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POOpenGL Clip Space Frustum Culling Wrong Results
 

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  1. POOpenGL Clip Space Frustum Culling Wrong Results
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    <p>I implemented a Clip-Space Frustum Culling on the CPU. In a simple reduced case, I just create a rectangle based on 4 different points which I'm going to render in GL_LINES modes.</p> <p>But sometimes, it seems to me, I get wrong results. Here is an example:</p> <p><a href="http://s7.directupload.net/images/121205/imawbhoh.png" rel="nofollow noreferrer">http://s7.directupload.net/images/121205/imawbhoh.png</a></p> <p>In this render pass, my frustum culling computation detects, all points would be outside the positive y coordinate in NDC coordinates.</p> <p>Here is the input:</p> <p>Points:</p> <pre><code>P1: -5000, 3, -5000 P2: -5000, 3, 5000 P3: 5000, 3, 5000 P4: 5000, 3, -5000 </code></pre> <p>MVP (rounded):</p> <pre><code> 1.0550 0.0000 -1.4521 1138.9092 -1.1700 1.9331 -0.8500 -6573.4885 -0.6481 -0.5993 -0.4708 -2129.3858 -0.6478 -0.5990 -0.4707 -2108.5455 </code></pre> <p>And the calculations (MVP * Position)</p> <pre><code>P1 P2 P3 P4 3124 -11397 -847 13674 3532 -4968 -16668 -8168 3463 -1245 -7726 -3018 3482 -1225 -7703 -2996 </code></pre> <p>And finally, transformed by the perspective divide (w-component)</p> <pre><code>P1 P2 P3 P4 0.897 9.304 0.110 -4.564 1.014 4.056 2.164 2.726 0.995 1.016 1.003 1.007 1.0 1.0 1.0 1.0 </code></pre> <p>As you can see, all transformed points have their y component greater than 1 and should be outside the Viewing Frustum.</p> <p>I already double checked my matrices. I also made a transform feedback in the Vertex Shader to be sure, I use the same matrices for computation on CPU and GPU. Even the result of MVP * Point is the same in my Vertex Shader as on the GPU. My rendering pipeline is as simple as possible. </p> <p>The vertex Shader is </p> <pre><code>vColor = aColor; gl_Position = MVP * aVertexPosition; //With Transform Feedback enabled //transOut = MVP * aVertexPosition </code></pre> <p>And the fragment Shader</p> <pre><code>FragColor = vColor; </code></pre> <p>So the Vertex Shader has the same results as my CPU computations. But still, they are lines drawn on the screen! Any Ideas why there are lines? Do I do something wrong with the perspective divide?</p> <p>What do I have to do, to detect, that this rectangle should not be culled, because there is at least one line visible (basically a strip of another line is visible as well in this example)</p> <p>If it helps: The visible red line is the one between P1 and P2...</p> <p><strong>[Edit]</strong> Now I implemented a world space culling by computing the camera frustum normals and hesse normal form equations. This works fine with correct recognition. Sadly I do need correct computations in clip space, since I'm going to make other computations with that points. Someone any ideas?</p> <p>Here is my computation code:</p> <pre><code>int outOfBoundArray[6] = {0, 0, 0, 0, 0, 0}; std::vector&lt;glm::dvec4&gt; tileBounds = activeElem-&gt;getTileBounds(); //Same as I use for world space culling const glm::dmat4&amp; viewProj = cam-&gt;getCameraTransformations().viewProjectionMatrix; //Same camera which I use for world space culling. for (int i=0; i&lt;tileBounds.size(); i++) { //Apply ModelViewProjection Matrix, to Clip Space glm::dvec4 transformVec = viewProj * tileBounds[i]; //To NDC space [-1,1] transformVec = transformVec / transformVec[3]; //Culling test if ( transformVec.x &gt; 1.0 ) outOfBoundArray[0]++; if ( transformVec.x &lt; -1.0 ) outOfBoundArray[1]++; if ( transformVec.y &gt; 1.0 ) outOfBoundArray[2]++; if ( transformVec.y &lt; -1.0 ) outOfBoundArray[3]++; if ( transformVec.z &gt; 1.0 ) outOfBoundArray[4]++; if ( transformVec.z &lt; -1.0 ) outOfBoundArray[5]++; //Other computations... } for (int i=0; i&lt;6; i++) { if (outOfBoundArray[i] == tileBounds.size()) { return false; } } return true; </code></pre>
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