MATLAB: Error using rgb2gray

Question

I've made GUI with GUIDE which makes RGB image into binary image. Before I change the RGB image into binary image, I read the image from another axes not from folder.

%read multiple image at once and showed it in multiple axes
[files,folder,FilterIndex] = uigetfile('*.jpg*','MultiSelect','on');
handles.img=cell(1,numel(files));
for ifile = 1:numel(files)
    filename=fullfile(folder,files{ifile});
    img=imread(filename);
    ax = findall(gcf,'type','axes','tag',sprintf('axes%d',ifile));
    imshow(img,'Parent',ax);
    handles.img{ifile} = img;
end
guidata(hObject,handles);
%change RGB image in axes1 into binary image
proses=guidata(gcbo);
D=get(proses.citraasli,'Userdata'); 
abu=rgb2gray(D); 
cb=imclearborder(abu); 
thresh=graythresh(cb);
b=im2bw(cb,thresh); 
bw=bwareaopen(b,60); 
bwfill=imfill(bw,'holes'); 
[c,num]=bwlabel(bwfill,8);
for i=1:1:num
    axesName=sprintf('axes%d',i);
    ax=handles.(axesName);
    imshow((c==i),'Parent',ax);
end
guidata(hObject,handles);

This command works properly in another GUI program, but it gives that following error:

??? Error using ==> rgb2gray>parse_inputs at 82
MAP must be a m x 3 array.
Error in ==> rgb2gray at 35
X = parse_inputs(varargin{:});
Error in ==> guikedelaizulfa>ekstrakfitur_Callback at 1127
abu=rgb2gray(D);
Error in ==> gui_mainfcn at 96
        feval(varargin{:});
Error in ==> guikedelaizulfa at 42
    gui_mainfcn(gui_State, varargin{:});
Error in ==>
@(hObject,eventdata)guikedelaizulfa('ekstrakfitur_Callback',hObject,eventdata,guidata(hObject))
??? Error while evaluating uicontrol Callback
>>

I've checked the image is RGB image, and i can't find the problem to fix it. If somebody knows how to fix it, please help me! Thank you!

Answer 1

You are not getting an image from an axes with that code. You read it from a file, then find axes on your figure and display the image from the file into that axes. If you want to extract an image from an axes, use

theImage = getimage(handles.axesImage);

Don't use ax like you did - no reason to. Just use the actual "tag" name of the axes that you gave it in GUIDE.

What does this say in the command window:

[rows, columns, numberOfColorChannels] = size(D);
fprintf('Rows = %d, columns = %d, number of color channels = %d\n', rows, columns, numberOfColorChannels);
whos D
abu=rgb2gray(D);