How to implement the scanline access of TBitmap correctly?

Question

I am trying to access the scanline of a Bitmap according to an article on Embarcadero. Using scanlines like

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

I have implemented before. I noticed that accessing a scanline takes relatively much time and the article mentioned above offers a solution to that. I am not able to implement it correctly. My code is:

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.

And I always get an illegal access when y=1 and x=0. LineLength is negative (the width of the bitmap), but that might be expected. What am I doing wrong?

EDIT: The code above is changed to reflect the remarks processed till so far.

Answer 1

To not to access any negative index, I would do

procedure TForm1.Button1Click(Sender: TObject);
var Bitmap: TBitmap;
    q: TRGBQuad;
    x, y: LongInt;
    line{, FirstLine}: PRGBQuadArray;
    idx: NativeInt;
    LastLine: PRGBQuadArray;
    LineLength: NativeInt;
begin
   q.r := 0; q.g := 0;
   Bitmap := TBitmap.Create;
   Bitmap.Height := Image.Height;
   Bitmap.Width  := Image.Width;
   Bitmap.PixelFormat := pf32Bit;

   LastLine := Bitmap.ScanLine[Bitmap.Height - 1];
   LineLength := (NativeInt(Bitmap.Scanline[Bitmap.Height - 2]) - NativeInt(Lastline)) div SizeOf(TRGBQuad);
   try
      for y := 0 to Bitmap.Height - 1 do
      begin
         for x := 0 to Bitmap.Width - 1 do
         begin
            q.b := (x xor y) mod 255;
            idx := y * LineLength + x;
            LastLine [idx] := q;
         end; // for
      end; // for
      Image.Picture.Assign (Bitmap);
   finally
       Bitmap.Free;
   end; // try..finally
end;

Answer 2

LineLength is negative for a lot of bitmaps, because they frequently use bottom-up method to store lines. MSDN: BITMAPINFOHEADER. So this solution should be modified for such case.