# All cases covered Bresenham's line-algorithm

I need to check all pixels in a line, so I'm using Bresenham's algorithm to access each pixel in it. In particular I need to check if all pixels are located on valid pixel of a bitmap. This is the code:

```private void Bresenham(Point p1, Point p2, ref List<Point> track) {
int dx = p2.X - p1.X;
int dy = p2.Y - p1.Y;

int swaps = 0;
if (dy > dx) {
Swap(ref dx, ref dy);
swaps = 1;
}

int a = Math.Abs(dy);
int b = -Math.Abs(dx);

double d = 2*a + b;
int x = p1.X;
int y = p1.Y;
color_track = Color.Blue;
Check_Pixel(ref area, new Point(x,y));
track.Clear();

int s = 1;
int q = 1;
if (p1.X > p2.X) q = -1;
if (p1.Y > p2.Y) s = -1;

for(int k = 0; k < dx; k++) {
if (d >= 0) {
d = 2*(a+b) + d;
y = y + s;
x = x + q;
}
else {
if (swaps == 1) y = y + s;
else x = x + q;
d = 2 * a + d;
}
Check_Pixel(ref area, new Point(x, y));
}
}

private void Swap(ref int x, ref int y) {
int temp = x;
x = y;
y = temp;
}
```

Check_Pixel is used to check if the line pixel is on a valid bitmap one. I add the pixel founded to a List<Point> to render point by point his element and be sure Bresenham checks the correct pixels (it does). The problem is that my algorithm doesn't cover all the cases but about 50% of them.

P.S: my coordinates system origin is on the left-upper corner (x grows left to right, y up to down)

Find the complete version:

```public void line(int x,int y,int x2, int y2, int color) {
int w = x2 - x ;
int h = y2 - y ;
int dx1 = 0, dy1 = 0, dx2 = 0, dy2 = 0 ;
if (w<0) dx1 = -1 ; else if (w>0) dx1 = 1 ;
if (h<0) dy1 = -1 ; else if (h>0) dy1 = 1 ;
if (w<0) dx2 = -1 ; else if (w>0) dx2 = 1 ;
int longest = Math.Abs(w) ;
int shortest = Math.Abs(h) ;
if (!(longest>shortest)) {
longest = Math.Abs(h) ;
shortest = Math.Abs(w) ;
if (h<0) dy2 = -1 ; else if (h>0) dy2 = 1 ;
dx2 = 0 ;
}
int numerator = longest >> 1 ;
for (int i=0;i<=longest;i++) {
putpixel(x,y,color) ;
numerator += shortest ;
if (!(numerator<longest)) {
numerator -= longest ;
x += dx1 ;
y += dy1 ;
} else {
x += dx2 ;
y += dy2 ;
}
}
}
```