloat matrix[16];
GLfloat vertex[4]={1,2,-5,0}; //any co-ordinate for testing
GLfloat resultmatrix[4]={0,0,0,0};
 
glGetFloatv(GL_MODELVIEW_MATRIX, matrix); //grab the modelview matrix
 
MultiplyVMatrix(vertex,matrix,resultmatrix);

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Question by:Jamsb
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Expert Comment

by:ozo
ID: 21798598
No
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Author Comment

by:Jamsb
ID: 21798607
OK I will be more specific. If this does not work. What SHOULD I be doing?
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Author Comment

by:Jamsb
ID: 21798667
I would increase points on the question if i could...
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Author Comment

by:Jamsb
ID: 21798714
GLfloat vertex[4]={1,2,-5,0}; //any co-ordinate for testing

should read:
GLfloat vertex[4]={1,2,-5,1}; //any co-ordinate for testing
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ozo earned 300 total points
ID: 21798721
      for(int i=0;i<16;i++)
                ResultMatrix[i/4]+=Matrix[i]*Vertex[i%4];
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Author Comment

by:Jamsb
ID: 21800636
ozo:

Can you explain your code please? in particular the following  "ResultMatrix[i/4]".
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Expert Comment

by:ozo
ID: 21800675
The the first four results, (i=0,1,2,3) go into ResultMatrix[0]
the next four (i=4,5,6,7) go into ResultMatrix[1]
etc.
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Author Comment

by:Jamsb
ID: 21800681
Im going to assume that that should be a modulo. So what was broken? and how did your code fix it?
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Expert Comment

by:ozo
ID: 21800758
one is divide, one is moduo
I presumed that
ResultMatrix = Matrix *Vertex
and that Matrix was laid out as
m0  m4  m8  m12
m1  m5  m9  m13
m2  m6  m10 m14
m3  m7 m11  m15

the original code was equivalent to multiplying each element by
(m0+m1+m2+m3+m4+m5+...+m15)
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Expert Comment

by:ozo
ID: 21800808
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Author Closing Comment

by:Jamsb
ID: 31467816
OK. Figured it out. I'm still a newbie :)
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