窗体背景渐变

var i: word;
dy, y: real;
begin
dy := clientheight / 256;
y := 0;
for i := 255 downto 0 do
begin
canvas.brush.color := $00000000 + i * $10000;
canvas.fillrect(rect(0, round, clientwidth, round(y + dy)));
y := y + dy;
end;
end;