Canvas.h

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/**
 *\file         Canvas.h
 *
 *\brief        The Canvas class is a discrete simulation of the real life canvas. 
 *
 *              The canvas is cutted into grid structure, with <b><var>vReso</var></b> grids along
 *              vertical (<b><var>Row</var></b>) direction and <b><var>hReso</var></b> horizontal grids along 
 *              horizontal(<b><var>Col</var></b>) direction. Each rectangular grid is called a pixel, or picture
 *              element, and is specified by its lower-left <b><var>(row, col)</var></b> position. 
 *              The coordinate system has the lower-left corner of the canvas as its origin, 
 *              with <b><var>Row</var></b> going up and <b><var>Col</var></b> going right. The lower-left corner 
 *              pixel has a coordinate of <b><var>(0,0)</var></b>, and the upper-right one of <b><var>
 *              (hReso - 1, vReso - 1)</var></b>. 
 *
 *              A <b><var>(x, y)</var></b> pair means <b><var>col = x+hReso/2, and row = y+vReso/2</var></b>.
 *
 *              To each piexel <b><var>(row, col)</var></b> on the Canvas, a RGB color value is associated,
 *              which can be accessed via index operator [<b><var>row</var></b>][<b><var>col</var></b>].
 *
 *              WritePPM() writes the Canvas to an image file of PPM format.
 *
 * <H4>11 Jul 2004:</H4> Point structure and perspectiveProjection(Point const&) added. Now the Canvas is actually a virutal camera
 *              in the simplest setting: the Canvas plane is centered at z=0 with x-size of hReso, and y-size of vReso, 
 *              Eye at eye_at (>0) and looking along positive z direction.  eye_at defaults to hReso / 2.
 *
 *              Now the triangle primitives are updated so the given points are in 3D space. The other primitives are
 *              not updated though, and actually can be deleted from Canvas class, since for 3D graphics, everything 
 *              is usually tesselated by triangles.
 *
 *\author       Xianming Chen
 *
 */


#ifndef _CANVAS_H_
#define _CANVAS_H_

#include "RGB.h"
#include "xmath.h"
#include "Point.h"

#include <string>
#include <cassert>
#include <map>

using namespace std;

namespace columbia
{
  
  class Canvas
  { 
  public:
      explicit Canvas(int resoH = 512, int resoV = 512) : hReso(resoH), vReso(resoV), filename("tmp.ppm"), data(0)  { init(); }
      ~Canvas()                                  { delete [] data; }
      void Clear(RGB const&);
      
      RGB* operator[] (int r)                    { assert(r>=0 && r<vReso); return data + r*hReso; }
      RGB& Pixel(int row, int col);              //! Use this instead of the [] operator if both row and col number checking is needed.
      
      /**
       * Scan-convert a line segment given two end points. The scan conversion stop right before the second end point.
       * If output = 0, Draw a line segment between two end points, (x1,y1) and (x2,y2).  Each pixel color is interpolated from two end colors.
       * Else only return scanned result to output.
       */
      void ScanLineSegment(int x1, int y1, RGB const& color1, int x2, int y2, RGB const& color2, multimap<int, pair<int, RGB> >* output=0);  

      void WireTriangle(Point const& P1, RGB const& col1, Point const& P2, RGB const& col2, Point const& P3, RGB const& col3);
      void SolidTriangle(Point const& P1, RGB const& col1, Point const& P2, RGB const& col2, Point const& P3, RGB const& col3);
      
      void WritePPM(const char* = 0) const;

      int hReso, vReso;                          //! Resolutions (pixel #)at horizontal and vertical directions.
      std::string filename;                      //! write as PPM image file name.


      //! For 3D graphics, we can get rid of these methods-- everything should be triangles.
      void WireCircle(int center_x, int center_y, int radius, int reso, RGB const& col);
      void SolidCircle(int center_x, int center_y, int radius, RGB const& col);
      void SolidRectangle(int minx, int miny, int maxx, int maxy, RGB const& col);

  private:
      RGB* data;
      bool swap_xy, flip_y;                      // swap x&y if abs(slope) > 1.0, flip y if y<0.

      void init();

      void scanLineSegment(int x1, int y1, RGB const& col1, int x2, int y2, RGB const& col2, multimap<int, pair<int, RGB> >* output); 
      void writePixel(int X, int Y, RGB const& col, multimap<int, pair<int, RGB> >* output); 

      double eye_at;
      Point perspectiveProjection(Point const&); 
  };
  
  inline void Canvas :: writePixel(int X, int Y, RGB const& col, multimap<int, pair<int, RGB> >* output)
  {
    if(swap_xy) swap(X,Y);
    if(flip_y)  Y = -Y; 

    if(output) output->insert(make_pair(Y, make_pair(X, col)));

    else if( (X >= -hReso/2 && X <= hReso/2) &&
             (Y >= -vReso/2 && Y <= vReso/2) )
    {
      Pixel(Y + vReso/2, X + hReso/2) = col;
    }
  }
  

  inline RGB& Canvas :: Pixel(int row, int col)
  {
    if(row >= vReso) throw "row number overflow\n";
    if(row < 0)      throw "row number underflow\n";
    if(col >= hReso) throw "col number overrflow\n";
    if(col < 0)      throw "col number underflow\n";
    return *(data + row * hReso + col);
  }
  

  /* Geometry: Canvas at z=0. eye at eye_at(>0).
                    |
                    |  *
     eye<-----------|----------
                    |
                    |
  */
  inline Point Canvas :: perspectiveProjection(Point const& P)
  {
    double scale = eye_at / (eye_at - P.z);
    return Point(P.x * scale, P.y * scale, P.z);
  }


}

#endif

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