{
This code draws an anti-aliased line on a bitmap
This means that the line is not jagged like the
lines drawn using the LineTo() function
}

uses
Graphics, Windows;

type
TRGBTripleArray = array[0..1000] of TRGBTriple;
PRGBTripleArray = ^TRGBTripleArray;

// anti-aliased line
procedure WuLine(ABitmap : TBitmap ; Point1, Point2 : TPoint ; AColor : TColor);
var
deltax, deltay, loop, start, finish : integer;
dx, dy, dydx : single; // fractional parts
LR, LG, LB : byte;
x1, x2, y1, y2 : integer;
begin
x1 := Point1.X; y1 := Point1.Y;
x2 := Point2.X; y2 := Point2.Y;
deltax := abs(x2 - x1); // Calculate deltax and deltay for initialisation
deltay := abs(y2 - y1);
if (deltax = 0) or (deltay = 0) then begin // straight lines
ABitmap.Canvas.Pen.Color := AColor;
ABitmap.Canvas.MoveTo(x1, y1);
ABitmap.Canvas.LineTo(x2, y2);
exit;
end;
LR := (AColor and $000000FF);
LG := (AColor and $0000FF00) shr 8;
LB := (AColor and $00FF0000) shr 16;
if deltax > deltay then
begin // horizontal or vertical
if y2 > y1 then // determine rise and run
dydx := -(deltay / deltax)
else
dydx := deltay / deltax;
if x2 < x1 then
begin
start := x2; // right to left
finish := x1;
dy := y2;
end else
begin
start := x1; // left to right
finish := x2;
dy := y1;
dydx := -dydx; // inverse slope
end;
for loop := start to finish do
begin
AlphaBlendPixel(ABitmap, loop, trunc(dy), LR, LG, LB, 1 - frac(dy));
AlphaBlendPixel(ABitmap, loop, trunc(dy) + 1, LR, LG, LB, frac(dy));
dy := dy + dydx; // next point
end;
end else
begin
if x2 > x1 then // determine rise and run
dydx := -(deltax / deltay)
else
dydx := deltax / deltay;
if y2 < y1 then
begin
start := y2; // right to left
finish := y1;
dx := x2;
end else
begin
start := y1; // left to right
finish := y2;
dx := x1;
dydx := -dydx; // inverse slope
end;
for loop := start to finish do
begin
AlphaBlendPixel(ABitmap, trunc(dx), loop, LR, LG, LB, 1 - frac(dx));
AlphaBlendPixel(ABitmap, trunc(dx) + 1, loop, LR, LG, LB, frac(dx));
dx := dx + dydx; // next point
end;
end;
end;

// blend a pixel with the current colour
procedure AlphaBlendPixel(ABitmap : TBitmap ; X, Y : integer ; R, G, B : byte ; ARatio : Real);
Var
LBack, LNew : TRGBTriple;
LMinusRatio : Real;
LScan : PRGBTripleArray;
begin
if (X < 0) or (X > ABitmap.Width - 1) or (Y < 0) or (Y > ABitmap.Height - 1) then
Exit; // clipping
LScan := ABitmap.Scanline[Y];
LMinusRatio := 1 - ARatio;
LBack := LScan[X];
LNew.rgbtBlue := round(B*ARatio + LBack.rgbtBlue*LMinusRatio);
LNew.rgbtGreen := round(G*ARatio + LBack.rgbtGreen*LMinusRatio);
LNew.rgbtRed := round(R*ARatio + LBack.rgbtRed*LMinusRatio);
LScan[X] := LNew;
end;