DDA Line Drawing Algorithm

Question

I am using WinBGIm.

#include <iostream>
#include <cmath>
#include "graphics.h"
void Swap(double &a, double &b)
{
 double x = a;
 a = b;
 b = x;
}
double Round(double number)
{
 return (number >= 0) ? (int)(number + 0.5) : (int)(number - 0.5);
}
void PlotPixel(int x, int y, int color)
{
 const int radius = 1;
 setcolor(color);
 circle(400+x,300-y,radius);
}
void DDALine(double x1, double y1, double x2, double y2, int color)
{
 //Is it really required?
 if(x1>x2 || y1>y2)
 {
 Swap(x1, x2);
 Swap(y1, y2);
 }
 //////////////////////???
 double xi = x1;
 double yi = y1;
 double deltaY = y2 - y1;
 double deltaX = x2 - x1;
 double m = deltaY / deltaX;
 //Is it really required?
 /*if(x1 == x2)
 {
 m = 2;
 }*/
 if(abs(m)<=1)
 {
 while (xi<=x2)
 {
 yi = yi + m;
 yi = Round(yi);
 PlotPixel(xi, yi, color);
 xi = xi + 1;
 } 
 }
 if(abs(m)>1)
 {
 while (yi<=y2)
 {
 xi = xi + 1/m;
 xi = Round(xi);
 PlotPixel(xi, yi, color);
 yi = yi + 1;
 } 
 } 
}
int main()
{
 int gm=DETECT, gd=DETECT;
 initwindow(1024, 800,"Er");
 DDALine(-200, 0, 200, 0, BLUE);
 DDALine(0, 200, 0, -200, BLUE);
 ///////////////////////////////
 DDALine(-100, -100, -100, -100, WHITE);
 DDALine(-100, -100, -100, 100, WHITE);
 DDALine(-100, -100, 100, -100, WHITE);
 DDALine(-100, -100, 100, 100, WHITE);
 DDALine(-100, 100, -100, -100, WHITE);
 DDALine(-100, 100, -100, 100, WHITE);
 DDALine(-100, 100, 100, -100, WHITE);
 DDALine(-100, 100, 100, 100, WHITE);
 /////////////////////////////
 DDALine(100, -100, -100, -100, WHITE);
 DDALine(100, -100, -100, 100, WHITE);
 DDALine(100, -100, 100, -100, WHITE);
 DDALine(100, -100, 100, 100, WHITE);
 DDALine(100, 100, -100, -100, WHITE);
 DDALine(100, 100, -100, 100, WHITE);
 DDALine(100, 100, 100, -100, WHITE);
 DDALine(100, 100, 100, 100, WHITE);
 getch();
 closegraph();
}

Output:

enter image description here

Answer 1

Only draws straight or 45 degree lines

This code here seems wrong:

 while (xi<=x2)
 {
 yi = yi + m;
 yi = Round(yi);
 PlotPixel(xi, yi, color);
 xi = xi + 1;
 }

You are rounding yi at every step, which means that at every step, either you are going to add 0 or 1 to yi. That means you can only draw straight or 45 degree lines. What you should do instead is keep yi unrounded but plot the pixel at the rounded location, like this:

 while (xi<=x2)
 {
 yi = yi + m;
 PlotPixel(xi, Round(yi), color);
 xi = xi + 1;
 }

Of course the other loop needs to be fixed in the same way. I would also suggest inverting m once instead of once per loop. Additionally, you don't need that other if statement, it could just be an else. Full changes:

if(abs(m)<=1)
{
 while (xi<=x2)
 {
 yi = yi + m;
 PlotPixel(xi, Round(yi), color);
 xi = xi + 1;
 } 
}
else
{
 m = 1/m;
 while (yi<=y2)
 {
 xi = xi + m;
 PlotPixel(Round(xi), yi, color);
 yi = yi + 1;
 } 
}

• user3804: Is swapping required? Is divide by zero testing required?
• JS1: @anonymous Well, the way your code is written, your loops add 1 to xi or yi, so you need the swaps. If you didn't want to swap you could change your code to add either +1 or -1 on each loop. I would definitely include the divide by zero check. I'm not sure how your code managed to run without it.
• user3804: me neither..:-)
• JS1: @anonymous Actually, I think that a double divide by zero results in inf instead of throwing an exception.