# transforming nearplane

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Hello, I need your help solving this problem. There is a color coded cube with minimum coordinate (0,0,0) and maximum coordinate (1,1,1). Each vertex of this cube has the respecitve color. That means, Vertex 0,0,0 is colored with 0,0,0, vertex 1,0,0 is colored with 1,0,0. Please have a look at the link http://img80.imageshack.us/i/nearplane.jpg/ The top image shows such a color coded cube. The problem is, when moving the camera into the cube, there are no colors where the nearplane clips the cube (bottom image). To solve this problem i calculate the near plane vectors and multiply them by the inverse modelview matrix. In eyespace the camera is at 0,0,0. The tangent is tan(fovY/2) = (height/2) / nearplane_distance). So the height/2 is tan(fovY/2) * nearplane_distance
float fovy_half = deg_to_rad * fovYgrad * 0.5f;
float near_height_half = tan(fovy_half) * nearplane_dist;
float near_width_half = near_height_half * (aspect_ratio);

Vector4<float> nbl(-near_width_half, -near_height_half, -nearplane_dist, 1.0f)
Vector4<float> nbr( near_width_half, -near_height_half, -nearplane_dist, 1.0f)
Vector4<float> ntl( near_width_half,  near_height_half, -nearplane_dist, 1.0f)
Vector4<float> ntr(-near_width_half,  near_height_half, -nearplane_dist, 1.0f)

Matrix4<float> modelview;
glGetFloatv(GL_MODELVIEW_MATRIX, modelview.get());

Matrix4<float> mvi = Matrix4<float>::transpose(Matrix4<float>::inverse(modelview));

nbl = mvi * nbl;
nbr = mvi * nbr;
ntl = mvi * ntl;
ntr = mvi * ntr;


The nearplane vector are now in objectspace coordinates. With these 4 vectors I draw a fullscreen quad. Because the cube is from 0,0,0 to 1,1,1 and the colors as well, the 4 vectors nbl,nbr,ntl,ntr are use to represent the colors of the near plane. The goal is to fill the clipped area with the colors. Well, there is a colored "nearplane", but the colors are not correct.
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glMatrixMode(GL_PROJECTION);
glPushMatrix();

glColor3d(nbl.x, nbl.y, nbl.z); glVertex3f(-1.0f, -1.0f, -0.50f);
glColor3d(nbr.x, nbr.y, nbr.z); glVertex3f( 1.0f, -1.0f, -0.50f);
glColor3d(ntr.x, ntr.y, ntr.z); glVertex3f( 1.0f,  1.0f, -0.50f);
glColor3d(ntl.x, ntl.y, ntl.z); glVertex3f(-1.0f,  1.0f, -0.50f);
glEnd ();

glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glPopMatrix ();

I hope someone can help me, solving this problem regards, c-mos [Edited by - c-mos on April 23, 2010 7:02:13 AM]

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