Solved

Am I Doing this Matrix Multiplication Correctly?

Posted on 2008-06-16
11
1,946 Views
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);

Open in new window

translation.gif
0
Comment
Question by:Jamsb
  • 6
  • 5
11 Comments
 
LVL 84

Expert Comment

by:ozo
ID: 21798598
No
0
 
LVL 2

Author Comment

by:Jamsb
ID: 21798607
OK I will be more specific. If this does not work. What SHOULD I be doing?
0
 
LVL 2

Author Comment

by:Jamsb
ID: 21798667
I would increase points on the question if i could...
0
 
LVL 2

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
0
 
LVL 84

Accepted Solution

by:
ozo earned 100 total points
ID: 21798721
      for(int i=0;i<16;i++)
                ResultMatrix[i/4]+=Matrix[i]*Vertex[i%4];
0
Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

 
LVL 2

Author Comment

by:Jamsb
ID: 21800636
ozo:

Can you explain your code please? in particular the following  "ResultMatrix[i/4]".
0
 
LVL 84

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.
0
 
LVL 2

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?
0
 
LVL 84

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)
0
 
LVL 84

Expert Comment

by:ozo
ID: 21800808
0
 
LVL 2

Author Closing Comment

by:Jamsb
ID: 31467816
OK. Figured it out. I'm still a newbie :)
0

Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

As game developers, we quickly learn that Artificial Intelligence (AI) doesn’t need to be so tough.  To reference Space Ghost: “Moltar, I have a giant brain that is able to reduce any complex machine into a simple yes or no answer. (http://www.youtu…
Recently, in one of the tech-blogs I usually read, I saw a post about the best-selling video games through history. The first place in the list is for the classic, extremely addictive Tetris. Well, a long time ago, in a galaxy far far away, I was…
Internet Business Fax to Email Made Easy - With eFax Corporate (http://www.enterprise.efax.com), you'll receive a dedicated online fax number, which is used the same way as a typical analog fax number. You'll receive secure faxes in your email, fr…
As a trusted technology advisor to your customers you are likely getting the daily question of, ‘should I put this in the cloud?’ As customer demands for cloud services increases, companies will see a shift from traditional buying patterns to new…

863 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

20 Experts available now in Live!

Get 1:1 Help Now