Perbaikan kualitas citra (Image Enhancement) merupakan tahapan pre-processing yang umumnya dilakukan sebelum tahapan segmentasi. Dalam materi ini dibahas dua jenis perbaikan kualitas citra yaitu contrast stretching dan histogram equalization. Contrast stretching merupakan metode perbaikan kualitas citra yang bertujuan untuk meningkatkan atau menurunkan kontras suatu citra dengan cara memperlebar atau mempersempit range nilai intensitas piksel citra. Materi mengenai perbedaan kontras tinggi dan rendah dapat dilihat pada halaman berikut ini: Perbedaan citra gelap, terang, kontras rendah, dan kontras tinggi. Sedangkan histogram equalization merupakan metode perbaikan kualitas citra yang bertujuan untuk meratakan persebaran nilai intensitas piksel suatu citra. Materi mengenai histogram equalization lebih lanjut dapat dilihat pada halaman berikut ini: Ekualisasi Histogram pada Citra Digital

Pada materi ini, nilai PSNR dan MSE digunakan sebagai indikator perbandingan citra hasil perbaikan kualitas citra dengan citra asli.

Langkah-langkah pemrograman GUI matlab untuk melakukan contrast stretching dan histogram equalization pada citra digital adalah sebagai berikut:

1. Membuka tampilan GUI awal

2. Membuka citra RGB asli

3. Mengkonversi citra RGB menjadi grayscale dan menampilkan histogramnya

4. Melakukan contrast stretching terhadap citra grayscale dan menghitung nilai psnr dan mse

5. Melakukan histogram equalization terhadap citra grayscale dan menghitung nilai PSNR dan MSE

Secara visual, citra hasil contrast stretching tampak lebih mirip dengan citra grayscale asli dibandingkan dengan citra hasil histogram equalization. Pernyataan tersebut juga didukung oleh nilai indikator PSNR dan MSE yang menyatakan bahwa nilai PSNR citra hasil contrast stretching lebih tinggi dari pada citra hasil histogram equalization, sedangkan nilai MSE citra hasil contrast stretching lebih rendah dari pada citra hasil histogram equalization.

Tampilan source code untuk melakukan contrast stretching dan histogram equalization adalah sebagai berikut:

function varargout = contrast_stretch_and_histeq(varargin)
% CONTRAST_STRETCH_AND_HISTEQ MATLAB code for contrast_stretch_and_histeq.fig
%      CONTRAST_STRETCH_AND_HISTEQ, by itself, creates a new CONTRAST_STRETCH_AND_HISTEQ or raises the existing
%      singleton*.
%
%      H = CONTRAST_STRETCH_AND_HISTEQ returns the handle to a new CONTRAST_STRETCH_AND_HISTEQ or the handle to
%      the existing singleton*.
%
%      CONTRAST_STRETCH_AND_HISTEQ('CALLBACK',hObject,eventData,handles,...) calls the local
%      function named CALLBACK in CONTRAST_STRETCH_AND_HISTEQ.M with the given input arguments.
%
%      CONTRAST_STRETCH_AND_HISTEQ('Property','Value',...) creates a new CONTRAST_STRETCH_AND_HISTEQ or raises the
%      existing singleton*.  Starting from the left, property value pairs are
%      applied to the GUI before contrast_stretch_and_histeq_OpeningFcn gets called.  An
%      unrecognized property name or invalid value makes property application
%      stop.  All inputs are passed to contrast_stretch_and_histeq_OpeningFcn via varargin.
%
%      *See GUI Options on GUIDE's Tools menu.  Choose "GUI allows only one
%      instance to run (singleton)".
%
% See also: GUIDE, GUIDATA, GUIHANDLES

% Edit the above text to modify the response to help contrast_stretch_and_histeq

% Last Modified by GUIDE v2.5 12-Oct-2017 13:42:51

% Begin initialization code - DO NOT EDIT
gui_Singleton = 1;
gui_State = struct('gui_Name',       mfilename, ...
    'gui_Singleton',  gui_Singleton, ...
    'gui_OpeningFcn', @contrast_stretch_and_histeq_OpeningFcn, ...
    'gui_OutputFcn',  @contrast_stretch_and_histeq_OutputFcn, ...
    'gui_LayoutFcn',  [] , ...
    'gui_Callback',   []);
if nargin && ischar(varargin{1})
    gui_State.gui_Callback = str2func(varargin{1});
end

if nargout
    [varargout{1:nargout}] = gui_mainfcn(gui_State, varargin{:});
else
    gui_mainfcn(gui_State, varargin{:});
end
% End initialization code - DO NOT EDIT

% --- Executes just before contrast_stretch_and_histeq is made visible.
function contrast_stretch_and_histeq_OpeningFcn(hObject, eventdata, handles, varargin)
% This function has no output args, see OutputFcn.
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
% varargin   command line arguments to contrast_stretch_and_histeq (see VARARGIN)

% Choose default command line output for contrast_stretch_and_histeq
handles.output = hObject;

