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up vote -1 down vote favorite

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

share|improve this question
up vote 1 down vote

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;
    }   
}
share|improve this answer
    
Is swapping required? Is divide by zero testing required? – anonymous Aug 20 '15 at 17:20
    
@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. – JS1 Aug 20 '15 at 18:11
    
me neither..:-) – anonymous Aug 20 '15 at 18:15
    
@anonymous Actually, I think that a double divide by zero results in inf instead of throwing an exception. – JS1 Aug 20 '15 at 18:36

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