increasing brightness/contrast of image in different color channel using slider in GUI

I am trying to make sliders where I can change the brightness and contrast of an image using slider. However, I now want to make sliders that can change the brightness and contrast of the red channel only and then display the new merged image. How would I do that? This is my code for the whole image change:

 % --- Executes on slider movement.
function slider1_Callback(hObject, eventdata, handles)
% hObject    handle to slider1 (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
global im2
val=0.1*get(hObject,'Value')-0.1;
imbright=im2+val;
axes(handles.axes1);
imshow(imbright);
impixelinfo

and for contrast my code is:

   % --- Executes on slider movement.
function slider2_Callback(hObject, eventdata, handles)
% hObject    handle to slider2 (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
global im2
val=0.1*get(hObject,'Value')-0.1;
imcontrast=im2+val;
axes(handles.axes1);
imshow(imcontrast);
impixelinfo

Is my code to change the contrast even right, I just changed imbrightness to imcontrast?

But for how would I change the brightness and contrast for red channel using slider?

Do I need to?

   redChannel = im2(:, :, 1);
im2(:, :, 2) = 0
im2(:, :, 3); = 0

Also how do I make sure every slider is changing the current image and not the original one? Is that solved by calling global im2 at the beginning of every slider to make sure it is changing the current image and not the original one?

thanks

[Geoff Hayes]

Aa - don't use global variables. If you want all callbacks to have access to a particular image (the current one), then save it to the handles structure. For example, suppose you have loaded an image (however you have chosen to do this, in a callback, etc.) via something like

 img = imread(...);

You then decide to display this image (maybe using imshow or image) on one of your axes. Since img is the image being shown, you can now save it (in the same callback that read and displayed the image) to the handles structure as

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

Now, in any other callback where you want to update the currently displayed image you would do

 if isfield(handles, 'img')
     img = handles.img;
     % manipulate the image
     updatedImg = someFunction(img);
     % display the updated image
     imshow(updatedImg);
     % save it to handles
     handles.img = updatedImg;
     guidata(hObject, handles);
 end

This should allow you to avoid the use of globals.

[awezmm]

thank you for your response,

That works perfectly. Do you know what to do if I wanted multiple handles from one callback to another. Do i need if isfield statemnts for each because I wanted to update my image handle in a callback using another handle

Answer 1

[Image Analyst]

You need to split the color image up into the separate channels, then multiply the red channel by some factor - an equation of a line - then recombine the 3 color channels (the new red one plus the other two original ones) with cat(3, r, g, b) and call imshow(). Yous sliders could control the slope of the line (for contrast control) and offset of the line (for brightness control).

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);

% Change red
slope = handles.sldSlope.Value;
intercept = handles.sldIntercept.Value;
redChannel = uint8(double(redChannel) * slope + intercept);

% Recombine separate color channels into a single, true color RGB image.
rgbImage = cat(3, redChannel, greenChannel, blueChannel);

[awezmm]

thank you for your response, am I doing it right for the whole image brightness contrast though?

also is there a formula to calculate the brightness and contrast so i can start the slider at a specific value

[Image Analyst]

I don't know if you're doing it right because you have not shown anyone your code.

The offset (intercept) slider can go from -255 to +255. The slope can go from -90 degrees to +90 degrees. Start with a default of 0 for both sliders.

[awezmm]

ok thanks!

[Trinh Nguyen]

If offset and slope are set to 0 at the beginning then according to this formula:

redChannel = uint8(double(redChannel) * slope + intercept);

the redChannel will be 0, i.e. there is no redChannel at start?

[Image Analyst]

If it's an RGB image to begin with, there will be a red channel at the start, unless you cropped it off.

Correct, if offset(intercept) and slope are zero then that formula would set the redChannel variable to zero. If you then stuck that back into rgbImage, you'd have no red in the color image.