In Delphi, how does TBitmap.Monochrome and .PixelFormat influcence the format of .ScanLine?

I want to assign a given buffer with a bitmap in Mono8 format (Monochrome 8 Bits) to a bitmap. I then assign the resulting bitmap to a TImage component to display it. The pictures are screenshots of the resulting display.

The following code works but seems a bit wasteful:

procedure CopyToBitmapMono824(_Buffer: PByte; _Bmp: TBitmap);
var
  y: Integer;
  x: Integer;
  ScanLine: PdzRgbTripleArray;
begin
  for y := 0 to _Bmp.Height - 1 do begin
    ScanLine := _Bmp.ScanLine[y];
    for x := 0 to _Bmp.Width - 1 do begin
      // monochrome: all 3 colors set to the same value
      ScanLine[x].Red := _Buffer^;
      ScanLine[x].Green := _Buffer^;
      ScanLine[x].Blue := _Buffer^;
      Inc(_Buffer);
    end;
  end;
end;

// [...]
fBmp.PixelFormat := pf24Bit;
FBmp.Monochrome := False;
CopyToBitmap(Buffer, fBmp);

I would rather use a bitmap in pf8Bit format which I tried:

procedure CopyToBitmapMono8(_Buffer: PByte; _Bmp: TBitmap);
var
  y: Integer;
  x: Integer;
  ScanLine: PByteArray;
begin
  for y := 0 to _Bmp.Height - 1 do begin
    ScanLine := _Bmp.ScanLine[y];
    for x := 0 to _Bmp.Width - 1 do begin
      ScanLine[x] := _Buffer^;
      Inc(_Buffer);
    end;
  end;
end;

// [...]
FBmp.PixelFormat := pf8bit;
FBmp.Monochrome := False; // I also tried Monochrome := true
CopyToBitmapMono8(Buffer, FBmp)

If MonoChrome is true, the picture only has about 1/4 of the expected width, the rest is white.

If MonoChrome is false, the picture has the expected width, but the left 1/4 of it is monochrome, the rest contains false colors.

I'm obviously missing something, but what?

EDIT: The effect that the bitmap is only 1/4 of the expected size apparently was a side effect of converting it to a JPEG for saving prior to displaying it (code that I did not show above, mea culpa). So the problem was simply that I did not set a monochrome palette for the bitmap.

Monochrome has sense for pf1bit bitmaps.

Otherwise Monochrome := True changes bitmap format to DDB (pfDevice). Your screen is 32-bit, so call to Scanline caused DibNeeded call and transformation to 32bit, and using of function CopyToBitmapMono8 (intended for 8-bit) filled only 1/4 of screen.

For proper usage of 8-bit bitmaps you have to change standard weird palette (used in the right part of last image) to gray one.

procedure CopyToBitmapMono8(_Buffer: PByte; _Bmp: TBitmap);
var
  y: Integer;
  x: Integer;
  ScanLine: PByteArray;
begin
  for y := 0 to _Bmp.Height - 1 do begin
    ScanLine := _Bmp.ScanLine[y];
    for x := 0 to _Bmp.Width - 1 do begin
      ScanLine[x] := _Buffer^;
      Inc(_Buffer);
    end;
  end;
end;

var
   FBmp: TBitmap;
   Buffer: PbyteArray;
   i: integer;
begin
  GetMem(Buffer, 512 * 100);
  for i := 0 to 512 * 100 - 1 do
     Buffer[i] := (i and 511) div 2; // gray gradient

  FBmp := Tbitmap.Create;
  FBmp.Width := 512;
  FBmp.Height := 100;
  FBmp.PixelFormat := pf8bit;
  CopyToBitmapMono8(PByte(Buffer), FBmp);
  Canvas.Draw(0, 0, FBmp);

  //now right approach
  FBmp.Palette := MakeGrayPalette; // try to comment
  CopyToBitmapMono8(PByte(Buffer), FBmp);
  Canvas.Draw(0, 110, FBmp);

end;

function TForm1.MakeGrayPalette: HPalette;
var
  i: integer;
  lp:  TMaxLogPalette;
begin
  lp.palVersion    := $300;
  lp.palNumEntries := 256;
  for i := 0 TO 255 do begin
     lp.palPalEntry[i].peRed   := i;
     lp.palPalEntry[i].peGreen := i;
     lp.palPalEntry[i].peBlue  := i;
     lp.palPalEntry[i].peFlags := PC_RESERVED;
   end;
   Result := CreatePalette(pLogPalette(@lp)^);
end;

And example at efg2 page