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POIncorrect order of matrix values in glm?
 

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  1. POIncorrect order of matrix values in glm?
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    <p>I started using <a href="http://glm.g-truc.net" rel="nofollow noreferrer">GLM</a> library to do mathematics operations over OpenGL 3 and GLSL. I need an orthographic projection to draw 2D graphics, so I writed this simple code:</p> <pre><code>glm::mat4 projection(1.0); projection = glm::ortho( 0.0f, 640.0f, 480.0f, 0.0f, 0.0f, 500.0f); </code></pre> <p>Printing on screen the values that glm::ortho has created I get:</p> <pre><code> 0.00313 0.00000 0.00000 0.00000 0.00000 -0.00417 0.00000 0.00000 0.00000 0.00000 -0.00200 0.00000 -1.00000 1.00000 -1.00000 1.00000 </code></pre> <p>As I know this is not the correct order for the values in OpenGL, because multiplying this matrix by a position vector will ignore all translation values.</p> <p>I tested that matrix with my shader and some primitives and I only get a blank screen. But if I modify by hand the matrix as follows it works ok:</p> <pre><code> 0.00313 0.00000 0.00000 -1.00000 0.00000 -0.00417 0.00000 1.00000 0.00000 0.00000 -0.00200 -1.00000 0.00000 0.00000 0.00000 1.00000 </code></pre> <p>Moreover, looking at the function "ortho" in the "glm/gtc/matrix_transform.inl" file:</p> <pre><code>template &lt;typename valType&gt; inline detail::tmat4x4&lt;valType&gt; ortho( valType const &amp; left, valType const &amp; right, valType const &amp; bottom, valType const &amp; top, valType const &amp; zNear, valType const &amp; zFar) { detail::tmat4x4&lt;valType&gt; Result(1); Result[0][0] = valType(2) / (right - left); Result[1][1] = valType(2) / (top - bottom); Result[2][2] = - valType(2) / (zFar - zNear); Result[3][0] = - (right + left) / (right - left); Result[3][1] = - (top + bottom) / (top - bottom); Result[3][2] = - (zFar + zNear) / (zFar - zNear); return Result; } </code></pre> <p>I have replaced the last 3 initialization lines by the following code and also worked ok:</p> <pre><code> Result[0][3] = - (right + left) / (right - left); Result[1][3] = - (top + bottom) / (top - bottom); Result[2][3] = - (zFar + zNear) / (zFar - zNear); </code></pre> <p>This is a minimal vertex shader that I'm using for test (note that at this moment the uni_MVP is only the projection matrix explained above):</p> <pre><code>uniform mat4 uni_MVP; in vec2 in_Position; void main(void) { gl_Position = uni_MVP * vec4(in_Position.xy,0.0, 1.0); } </code></pre> <p>I thik that this is not a bug, because all functions works the same way. Maybe is an issue of my C++ compiler that inverts the order of multidimensional arrays? How can I solve this without modifying all GLM source code?</p> <p>I'm using the last version of GLM library (0.9.1) with Code::Blocks and MinGW running on Windows Vista.</p>
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