如何让我这样定义的指针指向bmp图的第一个象素？

......
type
    TRGB=record
         R,G,B:BYTE;
         end;
    TRGBLine=array[0..65535]of TRGB;
    PRGBLine=^TRGBLine;
......
procedure TForm1.Button2Click(Sender: TObject);
var
    i,j,k:integer;
    scanline:PRGBLine;//我指的就是这个指针，我想让它指向image2的第一个象素；
    a,d,c,t,z:byte;
    m,n,m1,n1:double;
    h:Thue;
    r,g,b:byte;
begin
    j:=0;
    quantization(image2);
    for k:=0 to ((image2.Width)*(image2.height)-1) do
    begin
    a:=(hsvdata[k]) div 32;
    d:=(hsvdata[k] mod 32)div 4;
    c:=(hsvdata[k] mod 128) ;
        if a=0 then
          h:=10
        else if a=1 then
          h:=30
        else if a=2 then
          h:=60
        else if a=3 then
          h:=100
        else if a=4 then
          h:=180
        else if a=5 then
          h:=250
        else if a=6 then
          h:=280
        else if a=7 then
          h:=310;        if d=0 then
          m:=0.1
        else if d=1 then
          m:=0.5
        else if d=2 then
          m:=0.8
        else m:=0;        if c=0 then
          n:=0.1
        else if c=1 then
          n:=0.5
        else if c=2 then
          n:=0.8
        else n:=0;
        m1:=m*255;
        n1:=n*255;
        t:=trunc(m1);
        z:=trunc(n1);
        hsvtorgb(h,t,z,r,g,b);
        scanline[j].R:=r;
        scanline[j].G:=g;
        scanline[j].B:=b;
        inc(j);
    end;
    Image2.Refresh;
end;
end