Solved

Multiplying by Modelview Matrix Gives Unexpected result

Posted on 2008-06-17
5
1,032 Views
Last Modified: 2013-12-26
In a previous question (http://www.experts-exchange.com/Programming/Game/3D_Prog./Q_23489987.html)
I asked how to find the coordinates of an object after undergoing a transform.

To do this I had to multiply the point I wanted to translate by the modelview matrix. The issue is that the values I get back are the opposite to what's occuring on the screen. ie. if the objects are rotated clockwise on the screen the results I get back are moving anticlockwise.

Do I need to Invert the modelview matrix or somthing like that? Could someone provide code to aid me in this?

//function to do multiplication

 

#include <gl/gl.h>

 

void MultiplyVMatrix(GLfloat* Vertex, GLfloat* Matrix, GLfloat* ResultMatrix)

{

     

      for(int i=0;i<16;i++)

                ResultMatrix[i/4]+=Matrix[i]*Vertex[i%4];

 

}        

 

**************************************************************

//modelview matrix and multiply co-ordinate of cube

 

GLfloat matrix[16];

GLfloat vertex[4]={1,2,-5,1}; //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

0
Comment
Question by:Jamsb
  • 3
  • 2
5 Comments
 
LVL 2

Author Comment

by:Jamsb
ID: 21803397
That's twice I've posted wrongly.

*Correction*
GLfloat vertex[4]={1,2,-5,1};
0
 
LVL 22

Accepted Solution

by:
NovaDenizen earned 150 total points
ID: 21813276
I think your problem is that you think the matrix is in row major order, but it is actually in column major order.  

see http://www.opengl.org/sdk/docs/man/xhtml/glLoadMatrix.xml to see this illustrated.

That would mean that tou want ResultMatrix[i%4] += Matrix[i]*Vertex[i/4]
0
 
LVL 2

Author Comment

by:Jamsb
ID: 21816770
I feel so stupid now! Its hard trying to imagine what's going wrong with all this Matrix math!  Thanks very much :)
0
 
LVL 2

Author Closing Comment

by:Jamsb
ID: 31467925
Exactly what I wanted.
0
 
LVL 22

Expert Comment

by:NovaDenizen
ID: 21816875
The only reason I know to look for this is that I did something very similar a few weeks ago, so don't feel too bad.
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

Foreword (May 2015) This web page has appeared at Google.  It's definitely worth considering! https://www.google.com/about/careers/students/guide-to-technical-development.html How to Know You are Making a Difference at EE In August, 2013, one …
Lithium-ion batteries area cornerstone of today's portable electronic devices, and even though they are relied upon heavily, their chemistry and origin are not of common knowledge. This article is about a device on which every smartphone, laptop, an…
Many functions in Excel can make decisions. The most simple of these is the IF function: it returns a value depending on whether a condition you describe is true or false. Once you get the hang of using the IF function, you will find it easier to us…
This is a video describing the growing solar energy use in Utah. This is a topic that greatly interests me and so I decided to produce a video about it.

867 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

19 Experts available now in Live!

Get 1:1 Help Now