Computing and Displaying Image Histogram in Delphi

To make our own histogram analysis using Delphi, first you need to make (or download the source code) of the graphical user interface for image processing we described in the previous chapter. To compute the image histogram, you need to make a vector variable to store the count of each pixel level. After that, you may begin to check the pixel value, and increment the variable that store the pixels count for that level. Histogram for color pixel is computed in the same way, but independently done for each R, G, and B element.

Creating Histogram Form

1. Open the GUI project created in the previous chapter.
2. Create a new form, change the form name property to HistogramForm, change the Caption to 'Histogram', set the ClientHeight property to 170, set the ClientWidth property to 275, and set the form style property to fsMDIChild.Make OnClose event handler, click Events tab on the object inspector, double click the OnClose event handler edit box. You'll be directed to the event handler, and change the Action variable to caFree between begin and end.
   

   
   procedure THistogramForm.FormClose(Sender: TObject;
   var Action: TCloseAction);
   begin
   Action:=caFree;
   end;
   

3. Go to menu Project->Options, on the Forms page, click HistogramForm to select, and press > to move the it from auto created forms list to available forms list.
   
4. Save the unit (File->Save) as HistogramUnit.

Adding Vector Variables to Store The Histogram

To add variables to store the histogram, add the variables to the original HistogramForm abstraction in private section. Also add a procedure ShowHistogram() to compute histogram in the public section. Add ExtCtrls in under the uses of the interface section, as we're going to use TImage in the form class abstraction.

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

type
THistogramForm = class(TForm)
procedure FormClose(Sender: TObject; var Action: TCloseAction);
private
{ Private declarations }
MaxCount:Integer;
HistogramGray:Array[0..255]of Integer;
HistogramRed:Array[0..255]of Integer;
HistogramGreen:Array[0..255]of Integer;
HistogramBlue:Array[0..255]of Integer;
public
{ Public declarations }
procedure ShowHistogram(Image:TImage);
end;

Writing The Histogram Computation Procedure
In the implementation, write the ShowHistogram procedure as shown below:

procedure THistogramForm.ShowHistogram(Image:TImage);
var
i,j:integer;
pixelPointer:PByteArray;
begin
try
begin
 for i:=0 to 255 do
 begin
   HistogramGray[i]:=0;
   HistogramRed[i]:=0;
   HistogramGreen[i]:=0;
   HistogramBlue[i]:=0;
 end;
 if Image.Picture.Bitmap.PixelFormat=pf8bit then
 begin
   for i:=0 to Image.Height-1 do
   begin
     pixelPointer:=Image.Picture.Bitmap.ScanLine[i];
     for j:=0 to Image.Width-1 do
     begin
       Inc(HistogramGray[pixelPointer[j]]);
     end;
   end;
   MaxCount:=0;
   for i:=0 to 255 do
     if HistogramGray[i]>MaxCount then
   MaxCount:=HistogramGray[i];
 end;
 if Image.Picture.Bitmap.PixelFormat=pf24bit then
 begin
   for i:=0 to Image.Height-1 do
   begin
     pixelPointer:=Image.Picture.Bitmap.ScanLine[i];
     for j:=0 to Image.Width-1 do
     begin
       Inc(HistogramBlue[pixelPointer[3*j]]);
       Inc(HistogramGreen[pixelPointer[3*j+1]]);
       Inc(HistogramRed[pixelPointer[3*j+2]]);
     end;
   end;
   for i:=0 to 255  do
   begin
     if HistogramRed[i]>MaxCount then
       MaxCount:=HistogramRed[i];
     if HistogramGreen[i]>MaxCount then
       MaxCount:=HistogramGreen[i];
     if HistogramBlue[i]>MaxCount then
       MaxCount:=HistogramBlue[i];
   end;
 end;
 Canvas.MoveTo(10, 160);
 Canvas.Pen.Color:=clBlack;
 for i:=0 to 255 do
   Canvas.LineTo(10+i,
     160-round(150*HistogramGray[i]/MaxCount));
 Canvas.Pen.Color:=clRed;
 Canvas.MoveTo(10, 160);
 for i:=0 to 255 do
   Canvas.LineTo(10+i,
     160-(round(150*HistogramRed[i]/MaxCount)));
 Canvas.Pen.Color:=clGreen;
 Canvas.MoveTo(10, 160);
 for i:=0 to 255 do
   Canvas.LineTo(10+i,
     160-(round(150*HistogramGreen[i]/MaxCount)));
 Canvas.Pen.Color:=clBlue;
 Canvas.MoveTo(10, 160);
 for i:=0 to 255 do
   Canvas.LineTo(10+i,
     160-(round(150*HistogramBlue[i]/MaxCount)));
end;
except
 Free; //free the histogram form if an exception happens
 ShowMessage('Operation is not completed');
end;
end;

Histogram Form Repainting
When the form is activated from minimized condition, we need to redraw the canvas we have drawn on ShowHistogram procedure. Place a focus on the histogram form, go to object inspector, on the Events tab, double click OnPaint event and we will be directed to the event handler, edit as shown below:

procedure THistogramForm.FormPaint(Sender: TObject);
var
i:integer;
begin
Canvas.MoveTo(10, 160);;
Canvas.Pen.Color:=clBlack;
for i:=0 to 255 do
Canvas.LineTo(10+i,
 160-round(150*HistogramGray[i]/MaxCount));
Canvas.Pen.Color:=clRed;
Canvas.MoveTo(10, 160);
for i:=0 to 255 do
 Canvas.LineTo(10+i,
  160-(round(150*HistogramRed[i]/MaxCount)));
Canvas.Pen.Color:=clGreen;
Canvas.MoveTo(10, 160);
for i:=0 to 255 do
 Canvas.LineTo(10+i,
   160-(round(150*HistogramGreen[i]/MaxCount)));
Canvas.Pen.Color:=clBlue;
Canvas.MoveTo(10, 160);
for i:=0 to 255 do
 Canvas.LineTo(10+i,
   160-(round(150*HistogramBlue[i]/MaxCount)));
end;

Calling Histogram Procedure from Main Form
To call showHistogram procedure, we need the user interface in the main form. On the main form, double click the MainMenu component, and add menu item Image->Histogram (see figure 1), then double click the Histogram item, so we will be directed to its event handler, and edit as shown below:

procedure TMainForm.Histogram1Click(Sender: TObject);
begin
if ImageForm<>nil then
begin
  ImageForm:=TImageForm(ActiveMDIChild);
  try
  begin
    Application.CreateForm(THistogramForm,HistogramForm);
    HistogramForm.ShowHistogram(ImageForm.Image1);
  end;
  except
    HistogramForm.Free;
    ShowMessage('Cannot complete the operation');
  end;
end;
end;

Figure 1. Image Histogram Main Menu

Make sure that in that main unit uses HistogramUnit under the uses of implementation section. Now you can save all form, compile, and run the executable. The example of executable running is shown in figure 2.

Figure 2. Image Histogram Execution

Source Code Download

You can download the source code for both Delphi 7 and Turbo Delphi Explorer. The Delphi 7 source code can be downloaded here, and the Turbo Delphi Explorer source code here.