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POWhy does my translation matrix needs to be transposed?
 

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  1. POWhy does my translation matrix needs to be transposed?
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    <p>I'm working on a small graphics engine using OpenGL and I'm having some issues with my translation matrix. I'm using OpenGL 3.3, GLSL and C++. The situation is this: I have defined a small cube which I want to render on screen. The cube uses it's own coordinate system, so I created a model matrix to be able to transform the cube. To make it myself a bit easier I started out with just a translation matrix as the cube's model matrix and after a bit of coding I've managed to make everything work and the cube appears on the screen. Nothing all to special, but there is one thing about my translation matrix that I find a bit odd.</p> <p>Now as far as I know, a translation matrix is defined as follows:</p> <pre><code>1, 0, 0, x 0, 1, 0, y 0, 0, 1, z 0, 0, 0, 1 </code></pre> <p>However, this does not work for me. When I define my translation matrix this way, nothing appears on the screen. It only works when I define my translation matrix like this:</p> <pre><code>1, 0, 0, 0 0, 1, 0, 0 0, 0, 1, 0 x, y, z, 1 </code></pre> <p>Now I've been over my code several times to find out why this is the case, but I can't seem to find out why or am I just wrong and does a translation matrix needs to be defined like the transposed one here above?</p> <p>My matrices are defined as a one-dimensional array going from left to right, top to bottom.</p> <p>Here is some of my code that might help:</p> <pre><code>//this is called just before cube is being rendered void DisplayObject::updateMatrices() { modelMatrix = identityMatrix(); modelMatrix = modelMatrix * translateMatrix( xPos, yPos, zPos ); /* update modelview-projection matrix */ mvpMatrix = modelMatrix * (*projMatrix); } //this creates my translation matrix which causes the cube to disappear const Matrix4 translateMatrix( float x, float y, float z ) { Matrix4 tranMatrix = identityMatrix(); tranMatrix.data[3] = x; tranMatrix.data[7] = y; tranMatrix.data[11] = z; return Matrix4(tranMatrix); } </code></pre> <p>This is my simple test vertex shader:</p> <pre><code>#version 150 core in vec3 vPos; uniform mat4 mvpMatrix; void main() { gl_Position = mvpMatrix * vec4(vPos, 1.0); } </code></pre> <p>I've also did tests to check if my matrix multiplication works and it does. I * randomMatrix is still just randomMatrix</p> <p>I hope you guys can help. Thanks</p> <p>EDIT:</p> <p>This is how I send the matrix data to OpenGL:</p> <pre><code>void DisplayObject::render() { updateMatrices(); glBindVertexArray(vaoID); glUseProgram(progID); glUniformMatrix4fv( glGetUniformLocation(progID, "mvpMatrix"), 1, GL_FALSE, &amp;mvpMatrix.data[0] ); glDrawElements(GL_TRIANGLES, bufferSize[index], GL_UNSIGNED_INT, 0); } </code></pre> <p>mvpMatrix.data is a std::vector:</p>
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      1. POWhy does my translation matrix needs to be transposed?
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    1. COYour code doesn't actually show any interaction with OpenGL, does it? How do you feed the matrix to OpenGL?
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      1. USDavid Grayson
    2. COIs there a reason you aren't using glTranslatef?
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      1. USEric B
    3. COI also think G_G is right, your translation matrix should be pre-multiplied, not post-multiplied
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      1. USEric B
 

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