修改TPngImage的每像素alpha数据

var
  png: TPNGImage;
  sl: PByteArray;

...

for y := 0 to png.Height div 2 do
begin
  sl := png.AlphaScanline[y];
  FillChar(sl^, png.Width, 0);
end;

for y := 0 to png.Height do
begin
  sl := png.AlphaScanline[y];
  for x := 0 to png.Width - 1 do
    sl^[x] := byte(round(255*x/png.Width));
end;

(png.AlphaScanline[y]^)[x]

for Y := 0 to Image.Height - 1 do begin
  Line := Image.AlphaScanline[Y];
  for X := 0 to Image.Width - 1 do begin        
      Line[X] := ALPHA        
  end;
end;