🔒 Locked

OpenGL transform matrix

Started by space warp Mar 31, 2007 at 8:58 AM 2 replies 8.5k views

Original Post

Mar 31, 2007 at 8:58 AM

I'm making a space-shooter-demo for fun and I have built a matrix that stores rotation and position of the ship. I've tried it and it rotates correctly and everything but when I change the position nothing happens, the ship is still at the origin. I'm using glMultMatrix() for this and I would like to know what format OpenGL uses for the transformation matrix and why I can't move the ship.

Yann L

Mar 31, 2007 at 9:43 AM

Since unfortunately we're not psychic, we can't know what is wrong with your matrix without seeing it (ie. the code that generates it).

OpenGL uses column major order (by default). A 4x4 matrix looks like this in memory:

[0  4  8 12][1  5  9 13][2  6 10 14][3  7 11 15]

The upper left 3x3 part represents rotations and scaling (and shearing), the top right 3 element column (12, 13, 14) the translation:

[R0 R3 R6 Tx][R1 R4 R7 Ty][R2 R5 R8 Tz][ 0  0  0  1]

Now show us the code that generates your matrix, and we can see further.

space warp

Mar 31, 2007 at 10:04 AM

It is what you described.

double mat[16];mat[0] = Cell(0,0); mat[4] = Cell(1,0); mat[8] = Cell(2,0); mat[12] = Cell(3,0);mat[1] = Cell(0,1); mat[5] = Cell(1,1); mat[9] = Cell(2,1); mat[13] = Cell(3,1);mat[2] = Cell(0,2); mat[6] = Cell(1,2); mat[10] = Cell(2,2); mat[14] = Cell(3,2);mat[3] = Cell(0,3); mat[7] = Cell(1,3); mat[11] = Cell(2,3); mat[15] = Cell(3,3);glMultMatrixd(mat);

Cell(x,y) is a function that multiplies the rotation matrix and the translation matrix so that I get the transformation matrix.

double TRMATRIX::Cell(int x, int y) {    return rot[0][y] * pos[x][0] +           rot[1][y] * pos[x][1] +           rot[2][y] * pos[x][2] +           rot[3][y] * pos[x][3];}

The rotation matrix looks like this:

[R1 R2 R3 0][R4 R5 R6 0][R7 R8 R9 0][0  0  0  1]

and the translation matrix looks like this:

[1 0 0 X][0 1 0 Y][0 0 1 Z][0 0 0 1]

space warp

Mar 31, 2007 at 12:17 PM

Solved.

Topic Locked

This topic has been locked by a moderator. New replies are not allowed.