Task

Implement the   Xiaolin Wu's line algorithm   described in Wikipedia.



This algorithm draws anti-aliased lines.



Related task

   See   Bresenham's line algorithm   for aliased lines.


package raster
 
import "math"
 
func ipart(x float64) float64 {
    return math.Floor(x)
}
 
func round(x float64) float64 {
    return ipart(x + .5)
}
 
func fpart(x float64) float64 {
    return x - ipart(x)
}
 
func rfpart(x float64) float64 {
    return 1 - fpart(x)
}
 
// AaLine plots anti-aliased line by Xiaolin Wu's line algorithm.
func (g *Grmap) AaLine(x1, y1, x2, y2 float64) {
    // straight translation of WP pseudocode
    dx := x2 - x1
    dy := y2 - y1
    ax := dx
    if ax < 0 {
        ax = -ax
    }
    ay := dy
    if ay < 0 {
        ay = -ay
    }
    // plot function set here to handle the two cases of slope
    var plot func(int, int, float64)
    if ax < ay {
        x1, y1 = y1, x1
        x2, y2 = y2, x2
        dx, dy = dy, dx
        plot = func(x, y int, c float64) {
            g.SetPx(y, x, uint16(c*math.MaxUint16))
        }
    } else {
        plot = func(x, y int, c float64) {
            g.SetPx(x, y, uint16(c*math.MaxUint16))
        }
    }
    if x2 < x1 {
        x1, x2 = x2, x1
        y1, y2 = y2, y1
    }
    gradient := dy / dx
 
    // handle first endpoint
    xend := round(x1)
    yend := y1 + gradient*(xend-x1)
    xgap := rfpart(x1 + .5)
    xpxl1 := int(xend) // this will be used in the main loop
    ypxl1 := int(ipart(yend))
    plot(xpxl1, ypxl1, rfpart(yend)*xgap)
    plot(xpxl1, ypxl1+1, fpart(yend)*xgap)
    intery := yend + gradient // first y-intersection for the main loop
 
    // handle second endpoint
    xend = round(x2)
    yend = y2 + gradient*(xend-x2)
    xgap = fpart(x2 + 0.5)
    xpxl2 := int(xend) // this will be used in the main loop
    ypxl2 := int(ipart(yend))
    plot(xpxl2, ypxl2, rfpart(yend)*xgap)
    plot(xpxl2, ypxl2+1, fpart(yend)*xgap)
 
    // main loop
    for x := xpxl1 + 1; x <= xpxl2-1; x++ {
        plot(x, int(ipart(intery)), rfpart(intery))
        plot(x, int(ipart(intery))+1, fpart(intery))
        intery = intery + gradient
    }
}