# Bresenham's Algorithm not drawing horizontal lines

I'm getting started with graphics and I'm having trouble implementing the Bresenham's algorithm. Now, I know that my implementation is incomplente, but I can't see why it doesn't make horizontal lines(same height(y) values, different width(x values). I get a floating point exception. Here's the algo:

int draw_line(void *mlx_ptr, void*win_ptr, t_2dpoint *initial_p, t_2dpoint *final_p)
{
53     int delta_x;

53     int delta_x;
54     int y;
55     int delta_y;
56
57     delta_x = final_p->x - initial_p->x;
58     delta_y = final_p->y - initial_p->y;
59
60     x = initial_p->x;
61     y = initial_p->y;
62     e = (float)(delta_x / delta_y) - 1.0;
63
64     while(x < final_p->x - 1)
65     {
66         mlx_pixel_put(mlx_ptr, win_ptr, x, y, 0XFF00FF);
67         if ( e >= 0)
68         {
69             y += 1;
70             e -= 1.0;
71         }
72         x += 1;
73         e += (float)delta_x / delta_y;
74     }
75     mlx_pixel_put(mlx_ptr, win_ptr, x, y, 0XFF00FF);
76 }


e = (float)(delta_x / delta_y) - 1.0;

e += (float)delta_x / delta_y;


Either of these lines will cause a divide-by-zero error when delta_y is zero, which is when the line is horizontal (or zero-length). You need to check for this condition and have some special-case code.

As an aside, I don't know why people still teach Bresenham's algorithm to new CG students. It's a good example of an algorithm but it's not very useful knowledge in itself: you're unlikely to need to know it unless it's for a job interview.

• The underlying principle of Bresenham (the error value getting accumulated and moved back to 0 as events happen) is nice but no-one seems to teach it – ratchet freak Aug 7 '18 at 14:58
• Yep, that fixes the floating point exception, now I just need to find the math logic. What algorithm you'd recommend? – koperandus Aug 8 '18 at 16:38
• Well, if you want to draw lines one pixel at a time, Bresenham's algorithm is the right one. But why do you want to draw lines one pixel at a time? Any OpenGL implementation will draw them all at once for you. – Dan Hulme Aug 8 '18 at 17:04
• @DanHulme Sometimes it's useful to know a little history.... even if it's only to give you a greater appreciation of the present ;-) – Simon F Sep 7 '18 at 8:20