Clipper.cc

00001 /*
00002 File: Clipper.cc
00003 
00004 Function: Clips homogeneous coordinates to the unit cube
00005 between [0 0 0] and [1 1 1].
00006 
00007 Author: Andrew Willmott
00008 
00009 Notes: 
00010 */
00011 
00012 #include "gcl/Clipper.h"
00013 #include "cl/String.h"
00014 
00015  #define DBG_COUT if (0) cerr
00016 
00041  Clipper::Clipper()
00042 {
00043 lastOutcode = 0;
00044 lastP = vl_0;
00045 }
00046 
00047  Void Clipper::ClipPoint(const HPoint &p, Bool draw)
00048 {
00049 Int outcode;
00050 BdryCoords bc;
00051 
00052 // calc BC for [-1, 1] x [-1, 1] x [0, 1]
00053 // bc is -ve if:
00054 bc[0] = p[3] + p[0]; // x < -1
00055 bc[1] = p[3] - p[0]; // x > 1
00056 bc[2] = p[3] + p[1]; // y < -1
00057 bc[3] = p[3] - p[1]; // y > 1
00058 bc[4] = p[2]; // z < 0
00059 bc[5] = p[3] - p[2]; // z > 1
00060 
00061 // should replace this with non-branching code where possible
00062 outcode = 0;
00063 if (bc[0] < 0) outcode |= 0x01;
00064 if (bc[1] < 0) outcode |= 0x02;
00065 if (bc[2] < 0) outcode |= 0x04;
00066 if (bc[3] < 0) outcode |= 0x08;
00067 if (bc[4] < 0) outcode |= 0x10;
00068 if (bc[5] < 0) outcode |= 0x20;
00069 
00070 DBG_COUT << String().Printf("outcode = %02x", outcode) << endl;
00071 
00072 // do the clipping
00073 
00074 if (!draw)
00075 {
00076 // move if we're inside the clip volume
00077 if (outcode == 0)
00078 ViewPoint(p, draw);
00079 }
00080 else
00081 {
00082 if ((outcode & lastOutcode) == 0)
00083 // we have at least one endpoint inside all planes
00084 if ((outcode | lastOutcode) == 0)
00085 // both are inside: trivial accept
00086 ViewPoint(p, draw);
00087 else
00088 {
00089 Int i, clip = outcode | lastOutcode, mask = 1;
00090 GCLReal alpha, alpha0 = 0.0, alpha1 = 1.0;
00091 
00092 DBG_COUT << String().Printf("clipping: clip = %02x", clip) << endl;
00093 
00094 // must clip line segment to the volume.
00095 
00096 // step through each plane 
00097 for (i = 0; i < 6; i++, mask <<= 1)
00098 {
00099 if (mask & clip)
00100 // we straddled this boundary
00101 {
00102 // find the alpha of the intersection with this plane
00103 alpha = lastBC[i] / (lastBC[i] - bc[i]);
00104 if (lastOutcode & mask)
00105 // heading in: clip alpha0
00106 alpha0 = Max(alpha0, alpha);
00107 else
00108 // heading out: clip alpha1
00109 alpha1 = Min(alpha1, alpha);
00110 }
00111 }
00112 
00113 DBG_COUT << "alpha0 = " << alpha0 << " alpha1 = " << alpha1 << endl; 
00114 
00115 if (lastOutcode)
00116 // heading in, so move to entry point
00117 ViewPoint(lastP + alpha0 * (p - lastP), false);
00118 if (outcode)
00119 // heading out, so draw to exit point
00120 ViewPoint(lastP + alpha1 * (p - lastP), true);
00121 else
00122 // terminate inside clipping volume, so just draw!
00123 ViewPoint(p, true);
00124 }
00125 }
00126 
00127 lastOutcode = outcode;
00128 lastBC[0] = bc[0];
00129 lastBC[1] = bc[1];
00130 lastBC[2] = bc[2];
00131 lastBC[3] = bc[3];
00132 lastBC[4] = bc[4];
00133 lastBC[5] = bc[5];
00134 lastP = p;
00135 }