How to save a value from a slider

I am trying to extra a value from a slider in a GUI. I have followed many suggestions buy none of them seem to work on the code I already have. The intention is that a video frame will appear as a gray frame. The user will then move the slider which will produce a binary image and adjust the black and white threshold. When the threshold is correct, the user will press a button to close the GUI and save the threshold, which will then be used in another section of code.

Here is what I have so far:

function buttonPlot(Frame)
%display a video frame
Frame=rgb2gray(Frame);
fig=uifigure('Name', 'Set the black-white threshold');
fig.Position=[100 100 1200 800]; %this sets the size of the gui
cg=uiaxes(fig, 'Position', [50 150 1920*.5 1080*.5],'Visible', 'off');
imshow(Frame, 'parent', cg);

%Slider
sld=uislider(fig, 'Position', [50 100 1920*.5 3], 'Value', 50, 'Limits', [20, 200],'ValueChangingFCN',@(sld,event) changeThreshold(event,cg,Frame),'BusyAction','cancel');
thresholdValue=[];

%Button
btn=uibutton(fig,'push', 'Text', 'Save Value', 'Position', [75+1920*.5, 1080*.5, 100, 22],'ButtonPushedFcn', @(btn,event) plotButtonPushed(btn,event,fig,thresholdValue));

%% Slider callback
    function slider=changeThreshold(event,cg,Frame)
        % hobj is the image object handle
        threshold=event.Value;
        
        %display frame as a binary image. Changing the slider adjust the
        %threshold
        binaryImage = Frame < threshold;
        binaryImage = imfill(binaryImage, 'holes');
        
        fish_area=200;
        bw=bwareaopen(binaryImage,fish_area);
        L=bwlabel(bw);
        
        s=regionprops(L,'area','centroid');
        area_vector=[s.Area];
        [tmp,idx]=max(area_vector);
        
        imshow(bw,'parent', cg);
        
        %% Button to close GUI and save the threshold value
        function button=plotButtonPushed(btn,event,fig,thresholdValue)
            
            close(fig)
            
        end
    end
end

Answer 1

[Voss]

One thing to note is that plotButtonPushed() is nested inside changeThreshold(), so the main function buttonPlot() will not see it and you'll get an error when pushing the button. Instead, both changeThreshold() and plotButtonPushed() should be nested inside buttonPlot(), like this:

function buttonPlot(Frame)
%display a video frame
Frame=rgb2gray(Frame);
fig=uifigure('Name', 'Set the black-white threshold');
fig.Position=[100 100 1200 800]; %this sets the size of the gui
cg=uiaxes(fig, 'Position', [50 150 1920*.5 1080*.5],'Visible', 'off');
imshow(Frame, 'parent', cg);

%Slider
sld=uislider(fig, 'Position', [50 100 1920*.5 3], 'Value', 50, 'Limits', [20, 200],'ValueChangingFCN',@(sld,event) changeThreshold(event,cg,Frame),'BusyAction','cancel');
thresholdValue=[];

%Button
btn=uibutton(fig,'push', 'Text', 'Save Value', 'Position', [75+1920*.5, 1080*.5, 100, 22],'ButtonPushedFcn', @(btn,event) plotButtonPushed(btn,event,fig,thresholdValue));

%% Slider callback
    function slider=changeThreshold(event,cg,Frame)
        % hobj is the image object handle
        threshold=event.Value;
        
        %display frame as a binary image. Changing the slider adjust the
        %threshold
        binaryImage = Frame < threshold;
        binaryImage = imfill(binaryImage, 'holes');
        
        fish_area=200;
        bw=bwareaopen(binaryImage,fish_area);
        L=bwlabel(bw);
        
        s=regionprops(L,'area','centroid');
        area_vector=[s.Area];
        [tmp,idx]=max(area_vector);
        
        imshow(bw,'parent', cg);
        
    end
    
    %% Button to close GUI and save the threshold value
    function button=plotButtonPushed(btn,event,fig,thresholdValue)
    
        close(fig)
        
    end
    
end

Another thing to consider is that rgb2gray() returns a grayscale image with values between 0 and 1, but the threshold is defined to be between 20 and 200, so you'll need to scale one or the other in order to compare them on the same basis, e.g., Frame = 255*rgb2gray(Frame/255); I'm not sure of the exact transform because I don't know what your Frame data is.