delphi – 如何正确实现TBitmap的扫描线访问？

for y := 0 to n do
begin
   line := bitmap.scanline [y];
   for x := 0 to n do line [x] := value;

我之前已经实施过.我注意到访问扫描线需要相当多的时间,上面提到的文章提供了解决方案.我无法正确实现它.我的代码是：

unit SCTester;

interface

uses Windows,Messages,SysUtils,Variants,Classes,Graphics,Controls,Forms,ExtCtrls;

type
   TRGBQuad = packed record
      b: uInt8;
      g: uInt8;
      r: uInt8;
      alpha: uInt8;
   end; // Record: TQuad //

// Override the definitions in Graphics.pas
   TRGBQuadArray = packed array [0..MaxInt div SizeOf (TRGBQuad) - 1] of TRGBQuad;
   PRGBQuadArray = ^TRGBQuadArray;

  TForm1 = class(TForm)
    Image: TImage;
    procedure ImageDblClick(Sender: TObject);
  end;

var Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.ImageDblClick(Sender: TObject);
var Bitmap: TBitmap;
    q: TRGBQuad;
    x,y: NativeInt;
    FirstLine: PRGBQuadArray;
    idx: NativeInt;
    LineLength: NativeInt;
begin
   q.r := 0; q.g := 0;
   Bitmap := TBitmap.Create;
   Bitmap.Height := Image.Height;
   Bitmap.Width  := Image.Width;
   Bitmap.PixelFormat := pf32Bit;
   FirstLine := Bitmap.ScanLine [0];
   LineLength := (NativeInt (Bitmap.Scanline [1]) - NativeInt (FirstLine)) div SizeOf (TRGBQuad);
   try
      for y := Bitmap.Height - 1 downto 0 do
      begin
         for x := 0 to Bitmap.Width - 1 do
         begin
            q.b := (x xor y) mod 255;
            idx := y * LineLength + x;
            FirstLine [idx] := q;
         end; // for
      end; // for
      Image.Picture.Assign (Bitmap);
   finally
       Bitmap.Free;
   end; // try..finally
end;

end.

当y = 1且x = 0时,我总是得到非法访问. LineLength是负数(位图的宽度),但可能是预期的.我究竟做错了什么？

编辑：上面的代码已更改,以反映到目前为止处理的备注.