Change image contrast and brightness - GUI and sliders

I am building a simple GUI in Matlab that I aim to use in order to change image brightness and contrast using sliders (similar to Brightness / Contrast in paint.net).

I was calculating the the brightness via mean2 and set it as the slider value. Unfortunately, when I move the slider (set 0...255) the I = I + BrightSliderValue does not seems to be the right operation. Any idea how to solve it? Furthermore, how do I proceed with the contrast issue? How do I calculate it and how do I change it?

Code:

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

      I = getappdata(handles.imageAxes , 'yourVariable');
      disp(size(I, 3));
      if size(I, 3)>1
          I = rgb2gray(I);
      else
          h = msgbox('A none color image'); 
      end

      axes(handles.imageAxes);
      imshow(I);
      brightness = mean2(I)
      set(handles.BrightSlider, 'value', brightness);
      %contrast = (I) % to be calculated
      %set(handles.ContrastSlider, 'value', contrast);
      setappdata(handles.imageAxes, 'yourVariable', I);

      % --- Executes on slider movement.
      function BrightSlider_Callback(hObject, eventdata, handles)
      % hObject    handle to BrightSlider (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,'Value') returns position of slider
      %        get(hObject,'Min') and get(hObject,'Max') to determine range of slider
      I = getappdata(handles.imageAxes , 'yourVariable');
      BrightSliderValue = get(handles.BrightSlider, 'Value');
      I = I + BrightSliderValue ; % to be change
      axes(handles.imageAxes);
      imshow(I);
      setappdata(handles.imageAxes, 'yourVariable', I);

      % --- Executes on slider movement.
      function ContrastSlider_Callback(hObject, eventdata, handles)
      % hObject    handle to ContrastSlider (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,'Value') returns position of slider
      %        get(hObject,'Min') and get(hObject,'Max') to determine range of slider
      I = getappdata(handles.imageAxes , 'yourVariable');
      ContrastSliderValue = get(handles.ContrastSlider, 'Value');
      I = I + ContrastSliderValue; % to be change
      I = getappdata(handles.imageAxes , 'yourVariable');
      % I = %change contrast
      axes(handles.imageAxes);
      imshow(I);
      setappdata(handles.imageAxes, 'yourVariable', I);

Answer 1

[Image Analyst]

I recommend you don't change the image and don't use getappdata and setappdata. Simply read the sliders in the callback functions, create a lookup table, and apply the lookup table to the axes.

[hu]

I wish to do brighten and contrast an image dynamically on the same image axes. Is it possible via the code above?

[Image Analyst]

Of course. Something like (untested):

slope = handles.sliderSlope.Value; % Get slope (contrast) from slider.
offset = handles.sliderOffset.value; % Get offset (brightness) from slider.
x = linspace(0, 1, 256); % Input gray level.
y = slope * (x - offset) + 0.5; % Output gray level
y(y>1) = 1; Clip colormap to range 0-1.
y(y<0) = 0;
cmap = [y', y', y'];
colormap(cmap);

[Ely Raz]

Is there a way to use imadjust instead?

[Image Analyst]

Yes, use the slider values and pass them into imadjust. Unlike colormapping, it creates a brand new image that you'll have to display with imshow(). You also will not use a colormap if you use imadjust() since essentially the new image already was created by applying the colormap and so you don't want to do it twice.

[Ely Raz]

In the script above I already implemented the imshow() but I do not know how to add to the script the imadjust for contrast and brightness. I will appreciate a lot if you can provide a more explanations and it will be great if you can add some code so I can learn from it.