MATLAB: Can’t get Slider Step in GUI to move in step sizes that I want

figurefunctionguiMATLABslidersliderstepuicontrol

Here's my complete code for a gui where I want to adjust the threshhold of an image with a slider in the figure. I have everything working except for the slider step size. When I keep the line in that specifies what step size the slider should take, the slider doesn't work at all. When I leave that line out, things work exactly as I want (except for the step size of course).
Any help or input would be greatly appreciated!
 function final_image_threshhold = GetImageThreshhold(image,flag)
    if (strcmp(flag,'None') == 1)
        function_marker = 1;
    elseif (strcmp(flag,'AOMDEL') == 1)
        function_marker = 2; 
    else
        error('Unknown flag in GetImageThreshhold');
    end
    max_threshhold = max(image(:));
    se = strel('disk',10,4);
    if (function_marker == 1)
        new_image = 255*(double(image > round(max_threshhold/2))); 
        default_colormap = 1;
    elseif (function_marker == 2) 
        new_image = image > round(max_threshhold/2); 
        new_image = bwareaopen(new_image,13); 
        new_image = bwmorph(new_image,'spur'); 
        new_image = imdilate(imerode(imdilate(new_image,se),se),se);
        new_image = bwlabel(new_image); 
        default_colormap = 1;
    else
    end
    current_figure = imshow(imadjust(uint8(new_image)));
    colormap_menu = uicontrol('Style','popupmenu',...
                              'Value',default_colormap,...
                              'String',{'ColorMap','winter','lines','jet','hsv','hot','cool','gray'},...
                              'Position',[20 -10 80 50],...
                              'Callback', @SetColorMap); %#ok<NASGU>                      
    threshhold_slider = uicontrol('Style','slider',...
                                  'Min',0,'Max',ceil(max_threshhold),'Value',round(max_threshhold/2),...
                                  'SliderStep',[100*round(1/(max_threshhold + 1)),250*round(1/(max_threshhold + 1))],...  %%%_______Line in Question_______%%%
                                  'Position',[110 15 410 25],...
                                  'Callback', {@SetThreshholdLevel,image,current_figure,function_marker});                     
    threshhold_button = uicontrol('Style', 'pushbutton',...
                                  'String', 'Continue',...
                                  'Position', [530 14 70 28],...
                                  'Callback', @ThreshButtonDone);  %#ok<NASGU> 
    figure(999); set(999,'Visible','off');                          
    waitfor(999);
    final_image_threshhold = get(threshhold_slider,'Value');
    close all; 
 end
 function SetColorMap(source,~)
    val = get(source,'Value');
    maps = get(source,'String');
    newmap = maps{val};
    colormap(newmap);
 end
 function SetThreshholdLevel(slider_uic,~,image,current_figure,function_marker)
    new_threshhold = round(get(slider_uic,'Value'))
    se = strel('disk',10,4);
    if (function_marker == 1)
        new_image = uint8(image > new_threshhold)*255;
    elseif (function_marker == 2)
        new_image = uint8(image > round(new_threshhold)); 
        new_image = bwareaopen(new_image,13); 
        new_image = bwmorph(new_image,'spur'); 
        new_image = imdilate(imerode(imdilate(new_image,se),se),se);
        new_image = bwlabel(new_image);         
    else
    end        
    set(current_figure,'cData',new_image);
 end
 function ThreshButtonDone(~,~)
    close(999);   
 end

Best Answer

Leonard - consider your line of code to set the step size
 'SliderStep',[100*round(1/(max_threshhold + 1)),250*round(1/(max_threshhold + 1))],…
where
 max_threshhold = max(image(:));
So depending upon the data type of your image, then the max_threshold could be 1 (for single or double input image), 255 (for 8-bit unsigned integer input), etc. Regardless, the code
 round(1/(max_threshhold + 1)
will always be zero, so your step sizes will be zero too. I think what you want to do is remove the round altogether since the slider steps are percent changes in the position of the slider (the first being the minor step when the user presses the left or right arrow, and the second being the major step which corresponds to pressing in the trough). Try the following instead
 'SliderStep',[100/(double(max_threshhold) + 1),250/(double(max_threshhold) + 1)],…
Note the casting of the max_threshold to the double data type. If our input image is an unsigned (or signed) integer, then 100 (or 250) divided by this integer will be an integer...and so will be zero (assuming max_threshold is greater than 100 and 250). By casting to a double, we allow the result to be a rational number.
With the above change and a max_threshold of 255, the minor step will be 0.3906 and the major step will be 0.976. You may want to change the minor step to 0.03906 so that you allow for more than 3-4 steps in your slider.
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