hist() function: Error using .* Error in hist (line 78)

When trying to generate with the HIST function a histogram I get an error message and do not know what I have to modify in my simple data analysis approach in order to prevent this. The error message is:

    Error using  .*
    Integers can only be combined with integers of the same class, or scalar doubles.
    Error in hist (line 78)
        xx = miny + binwidth*(0:x);

I load a TIF image into MATLAB 2013b (not having the Image Processing Toolbox, simply the MATLAB ore program, WinXP 32 bit) and receive the myimage variable of type "512x512x3 uint8"

myimage = imread(filename);

Trying to show a histogram produces the error message:

hist(myimage)

I then separated individual color channels to the variables R, G, and B, which result of type "512x512 uint8":

R= myimage(:,:,1);
G= myimage(:,:,2);
B= myimage(:,:,3);

Trying to show the histogram for one of these variables also produces the error message:

hist(R)

I tried to resample the content of a variable into a vector of type "262144x1 uint8", but still receive the same error message

test=R(:); hist(test)

Sorting then didn´t help neither:

test=sort(test); hist(test)

Neither worked finally changing the type of the variable:

test=int16(test); hist(test)
test=int8(test); hist(test)

What do I have to do, please, in order to get a histogram of the values of a color channel drawn, and especially, where am I not using the hist() function correctly? I am pretty sure that I do not completely understand what the error message wants to hint me to.

Answer 1

[Kevin Claytor]

It seems as though hist() prefers floating point values (single or double). Try;

hist(double(R(:))) % Histogram of red channel

Also, note that hist operates column-wise;

hist(double(R))   % column-wise histogram

[Marco]

Thanks, this indeed works and solved my problem!

Answer 2

[Image Analyst]

If you have the Image Processing Toolbox, use imhist(). But you can only use it on one color channel at a time.