src/map/hillshading.cpp

   1 #include <cmath>
   2 #include "hillshading.h"
   3
   4 struct Constants
   5 {
   6         double a1;
   7         double a2;
   8         double a3;
   9 };
  10
  11 struct SubMatrix
  12 {
  13         double z1;
  14         double z2;
  15         double z3;
  16         double z4;
  17         double z6;
  18         double z7;
  19         double z8;
  20         double z9;
  21 };
  22
  23 struct Derivatives
  24 {
  25         double dzdx;
  26         double dzdy;
  27 };
  28
  29 static void getConstants(double azimuth, double elevation, Constants &c)
  30 {
  31         double alpha = (M_PI / 180.0) * azimuth;
  32         double beta = (M_PI / 180.0) * elevation;
  33
  34         c.a1 = sin(beta);
  35         c.a2 = cos(beta) * sin(alpha);
  36         c.a3 = cos(beta) * cos(alpha);
  37 }
  38
  39 static void getDerivativesHorn(const SubMatrix &sm, double z, Derivatives &d)
  40 {
  41         d.dzdx = (z * (sm.z3 + 2 * sm.z6 + sm.z9 - sm.z1 - 2 * sm.z4 - sm.z7)) / 8;
  42         d.dzdy = (z * (sm.z1 + 2 * sm.z2 + sm.z3 - sm.z7 - 2 * sm.z8 - sm.z9)) / 8;
  43 }
  44
  45 static void getSubmatrix(int x, int y, const Matrix &m, SubMatrix &sm)
  46 {
  47         int left = x - 1;
  48         int right = x + 1;
  49         int top = y - 1;
  50         int bottom = y + 1;
  51
  52         sm.z1 = m.m(top, left);
  53         sm.z2 = m.m(top, x);
  54         sm.z3 = m.m(top, right);
  55         sm.z4 = m.m(y, left);
  56         sm.z6 = m.m(y, right);
  57         sm.z7 = m.m(bottom, left);
  58         sm.z8 = m.m(bottom, x);
  59         sm.z9 = m.m(bottom, right);
  60 }
  61
  62 QImage HillShading::render(const Matrix &m, quint8 alpha, double z,
  63   double azimuth, double elevation)
  64 {
  65         QImage img(m.w() - 2, m.h() - 2, QImage::Format_ARGB32_Premultiplied);
  66         uchar *bits = img.bits();
  67         int bpl = img.bytesPerLine();
  68
  69         Constants c;
  70         SubMatrix sm;
  71         Derivatives d;
  72
  73         getConstants(azimuth, elevation, c);
  74
  75         for (int y = 1; y < m.h() - 1; y++) {
  76                 for (int x = 1; x < m.w() - 1; x++) {
  77                         getSubmatrix(x, y, m, sm);
  78                         getDerivativesHorn(sm, z, d);
  79
  80                         double L = (c.a1 - c.a2 * d.dzdx - c.a3 * d.dzdy)
  81                           / sqrt(1.0 + d.dzdx * d.dzdx + d.dzdy * d.dzdy);
  82
  83                         quint32 pixel;
  84                         if (std::isnan(L))
  85                                 pixel = 0;
  86                         else {
  87                                 quint8 val = (L < 0) ? 0 : L * alpha;
  88                                 pixel = (alpha - val)<<24;
  89                         }
  90
  91                         *(quint32*)(bits + (y - 1) * bpl + (x - 1) * 4) = pixel;
  92                 }
  93         }
  94
  95         return img;
  96 }