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POHow to create glClipPlanes matching the current projection matrix?
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copied!<p>I'm rendering a collection of points in space which the user has manually rotated, panned, zoomed, and finally drawn a selection rectangle over a desired section. When they do this I call <code>gluPickMatrix</code> and then <code>gluPerspective</code> to construct the projection matrix, before using the <code>GL_SELECT</code> render mode to find the points in their selected area.</p> <p>I want to only show the points in the selected area. For testing each point has a boolean <code>isSelected</code>, and I know selection works because the correct ones are getting selected. There are too many points for this to be feasible, though, so I'd like to use <code>glClipPlane</code> to construct four planes corresponding to the rectangular prism (<em>edit: I mean the frustum</em>) defined by the user's selection rectangle. How can I derive the coefficients to pass to <code>glClipPlane</code> after setting up the projection matrix?</p> <p>Right now I'm doing this, but it ends up clipping all the points:</p> <pre><code>// Set up projection matrix int viewport[4]; glGetIntegerv(GL_VIEWPORT, viewport); gluPickMatrix(x, y, w, h, viewport); gluPerspective(FOV_Y, ASPECT_RATIO, NEAR_PLANE, FAR_PLANE); // Derive clip planes - this doesn't work // Based on the paper "Fast Extraction of Viewing Frustum Planes from the World-View-Projection Matrix" // http://graphics.cs.ucf.edu/cap4720/fall2008/plane_extraction.pdf double t[4], r[4], b[4], l[4]; double m[16]; glGetDoublev(GL_PROJECTION_MATRIX, m); t[0] = m[3] - m[1]; t[1] = m[7] - m[5]; t[2] = m[11] - m[9]; t[3] = m[15] - m[13]; r[0] = m[3] - m[0]; r[1] = m[7] - m[4]; r[2] = m[11] - m[8]; r[3] = m[15] - m[12]; b[0] = m[3] + m[1]; b[1] = m[7] + m[5]; b[2] = m[11] + m[9]; b[3] = m[15] + m[13]; l[0] = m[3] + m[0]; l[1] = m[7] + m[4]; l[2] = m[11] + m[8]; l[3] = m[15] + m[12]; // ... Render points in GL_SELECT mode, then go back to GL_RENDER mode ... glMatrixMode(GL_MODELVIEW); glLoadIdentity(); // ... Apply user's pan/rotate/zoom choices here ... // Apply clip planes glEnable(GL_CLIP_PLANE0); glClipPlane(GL_CLIP_PLANE0, t); glEnable(GL_CLIP_PLANE1); glClipPlane(GL_CLIP_PLANE1, r); glEnable(GL_CLIP_PLANE2); glClipPlane(GL_CLIP_PLANE2, b); glEnable(GL_CLIP_PLANE3); glClipPlane(GL_CLIP_PLANE3, l); </code></pre> <p>I also tried normalizing and/or negating the plane coordinates, which still seems to end up clipping all the points.</p>

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