Here's the source code. ANSWER
is the correct answer, RESULT
is the actual result.
Am I blind, or is it calculating a -1
for entry 33
when it should be a 1
?
Here's the code:
GLKMatrix4 a = GLKMatrix4Make(-1.000000, 0.000000, 0.000000, 0.000000,
0.000000, 1.000000, 0.000000, 0.000000,
0.000000, 0.000000, -1.000000, 0.000000,
0.000000, 0.000000, 0.000000, 1.000000);
GLKMatrix4 b = GLKMatrix4Make(1.000000, 0.000000, 0.000000, 0.000000,
0.000000, 1.000000, 0.000000, -1.000000,
0.000000, 0.000000, 1.000000, -1.000000,
0.000000, 0.000000, 0.000000, 1.000000);
GLKMatrix4 ANSWER = GLKMatrix4Make(-1.000000, 0.000000, 0.000000, 0.000000,
0.000000, 1.000000, 0.000000, -1.000000,
0.000000, 0.000000, -1.000000, 1.000000,
0.000000, 0.000000, 0.000000, 1.000000);
NSLog(@"##################################################");
GLKMatrix4 RESULT = GLKMatrix4Multiply(a,b);
NSLog(@"Result:");
NSLog(@" %f %f %f %f",RESULT.m00,RESULT.m01,RESULT.m02,RESULT.m03);
NSLog(@" %f %f %f %f",RESULT.m10,RESULT.m11,RESULT.m12,RESULT.m13);
NSLog(@" %f %f %f %f",RESULT.m20,RESULT.m21,RESULT.m22,RESULT.m23);
NSLog(@" %f %f %f %f",RESULT.m30,RESULT.m31,RESULT.m32,RESULT.m33);
NSLog(@"Answer:");
NSLog(@" %f %f %f %f",ANSWER.m00,ANSWER.m01,ANSWER.m02,ANSWER.m03);
NSLog(@" %f %f %f %f",ANSWER.m10,ANSWER.m11,ANSWER.m12,ANSWER.m13);
NSLog(@" %f %f %f %f",ANSWER.m20,ANSWER.m21,ANSWER.m22,ANSWER.m23);
NSLog(@" %f %f %f %f",ANSWER.m30,ANSWER.m31,ANSWER.m32,ANSWER.m33);
NSLog(@"##################################################");
Here's the output:
##################################################
Result:
-1.000000 0.000000 0.000000 0.000000
0.000000 1.000000 0.000000 -1.000000
0.000000 0.000000 -1.000000 -1.000000
0.000000 0.000000 0.000000 1.000000
Answer:
-1.000000 0.000000 0.000000 0.000000
0.000000 1.000000 0.000000 -1.000000
0.000000 0.000000 -1.000000 1.000000
0.000000 0.000000 0.000000 1.000000
##################################################
I've spent the last 5 hours trying to debug some OpenGL code only to find this is the problem. I must be missing something, surely. Can anyone spot what's going on, or verify this shouldn't be happening?
Um, I got the same result as your "RESULT" matrix.
I did the matrix multiplication with pen and paper, following these rules:
http://www.mathsisfun.com/algebra/matrix-multiplying.html
This is how I did it using your two matrix a and b:
OK, I think I know where you went wrong, you must have listed the numbers in the wrong position:
The way you list the number in code is horizontal e.g Coordinate(x,y,z) but in a Matrix, these numbers get moved to vertical positions.
So for example, [ X Y Z W ] should become vertically:
Now IF we were to list the your numbers in the wrong position and multiply Matrix a with Matrix b, we get the same answer as your "ANSWER" matrix:
Little bit more details on this.
OpenGL ES suppose matrices have column major order. So element order how OpenGL ES reads matrix is:
Of course Apple's GLKit Math Utilities are also built according to this column majored order. And it may be little bit confusing while you realise it. :)