{drop two images, a button and an openpicturedialog on a form
and then delete the unit code and paste in the code below
remember to double click the button to initiate the event handler
and then run to generate a histogram of an 8 bit grey scale bitmap}

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ExtCtrls, StdCtrls, ExtDlgs;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Image1: TImage;
    Image2: TImage;
    OpenPictureDialog1: TOpenPictureDialog;
    procedure Button1Click(Sender: TObject);
    procedure Histogram(var Bitmap : TBitmap;
                         var Histcount : array of integer);
    procedure Histogram100(var HistCount : array of integer);
    Procedure DrawHistogram(var Histcount : array of integer);

  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}


{the histogram function here only works on 8 bit grey scale bitmaps
so be sure to load a bitmap of this type to generate a valid histogram}

procedure TForm1.Button1Click(Sender: TObject);
var Bitmap : TBitmap;
    HistCount : array[0..255] of integer;
begin
     If OpenPictureDialog1.execute then begin
        Bitmap := TBitmap.create;
        Bitmap.LoadFromFile(OpenPictureDialog1.Filename);
        Image1.Picture.Bitmap := Bitmap;
        Histogram(Bitmap, HistCount);
        Histogram100(HistCount);
        DrawHistogram(Histcount);
     end;
end;


{to calculate a histogram of an 8 bit grey scale image in this example
we check each byte and then we increment the corresponding element in an array
each time we count a byte of a certain intensity value
for example if a byte has intensity 102 we increment array position 102 by one
each time we find the byte 102 in the bitmap}

procedure TForm1.Histogram(var Bitmap : TBitmap;
                           var Histcount : array of integer);
var
   x, y, i, j : integer;
   pb : pbytearray;
begin
      For i := 0 to 255 do begin
          HistCount[i] := 0;
      end;
      for y := 0 to Bitmap.height - 1 do begin
          pb := Bitmap.scanline[y];
          for x := 0 to Bitmap.width - 1 do begin

              {the C++ code to calculate the totals for the histogram
              j = *(Image->data+x+(long)y*image->cols);
               dereference the pointer to extract the byte in the bitmap}

              j := pb[x]; {that's it in Delphi, same thing}

              {next Increment the pixel count in the histogram for this
              pixel intensity, using j, the byte value as the index into
              the array}

              inc(HistCount[j]);
          end;
      end;
end;

{a histogram can have thousands of points of the same intensity
in order to output the histogram to a bitmap for display
the points will interpreted as a percentage of the maximum value
so the points will will be output as values between 0 and 100}

procedure TForm1.Histogram100(var HistCount : array of integer);
var maxi, i : integer;
begin
     maxi := 0;
     for i := 0 to 255 do begin    {find maximum value in array}
         if HistCount[i] > maxi then maxi := Histcount[i];
     end;
     If maxi < 100 then exit     {else make each histogram point <= 100}
     else begin
        for i := 0 to 255 do begin
            {express each histogram point as a percentage of maxi so that
            the histogram points will vary from 0 to 100}
            HistCount[i] := ((HistCount[i] * 100) div maxi);
        end;
     end;
end;

{draw the histogram on a bitmap}

Procedure TForm1.DrawHistogram(var Histcount : array of integer);
var
    x : integer;
    rect : Trect;
begin
    Image2.picture.Bitmap.pixelformat := pf8bit;
    Image2.Width := 256;
    Image2.Height := 200;
    Image2.picture.Bitmap.height := 200;
    Image2.picture.Bitmap.width := 256;

    rect.Left := 0;
    rect.right := Image2.Width - 1;
    rect.Top := 0;
    rect.Bottom := Image2.height - 1;
    with Image2.Picture.Bitmap do begin
       Canvas.Brush.Color := clWhite;
       Canvas.Brush.Style := bsSolid;
       Canvas.FillRect(rect);

       Canvas.Pen.Color := clBlack;
       Canvas.Pen.Style := psSolid;
       Canvas.Pen.Width := 1;
       Canvas.MoveTo(0, HistCount[0]);

       for x := 1 to 255 do begin
          Canvas.LineTo(x, 200 - HistCount[x]);
       end;
    end;
end;



end.


{included here is the C++ code which is discussed on this web page
http://www.awitness.org/delphi_pascal_tutorial/c++_delphi/grey_scale_histogram.html
This C++ code is equivalent to the Delphi Histogram procedure above..

void Histogram(struct Image *IMAGE, int HISTCOUNT[])
  {
     int x, y, i, j;
     for (i=0;i<=255;i++) HISTCOUNT[i] = 0;
     for (y=0; y<IMAGE->Rows; y++)
        {
        for (x=0; x<IMAGE->Cols; x++)
          {
             j = *(IMAGE->Data+x+(long)y*IMAGE->Cols);
             HISTCOUNT[j] = HISTCOUNT[j]+1;
          }
        }  
}