Drawing lines with Bresenham's Line Algorithm

Question

My computer graphics homework is to implement OpenGL algorithms using only the ability to draw points.

So obviously I need to get drawLine() to work bef

Answer 1

You can find the complete code in C++ to draw a line using Bresenham Algorithm at http://www.etechplanet.com/codesnippets/computer-graphics-draw-a-line-using-bresenham-algorithm.aspx:

/*BRESENHAAM ALGORITHM FOR LINE DRAWING*/
#include<iostream.h>
#include<graphics.h>
#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<math.h>
#include<dos.h>
void bhm_line(int,int,int,int,int);
void main()
{
 int ghdriver=DETECT,ghmode,errorcode,x1,x2,y1,y2;
 initgraph(&ghdriver,&ghmode,"..\\bgi");
 errorcode = graphresult();
 if(errorcode !=grOk)
 {
  cout<<"Graphics error:%s\n"<<grapherrormsg(errorcode);
  cout<<"Press any key to halt:";
  getch();
  exit(1);
 }
 clrscr();
 cout<<"Enter the coordinates (x1,y1): ";
 cin>>x1>>y1;
 cout<<"Enter the coordinates (x2,y2): ";
 cin>>x2>>y2;
 bhm_line(x1,y1,x2,y2,1);
 getch();
}
void bhm_line(int x1,int y1,int x2,int y2,int c)
{
 int x,y,dx,dy,dx1,dy1,px,py,xe,ye,i;
 dx=x2-x1;
 dy=y2-y1;
 dx1=fabs(dx);
 dy1=fabs(dy);
 px=2*dy1-dx1;
 py=2*dx1-dy1;
 if(dy1<=dx1)
 {
  if(dx>=0)
  {
   x=x1;
   y=y1;
   xe=x2;
  }
  else
  {
   x=x2;
   y=y2;
   xe=x1;
  }
  putpixel(x,y,c);
  for(i=0;x<xe;i++)
  {
   x=x+1;
   if(px<0)
   {
    px=px+2*dy1;
   }
   else
   {
    if((dx<0 && dy<0) || (dx>0 && dy>0))
    {
     y=y+1;
    }
    else
    {
     y=y-1;
    }
    px=px+2*(dy1-dx1);
   }
   delay(0);
   putpixel(x,y,c);
  }
 }
 else
 {
  if(dy>=0)
  {
   x=x1;
   y=y1;
   ye=y2;
  }
  else
  {
   x=x2;
   y=y2;
   ye=y1;
  }
  putpixel(x,y,c);
  for(i=0;y<ye;i++)
  {
   y=y+1;
   if(py<=0)
   {
    py=py+2*dx1;
   }
   else
   {
    if((dx<0 && dy<0) || (dx>0 && dy>0))
    {
     x=x+1;
    }
    else
    {
     x=x-1;
    }
    py=py+2*(dx1-dy1);
   }
   delay(0);
   putpixel(x,y,c);
  }
 }
}

Answer 2

In case anyone was wondering what the problem was, I still don't know what it was. What I ended up doing was re-factored my code so that the -shallow and -steep used the same algorithm as +shallow and +steep, respectively. After adjusting the x,y coordinates (negating the x or y coordinate), when I went to plot them I negated my original negation so that it plotted in the right spot.

Answer 3

I implemented the original Bresenham's algorithm in C++ and tried to optimize as much as I could (especially regarding removing the IF from the interior loop).

It draws in a linear buffer instead of a surface, and for this matter, this implementation was almost as fast as EFLA (Extremely Fast Line Algorithm) (maybe 5% slower).

#include <vector>
#include <math.h>
using namespace std;

vector<unsigned char> buffer;

int imageSide = 2048; // the width of the surface

struct Point2Di
{
    int x;
    int y;
    Point2Di(const int &x, const int &y): x(x), y(y){}
    Point2Di(){}

};

void drawLine(const Point2Di &p0, const Point2Di &p1)
{
    int dx = p1.x - p0.x;
    int dy = p1.y - p0.y;

    int dLong = abs(dx);
    int dShort = abs(dy);

    int offsetLong = dx > 0 ? 1 : -1;
    int offsetShort = dy > 0 ? imageSide : -imageSide;

    if(dLong < dShort)
    {
        swap(dShort, dLong);
        swap(offsetShort, offsetLong);
    }

    int error = dLong/2;
    int index = p0.y*imageSide + p0.x;
    const int offset[] = {offsetLong, offsetLong + offsetShort};
    const int abs_d[]  = {dShort, dShort - dLong};
    for(int i = 0; i <= dLong; ++i)
    {
        buffer[index] = 255;  // or a call to your painting method
        const int errorIsTooBig = error >= dLong;
        index += offset[errorIsTooBig];
        error += abs_d[errorIsTooBig];
    }
}

The EFLA implementation that I am using is:

void drawLine(Point2Di p0,  Point2Di p1)
{
    bool yLonger=false;
    int shortLen=p1.y-p0.y;
    int longLen=p1.x-p0.x;
    if (abs(shortLen)>abs(longLen)) {
        swap(shortLen, longLen);
        yLonger=true;
    }
    int decInc = longLen==0 ?  decInc=0 : ((shortLen << 16) / longLen);

    if (yLonger) {
        p0.y*=imageSide;
        p1.y*=imageSide;
        if (longLen>0)
            for (int j=0x8000+(p0.x<<16);p0.y<=p1.y;p0.y+=imageSide, j+=decInc)
                buffer[p0.y + (j >> 16)] = 255;  // or a call to your painting method
        else
            for (int j=0x8000+(p0.x<<16);p0.y>=p1.y;p0.y-=imageSide, j-=decInc)
                buffer[p0.y + (j >> 16)] = 255;  // or a call to your painting method
    }
    else
    {
        if (longLen>0)
            for (int j=0x8000+(p0.y<<16);p0.x<=p1.x;++p0.x, j+=decInc)
                buffer[(j >> 16) * imageSide + p0.x] = 255;  // or a call to your painting method
        else
            for (int j=0x8000+(p0.y<<16);p0.x>=p1.x;--p0.x, j-=decInc)
                buffer[(j >> 16) * imageSide + p0.x] = 255;  // or a call to your painting method
    }
}