请问如何读取一幅JPEG图像的RGB值?(100分)

我使用的是TJEPGIMAGE类,图像已经读入
var
JpegImage1:TJPegImage;
Bitmap1:Tbitmap;
begin
jpegimage1:=TJpegimage.Create;
jpegimage1.LoadFromFile('hi.jpg');

////////////此处读取图像的RGB值并处理，请问如何做？

jpegimage1.Free;

end;

转为bmp

请问怎么转啊？

procedure TForm1.Button1Click(Sender: TObject);
var
jpeg: TJPEGImage;
bmp: TBitmap;
begin
if OpenPictureDialog1.Execute then
begin
jpeg := TJPEGImage.Create;
try
jpeg.LoadFromFile(OpenPictureDialog1.filename);
bmp := TBitmap.Create;
try
bmp.Assign(jpeg);
Image1.Picture.Graphic := bmp;
//此处读取图像的RGB值并处理
finally
bmp.free
end;
finally
jpeg.free
end;
end;
end;

谢谢linsb和影子，再请问得到bmp图像后怎么读取每个像素的RGB值呢,需要转换吗?

GetRValue
GetGValue
GetBValue

例子：
function ConvertBitmapToGrayscale1(const Bitmap: TBitmap): TBitmap;
var
i, j: Integer;
Grayshade, Red, Green, Blue: Byte;
PixelColor: Longint;
begin
with Bitmap do
for i := 0 to Width - 1 do
for j := 0 to Height - 1 do
begin
PixelColor := ColorToRGB(Canvas.Pixels[i, j]);
Red := PixelColor;
Green := PixelColor shr 8;
Blue := PixelColor shr 16;
Grayshade := Round(0.3 * Red + 0.6 * Green + 0.1 * Blue);
Canvas.Pixels[i, j] := RGB(Grayshade, Grayshade, Grayshade);
end;
Result := Bitmap;
end;

var
jpeg: TJPEGImage;
bmp: TBitmap;
begin
if OpenPictureDialog1.Execute then
begin
jpeg := TJPEGImage.Create;
try
jpeg.LoadFromFile(OpenPictureDialog1.filename);
bmp := TBitmap.Create;
try
bmp.Assign(jpeg);
//此处读取图像的RGB值并处理－图像的灰度处理
ConvertBitmapToGrayscale1(bmp);
Image1.Picture.Graphic := bmp;
finally
bmp.free
end;
finally
jpeg.free
end;
end;
end;

to linsb:
这段代码对小点的图片处理起来比较快,可对大点的图片进行灰度化比较慢了
能不能有个更好的办法?