% Update handles structure
guidata(hObject, handles);
movegui(hObject,'center');

% UIWAIT makes contrast_stretch_and_histeq wait for user response (see UIRESUME)
% uiwait(handles.figure1);

% --- Outputs from this function are returned to the command line.
function varargout = contrast_stretch_and_histeq_OutputFcn(hObject, eventdata, handles)
% varargout  cell array for returning output args (see VARARGOUT);
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Get default command line output from handles structure
varargout{1} = handles.output;

function edit3_Callback(hObject, eventdata, handles)
% hObject    handle to edit3 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of edit3 as text
%        str2double(get(hObject,'String')) returns contents of edit3 as a double

% --- Executes during object creation, after setting all properties.
function edit3_CreateFcn(hObject, eventdata, handles)
% hObject    handle to edit3 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end

function edit4_Callback(hObject, eventdata, handles)
% hObject    handle to edit4 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of edit4 as text
%        str2double(get(hObject,'String')) returns contents of edit4 as a double

% --- Executes during object creation, after setting all properties.
function edit4_CreateFcn(hObject, eventdata, handles)
% hObject    handle to edit4 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end

function edit1_Callback(hObject, eventdata, handles)
% hObject    handle to edit1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of edit1 as text
%        str2double(get(hObject,'String')) returns contents of edit1 as a double

% --- Executes during object creation, after setting all properties.
function edit1_CreateFcn(hObject, eventdata, handles)
% hObject    handle to edit1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end

function edit2_Callback(hObject, eventdata, handles)
% hObject    handle to edit2 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of edit2 as text
%        str2double(get(hObject,'String')) returns contents of edit2 as a double

% --- Executes during object creation, after setting all properties.
function edit2_CreateFcn(hObject, eventdata, handles)
% hObject    handle to edit2 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end

% --- Executes on button press in pushbutton1.
function pushbutton1_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
[filename,pathname] = uigetfile('*.jpg');

if ~isequal(filename,0)
    Img = imread(fullfile(pathname,filename));
    axes(handles.axes1)
    imshow(Img)
    title('Original Image')
else
    return
end

handles.Img = Img;
guidata(hObject, handles)

% --- Executes on button press in pushbutton2.
function pushbutton2_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton2 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
Img = handles.Img;

Img_gray = rgb2gray(Img);
axes(handles.axes2)
imshow(Img_gray)
title('Grayscale')

axes(handles.axes3)
imhist(Img_gray)
title('Histogram')
grid on

handles.Img_gray = Img_gray;
guidata(hObject, handles)

% --- Executes on button press in pushbutton3.
function pushbutton3_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton3 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
Img_gray = handles.Img_gray;
Img_stretch = imstretch(Img_gray);

axes(handles.axes4)
imshow(Img_stretch)
title('Contrast Strecthing')

axes(handles.axes5)
imhist(Img_stretch)
title('Histogram')
grid on

[row,col] = size(Img_gray);
MSE = sum(sum((Img_stretch-Img_gray)^2))/(row*col);
PSNR = 10*log(1/MSE);

set(handles.edit1,'String',[PSNR,' dB']);
set(handles.edit2,'String',MSE);

% --- Executes on button press in pushbutton4.
function pushbutton4_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton4 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
Img_gray = handles.Img_gray;
Img_histeq = histogramequalization(Img_gray);

axes(handles.axes6)
imshow(Img_histeq)
title('Hist. Equalization')

axes(handles.axes7)
imhist(Img_histeq)
title('Histogram')
grid on

[row,col] = size(Img_gray);
MSE = sum(sum((Img_histeq-Img_gray)^2))/(row*col);
PSNR = 10*log(1/MSE);

set(handles.edit3,'String',[PSNR,' dB']);
set(handles.edit4,'String',MSE);

% --- Executes on button press in pushbutton5.
function pushbutton5_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton5 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
axes(handles.axes1)
cla reset
set(gca,'XTick',[])
set(gca,'YTick',[])

axes(handles.axes2)
cla reset
set(gca,'XTick',[])
set(gca,'YTick',[])

axes(handles.axes3)
cla reset
set(gca,'XTick',[])
set(gca,'YTick',[])

axes(handles.axes4)
cla reset
set(gca,'XTick',[])
set(gca,'YTick',[])

axes(handles.axes5)
cla reset
set(gca,'XTick',[])
set(gca,'YTick',[])

axes(handles.axes6)
cla reset
set(gca,'XTick',[])
set(gca,'YTick',[])

axes(handles.axes7)
cla reset
set(gca,'XTick',[])
set(gca,'YTick',[])

set(handles.edit1,'String',[])
set(handles.edit2,'String',[])
set(handles.edit3,'String',[])
set(handles.edit4,'String',[])

File source code lengkap beserta citra pada pemrograman di atas dapat diperoleh melalui halaman berikut ini: Source Code

Tutorial pembuatan App Designer untuk meningkatkan contrast dan brightness (kecerahan) pada citra digital dapat dilihat pada video berikut ini: