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POOpenGL 3 (LWJGL) LookAt Matrix Confusion
 

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  1. POOpenGL 3 (LWJGL) LookAt Matrix Confusion
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    <p>I'm learning OpenGL 3 using LWJGL. I have tried to implement an equivalent to <code>gluLookAt()</code>, and although it works am I somewhat confused as to why.</p> <p>I confess to just copying this code from various sources on the web, but after much study I think understand the maths behind it, and that I understand what LWJGL is doing.</p> <p>However the 'correct' <code>gluLookAt</code> code behaved incorrectly in my application, as the camera seemed to be turning the wrong way. I only managed to get my code working by transposing the orthonormal vectors <code>forward</code>, <code>side</code>, and <code>up</code> (hope I'm using the correct terminology!), which I'm pretty sure is wrong...</p> <pre><code>private static final Vector3f forward = new Vector3f(); private static final Vector3f side = new Vector3f(); private static final Vector3f up = new Vector3f(); private static final Vector3f eye = new Vector3f(); public static Matrix4f lookAt(float eyeX, float eyeY, float eyeZ, float centerX, float centerY, float centerZ, float upX, float upY, float upZ) { forward.set(centerX - eyeX, centerY - eyeY, centerZ - eyeZ); forward.normalise(); up.set(upX, upY, upZ); Vector3f.cross(forward, up, side); side.normalise(); Vector3f.cross(side, forward, up); up.normalise(); Matrix4f matrix = new Matrix4f(); matrix.m00 = side.x; matrix.m01 = side.y; matrix.m02 = side.z; matrix.m10 = up.x; matrix.m11 = up.y; matrix.m12 = up.z; matrix.m20 = -forward.x; matrix.m21 = -forward.y; matrix.m22 = -forward.z; matrix.transpose(); // &lt;------ My dumb hack eye.set(-eyeX, -eyeY, -eyeZ); matrix.translate(eye); return matrix; } </code></pre> <p>I dont think I should be doing the transpose, but it doesn't work without it. I put <code>transpose()</code> because I couldn't be bothered to retype all the matrix cell positions btw!</p> <p>My understanding is that the form of the lookAt matrix should be as follows</p> <pre><code>[ side.x up.x fwd.x 0 ] [ 1 0 0 -eye.x ] [ side.y up.y fwd.y 0 ] [ 0 1 0 -eye.y ] [ side.z up.z fwd.z 0 ] [ 0 0 1 -eye.z ] [ 0 0 0 1 ] [ 0 0 0 1 ] </code></pre> <p>And I think that the LWJGL <code>Matrix4f</code> class represents matrix cells as <code>m&lt;col&gt;&lt;row&gt;</code>. The <code>translate(Vector3f)</code> method does the following</p> <pre><code>public static Matrix4f translate(Vector3f vec, Matrix4f src, Matrix4f dest) { ... dest.m30 += src.m00 * vec.x + src.m10 * vec.y + src.m20 * vec.z; dest.m31 += src.m01 * vec.x + src.m11 * vec.y + src.m21 * vec.z; dest.m32 += src.m02 * vec.x + src.m12 * vec.y + src.m22 * vec.z; dest.m33 += src.m03 * vec.x + src.m13 * vec.y + src.m23 * vec.z; ... } </code></pre> <p>So I am left hugely confused as to what part of this I have screwed up. Is it my understanding of the lookAt matrix, the column/row major-ness (is that a word?!) of <code>Matrix4f</code>, or something else? Is the rest of my code just broken? Is it actually correct and I'm just worrying too much? Am I just an idiot?</p> <p>Thanks.</p>
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