Solved

Am I Doing this Matrix Multiplication Correctly?

Posted on 2008-06-16
11
1,958 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
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 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
 Database Backup and Recovery Best Practices

Join Percona’s, Architect, Manjot Singh as he presents Database Backup and Recovery Best Practices (with a Focus on MySQL) on Thursday, July 27, 2017 at 11:00 am PDT / 2:00 pm EDT (UTC-7). In the case of a failure, do you know how long it will take to restore your database?

 
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
 
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

Database Solutions Engineer FAQs

In this series, we will discuss common questions received as a database Solutions Engineer at Percona. In this role, we speak with a wide array of MySQL and MongoDB users responsible for both extremely large and complex environments to smaller single-server environments.

Question has a verified solution.

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

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…
Performance in games development is paramount: every microsecond counts to be able to do everything in less than 33ms (aiming at 16ms). C# foreach statement is one of the worst performance killers, and here I explain why.
In this video we outline the Physical Segments view of NetCrunch network monitor. By following this brief how-to video, you will be able to learn how NetCrunch visualizes your network, how granular is the information collected, as well as where to f…
Michael from AdRem Software explains how to view the most utilized and worst performing nodes in your network, by accessing the Top Charts view in NetCrunch network monitor (https://www.adremsoft.com/). Top Charts is a view in which you can set seve…

615 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