Calling Matlab slider from within a for loop - matlab

Question

I've created a Matlab project where an image is read from file and then converted to binary using a predefined threshold value. This for loop is repeated for the entire folder of similar images.
I'm trying to implement a slider so that I can change the threshold value in realtime using the slider, which will in turn adjust the binary image. I'm having problems working out where to define the slider's uicontrol and also where / how to call the slider from within the loop.
I have the slider's uicontrol:
uicontrol(...
'tag', 'fff',...
'style', 'slider',...
'callback', #ui_slider_Callback,...
'position', [20 20 200 50],...
'tooltipstring', 'Colormap scaling relative to actual slice',...
'Max', 250,...
'Min', 0,...
'value', 230,...
'SliderStep', [0.002, 0.002]);
I also know somewhere I need to have this line, but I'm unsure whether I need to define a slider function:
thresholdValue = get(hObject,'Value');
A simplified version of my code follows:
function
yourfolder=path name;
d=dir([yourfolder '\*.jpg']);
files={d.name};
for q=1:numel(files);
I = imread(files{q});
J = rgb2gray(I);
thresholdValue = 230;
binaryImage = J < thresholdValue;
imshow(binaryImage);
drawnow;
end
end
Admittedly I have very limited knowledge of functions and calling them but any help would be greatly appreciated.

Answer 1

You need to define the callback function. One method is to define it in the same *.m file as your GUI, which MATLAB calls a local function.
A small functional example:
function testcode
% Initialize GUI
h.fig = figure('MenuBar', 'none', 'ToolBar', 'none');
h.ax = axes( ...
'Parent', h.fig, ...
'Units', 'Normalized', ...
'Position', [0.1 0.15 0.8 0.8] ...
);
h.slide = uicontrol( ...
'tag', 'fff', ...
'style', 'slider', ...
'Units', 'Normalized', ...
'position', [0.05 0.03 0.9 0.05], ...
'tooltipstring', 'Colormap scaling relative to actual slice', ...
'Max', 250, ...
'Min', 0, ...
'value', 230, ...
'SliderStep', [0.002, 0.002] ...
);
% Dummy image
I = imread('ngc6543a.jpg');
h.img = rgb2gray(I);
thresholdValue = get(h.slide, 'Value');
binaryImage = h.img < thresholdValue;
imshow(binaryImage, 'Parent', h.ax);
% Need to set callback after all our elements are initialized
set(h.slide, 'Callback', {#ui_slider_Callback, h});
end
function ui_slider_Callback(~, ~, h)
thresholdValue = get(h.slide, 'Value');
binaryImage = h.img < thresholdValue;
imshow(binaryImage, 'Parent', h.ax);
drawnow
end
By default, callbacks are always passed two variables, the handle of the invoking object and an eventdata structure, whose contents vary. As explained in the callback documentation, you can pass additional inputs to your callback by wrapping everything into a cell array. One thing to note is that the value of your variable being passed to the callback is its value as it exists when you define the callback. Because of this, you will see I have defined the callbacks once we have initialized all of our graphics objects.
I've used the slider handle explicitly rather than using hObj to get the threshold value. This is purely personal preference, either method will work fine.