Black image after imread

hello using this code:

I1 = imread('E:\Dropbox\Andrea VMS\Esame_di_Stato\Programmi\Spur_Gear\Spur11.PNG');
imshow(I1);
Y_j=input('Insert the geometrical coefficient of flexural resistance: '); 
close

appears a black window. using other images i have no problem. this is an image i scanned from a book

[Geoff Hayes]

Andrea - what are the dimensions of I1 and what is its data type? For example, what does the following return

 class(I1)

[andrea vironda]

hi Geoff. this is uint8, and also the not working one

Answer 1

[Image Analyst]

Try

imshow(I1, []);
message = sprintf('Max Value = %f\nMin Value = %f\n', max(I1(:)), min(I1(:)))
uiwait(helpdlg(message));

Also, check the values of I1 in the variable editor to see what they are.

[andrea vironda]

Using

imshow(I1, []); 

I can see the image, good!

In the help dialog I see

message =

Max Value = 1.000000

Min Value = 0.000000

[Image Analyst]

Glad that solved it. Maybe you could go ahead and "Accept this answer".

[Ammon McCarthy]

thank you

Answer 2

[Walter Roberson]

The PNG image might not be in RGB form. In particular it might be in indexed form, or it might be in grayscale. You should use the two-output form of imread() if it is an indexed image, and you should use colormap(gray(256)) if it is grayscale.

[andrea vironda]

yes, i scanned the paper from paint using greyscale. were can i put colormap(gray(256))?

[Mats Burgmans]

can you still answer the question Walter Robertson?

[Image Analyst]

You call that right after you call imshow() to display the image. Here's some snippets:

[imageArray, colorMap] = imread(fullImageFileName);
% colorMap will have something for an indexed image (gray scale image with a stored colormap).
% colorMap will be empty for a true color RGB image or a monochrome gray scale image.
% Here we actually display the image in the "axesImage" axes.
imshow(imageArray, 'InitialMagnification', 'fit', 'parent', handles.axesImage);

[rows, columns, numberOfColorChannels] = size(imageArray);
% Get the file date
fileInfo = dir(fullImageFileName);
txtInfo = sprintf('%s\n\n%d lines (rows) vertically\n%d columns across\n%d color channels\n', ...
  [basefilename extension], rows, columns, numberOfColorChannels);
% Tell user the type of image it is.
if numberOfColorChannels == 3
  colorbar 'off';  % get rid of colorbar.
  txtInfo = sprintf('%s\nThis is a true color, RGB image.', txtInfo);
elseif numberOfColorChannels == 1 && isempty(colorMap)
  colorbar 'off';  % get rid of colorbar.
  txtInfo = sprintf('%s\nThis is a gray scale image, with no stored color map.', txtInfo);
elseif numberOfColorChannels == 1 && ~isempty(colorMap)
  txtInfo = sprintf('%s\nThis is an indexed image.  It has one "value" channel with a stored color map that is used to pseudocolor it.', txtInfo);
  colormap(colorMap);
  whos colorMap;
  %fprintf('About to apply the colormap...\n');
  % Thanks to Jeff Mather at the Mathworks to helping to get this working for an indexed image.
  colorbar('peer', handles.axesImage);
  %fprintf('Done applying the colormap.\n');
end

uiwait(helpdlg(txtInfo));

Adapt as needed for your program.