Looking at data from image using Datacursormode and ROI (drawrectangle)

Hi there,

I am trying to make use of datacursormode to select points/regions I am interested in and displaying the x, y, and rgb values at that point. This is the code I have used and the function to generate the position at the location I have selected. However, I can't seem to be able to generate the RGB values when I umcomment the line CData and it shows "unable to update datatip using custom update function".

figure
imagesc; axis image;

datacursormode on
dcm = datacursormode(gcf);
set(dcm,'UpdateFcn',@myupdatefcn_example)

function txt = myupdatefcn_example(trash,event)
pos = get(event,'Position');
I = get(event, 'DataIndex');
% CData = get(event,'CData');

txt = {[num2str(pos(1))],...
       [num2str(pos(2))],...
       [num2str(I)]};
end

Next, I would also like to draw a rectangle and extract the mean RGB values within that rectangle using the datacursormode as well so that it is interactive. I found this drawrectangle function that allows me to draw the rectangle I want on the image, and also generates the position of the box. However, I am not quite sure how to link this up with the datacursormode.

figure
imagesc; axis image;
roi = drawrectangle('Color',[1 0 0]);
addlistener(roi,'MovingROI',@allevents);

function allevents(src,evt)
    evname = evt.EventName;
    switch(evname)
        case{'MovingROI'}
            disp(['ROI moving previous position: ' mat2str(evt.PreviousPosition)]);
            disp(['ROI moving current position: ' mat2str(round(evt.CurrentPosition))]);
        case{'ROIMoved'}
            disp(['ROI moved previous position: ' mat2str(evt.PreviousPosition)]);
            disp(['ROI moved current position: ' mat2str(evt.CurrentPosition)]);
    end
end

Could anyone please advise? Thank you for your time.

Best Regards,

Wayne

Answer 1

[Kiran Felix Robert]

Hi Wayne,

It is my understanding that you wish to know why

CData = get(event,'CData'); 

affects the operation of your custom update function. This is because the CData field does not belong to the events (info) object of UpdateFcn . The image pixel values in the UpdateFcn can be accessed by passing arguments to the callback function as shown below,

f = figure; 
imagesc(I); 
handles.I = I; 
dcm = datacursormode; 
dcm.Enable = 'on'; 
dcm.UpdateFcn = {@coordinates,handles}; 

function txt = coordinates(~,info,handles) 
x = info.Position(1); 
y = info.Position(2); 
R = handles.I(x,y,1); 
B = handles.I(x,y,2); 
G = handles.I(x,y,3); 
txt = ['(' num2str(x) ', ' num2str(y) ')'':(' num2str(R) ', ' num2str(G) ', ' num2str(B) ')']; 
end 

The datacursormode is only used to put up datatip in specific points in a figure or a plot. To access values from the ROI, the listener function can be used. Your ROI Listener can be modified as shown below to get the mean RGB values,

function allevents(src,evt) 
    evname = evt.EventName; 
    him = findobj(src.Parent,'Type','image'); 
    switch(evname) 
        case{'MovingROI'} 
            mask = src.createMask(him(1).CData); 
            [r,c] = find(mask); 
             P = impixel(him.CData,r,c); 
             RGBmean = mean(P,1) 
    end   
end 

The mean value of RGB is printed to command line for every ROI update.

Hope this Helps.

Kiran Felix Robert

[Wayne]

Sorry for the late reply - thank you for your help!