MakeShadesofGrayImage from RGB Composite Image

PROCEDURE TForm1.MakeShadesOfGrayImage;
VAR
Gray : INTEGER;
i : INTEGER;
j : INTEGER;
rowRGB : pRGBTripleArray;
rowGray: pRGBTripleArray;
BEGIN
Screen.Cursor := crHourGlass;
TRY
FOR j := BitmapRGB.Height-1 DOWNTO 0 DO
BEGIN
rowRGB := BitmapRGB.Scanline[j];
rowGray := BitmapGray.Scanline[j];
FOR i := BitmapRGB.Width-1 DOWNTO 0 DO
BEGIN
// Intensity = (R + G + B) DIV 3
WITH rowRGB[i] DO
Gray := (rgbtRed + rgbtGreen + rgbtBlue) DIV 3;

WITH rowGray[i] DO
BEGIN
rgbtRed := Gray;
rgbtGreen := Gray;
rgbtBlue := Gray
END

END
END;
FINALLY
Screen.Cursor := crDefault
END
END;