Xiaolin Wu's line algorithm is an algorithm for line antialiasing.
Xiaolin Wu's line algorithm was presented in the article "An Efficient Antialiasing Technique" in the July 1991 issue of Computer Graphics, as well as in the article "Fast Antialiasing" in the June 1992 issue of Dr. Dobb's Journal.
Bresenham's algorithm draws lines extremely quickly, but it does not perform anti-aliasing. In addition, it cannot handle any cases where the line endpoints do not lie exactly on integer points of the pixel grid. A naive approach to anti-aliasing the line would take an extremely long time. Wu's algorithm is comparatively fast, but is still slower than Bresenham's algorithm. The algorithm consists of drawing pairs of pixels straddling the line, each coloured according to its distance from the line. Pixels at the line ends are handled separately. Lines less than one pixel long are handled as a special case.
An extension to the algorithm for circle drawing was presented by Xiaolin Wu in the book Graphics Gems II. Just as the line drawing algorithm is a replacement for Bresenham's line drawing algorithm, the circle drawing algorithm is a replacement for Bresenham's circle drawing algorithm.
// integer part of xfunction ipart(x) is return floor(x)
function round(x) is return ipart(x + 0.5)
// fractional part of xfunction fpart(x) is return x - ipart(x)
function rfpart(x) is return 1 - fpart(x)
function drawLine(x0,y0,x1,y1) is boolean steep := abs(y1 - y0) > abs(x1 - x0) if steep then swap(x0, y0) swap(x1, y1) end if if x0 > x1 then swap(x0, x1) swap(y0, y1) end if dx := x1 - x0 dy := y1 - y0
if dx
// handle first endpoint xend := round(x0) yend := y0 + gradient * (xend - x0) xgap := rfpart(x0 + 0.5) xpxl1 := xend // this will be used in the main loop ypxl1 := ipart(yend) if steep then plot(ypxl1, xpxl1, rfpart(yend) * xgap) plot(ypxl1+1, xpxl1, fpart(yend) * xgap) else plot(xpxl1, ypxl1, rfpart(yend) * xgap) plot(xpxl1, ypxl1+1, fpart(yend) * xgap) end if intery := yend + gradient // first y-intersection for the main loop // handle second endpoint xend := round(x1) yend := y1 + gradient * (xend - x1) xgap := fpart(x1 + 0.5) xpxl2 := xend //this will be used in the main loop ypxl2 := ipart(yend) if steep then plot(ypxl2, xpxl2, rfpart(yend) * xgap) plot(ypxl2+1, xpxl2, fpart(yend) * xgap) else plot(xpxl2, ypxl2, rfpart(yend) * xgap) plot(xpxl2, ypxl2+1, fpart(yend) * xgap) end if // main loop if steep then for x from xpxl1 + 1 to xpxl2 - 1 do begin plot(ipart(intery), x, rfpart(intery)) plot(ipart(intery)+1, x, fpart(intery)) intery := intery + gradient end else for x from xpxl1 + 1 to xpxl2 - 1 do begin plot(x, ipart(intery), rfpart(intery)) plot(x, ipart(intery)+1, fpart(intery)) intery := intery + gradient end end ifend function