MATLAB Answers

# Black image after imread

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andrea vironda on 8 Mar 2016
Edited: Image Analyst on 5 Mar 2020
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

#### 2 Comments

Geoff Hayes on 8 Mar 2016
Andrea - what are the dimensions of I1 and what is its data type? For example, what does the following return
class(I1)
andrea vironda on 8 Mar 2016
hi Geoff. this is uint8, and also the not working one

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### Accepted Answer

Image Analyst on 8 Mar 2016
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.

#### 3 Comments

andrea vironda on 8 Mar 2016
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 on 8 Mar 2016
Glad that solved it. Maybe you could go ahead and "Accept this answer".
Ammon McCarthy on 5 Mar 2020
thank you

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### More Answers (1)

Walter Roberson on 8 Mar 2016
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.

#### 3 Comments

andrea vironda on 8 Mar 2016
yes, i scanned the paper from paint using greyscale. were can i put colormap(gray(256))?
Mats Burgmans on 23 May 2018
can you still answer the question Walter Robertson?
Image Analyst on 24 May 2018
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.

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