How to fix negative slope issue?

Question

My program to draw lines, running on Python 3.12 (latest version), is working in every way except for negative slopes that are not 1. It could be a stupid error, or something that is hard to find, but either way, I would like some help. This module is called Kandinsky; it's a module built-in to Numworks calculators. The only thing notable is that, like in Box2D, the Y-coordinates increase when going downwards, so negative-looking slopes are actually positive, and vice versa. By the way, the algorithm used is the Bresenham's line algorithm, which is pretty popular, from what I've heard. Here is the code:

from kandinsky import *
def draw_line(x1, y1, x2, y2, color):
    if x2 - x1 != 0:
        m = (y2 - y1) / (x2 - x1)
    elif y2 - y1 > 0:
        m = 500
    elif y2 - y1 < 0:
        m = -500
    if x1 > x2:
        x1, x2 = x2, x1
        y1, y2 = y2, y1
    if abs(m) < 1:
        A = 2 * (y2 - y1)
        B = A - 2 * (x2 - x1)
        P = A - (x2 - x1)
        set_pixel(x1, y1, color)
        for i in range(x1 + 1, x2 + 1):
            if P < 0:
                P += A
            else:
                P += B
                if m < 0:
                    y1 -= 1
                else:
                    y1 += 1
            set_pixel(i, y1, color)
    elif abs(m) > 1:
        A = 2 * (x2 - x1)
        B = A - 2 * (y2 - y1)
        P = A - (y2 - y1)
        set_pixel(x1, y1, color)
        for i in range(y1 + 1, y2 + 1):
            if P < 0:
                P += A
            else:
                P += B
                if m < 0:
                    x1 -= 1
                else:
                    x1 += 1
            set_pixel(x1, i, color)
    elif abs(m) == 1:
        for i in range(x1, x2 + 1):
            set_pixel(x1, y1, color)
            x1 += 1
            if m == 1:
                y1 += 1
            else:
                y1 -= 1

I don't know what's going wrong...

I tried checking the code, everything was fine as it looked to me. I did positive slopes, they work fine. Other lines with slope 1, -1, 0 or infinity in both sides of the number line work fine, thanks to the extra implementation. The only problem is the negative slopes. Please help!

Edit: I forgot to add what goes wrong when I try it anyways... oops! Basically the line doesn't slope at all, it just stays horizontal on the x1 coordinate.

Answer 1

When you are finding the slope 'm', you are not dealing with negative slopes correctly Instead of

m = (y2 - y1) / (x2 - x1)

Use integer division

m = (y2 - y1) // (x2 - x1)

Answer 2

All you need to do is a translation since the pixel cannot have negative coordinates.So assume your canvas is 1024x512 then just translate everything to 512 pixels right and 256 pixels top