Question

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?

Answer 1

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: