"""Script demonstrating drawing of anti-aliased lines using Xiaolin Wu's line
algorithm

usage: python xiaolinwu.py [output-file]

"""
from __future__ import division
import sys

from PIL import Image


def _fpart(x):
    return x - int(x)

def _rfpart(x):
    return 1 - _fpart(x)

def putpixel(img, xy, color, alpha=1):
    """Paints color over the background at the point xy in img.

    Use alpha for blending. alpha=1 means a completely opaque foreground.

    """
    c = tuple(map(lambda bg, fg: int(round(alpha * fg + (1-alpha) * bg)),
                  img.getpixel(xy), color))
    img.putpixel(xy, c)

def draw_line(img, p1, p2, color):
    """Draws an anti-aliased line in img from p1 to p2 with the given color."""
    x1, y1 = p1
    x2, y2 = p2
    dx, dy = x2-x1, y2-y1
    steep = abs(dx) < abs(dy)
    p = lambda px, py: ((px,py), (py,px))[steep]

    if steep:
        x1, y1, x2, y2, dx, dy = y1, x1, y2, x2, dy, dx
    if x2 < x1:
        x1, x2, y1, y2 = x2, x1, y2, y1

    grad = dy/dx
    intery = y1 + _rfpart(x1) * grad
    def draw_endpoint(pt):
        x, y = pt
        xend = round(x)
        yend = y + grad * (xend - x)
        xgap = _rfpart(x + 0.5)
        px, py = int(xend), int(yend)
        putpixel(img, p(px, py), color, _rfpart(yend) * xgap)
        putpixel(img, p(px, py+1), color, _fpart(yend) * xgap)
        return px

    xstart = draw_endpoint(p(*p1)) + 1
    xend = draw_endpoint(p(*p2))

    for x in range(xstart, xend):
        y = int(intery)
        putpixel(img, p(x, y), color, _rfpart(intery))
        putpixel(img, p(x, y+1), color, _fpart(intery))
        intery += grad


if __name__ == '__main__':
    if len(sys.argv) != 2:
        print 'usage: python xiaolinwu.py [output-file]'
        sys.exit(-1)

    blue = (0, 0, 255)
    yellow = (255, 255, 0)
    img = Image.new("RGB", (500,500), blue)
    for a in range(10, 431, 60):
        draw_line(img, (10, 10), (490, a), yellow)
        draw_line(img, (10, 10), (a, 490), yellow)
    draw_line(img, (10, 10), (490, 490), yellow)
    filename = sys.argv[1]
    img.save(filename)
    print 'image saved to', filename