Error opening file: Dot indexing is not supported for variables of this type. When opening a JPG image

Hi, I am very new in Matlab. Got this working code from my prof but I am facing some problems when running it on my machine (MacOS) that google haven't helped to solve. I am getting the following error:

imName: dataset/controlled/FB/QF-50/original-001-1012x1800.jpg
[dct_coef_hist.m]: jpeg_read() error: Attempt to execute SCRIPT jpeg_read as a function:
/Users/irene/Desktop/sharing-chains-reconstruction-main/src/jpegtbx_1.4/jpeg_read.m
Error opening file: Dot indexing is not supported for variables of this type.

Calling this function: (it fails in the first try-catch)

function histr = dct_coef_hist(imName)
%   This function extracts histogram of DCT coefficients
%   Input:
%       coeff_arrays: 1x3 cell array of coefficients
%       Nc: number of analyzed DCT coeff in each 8x8 blocks
%       BT: the limit of histogram ==> number of bins = 2*BT + 1


channel = 1;    % luminance channel
Nc = 9;         % number of AC coefficients (zig zag scan)
BT = 20;        % maximum bin value => number of bins = 2*BT+1
fprintf('imName: %s\n', imName);
jobj = [];             
coef_arrays = [];
quant_table = [];

% --------------------error-----------------------------------------
try 
    % the original code had no try-catch so I implemented it to see more details about the error
    jobj=jpeg_read(imName);         
    % it never gets here so the error must be in the line above (?!?)
    
    coef_arrays = jobj.coef_arrays;
    quant_table = jobj.quant_tables;
catch me
    fprintf("[dct_coef_hist.m]: jpeg_read() error: %s\n", me.message);
end
% -----------------------end--------------------------------------

coef_arrays = jobj.coef_arrays;
quant_table = jobj.quant_tables;

[h,w] = size(coef_arrays{channel});

nblks = (floor(h/8)-1)*(floor(w/8)-1);

coefs = coef_arrays{1};
hist = zeros(Nc, (2*BT+1));

zigzag_inds = zigzag(reshape(1:64, [8 8]));
zigzag_inds = zigzag_inds(2:Nc+1);

for b = 0:nblks-1
    i = 8*(floor(b / (floor(w/8)-1))) + 1;
    j = 8*mod(b, (floor(w/8)-1)) + 1;
    
    block = coefs(i:i+7, j:j+7);
    block = block(zigzag_inds);
    quant_values = quant_table{channel}(zigzag_inds);
    for n = 1:Nc
        v = block(n) * quant_values(n);
        if v > BT
            hist(n, end) = hist(n, end) + 1;
        elseif v < -BT
            hist(n, 1) = hist(n, 1) + 1;
        else
            hist(n, v+BT+1) = hist(n, v+BT+1) + 1;
        end
    end
end
hist = hist ./ nblks;
histr = reshape(hist, 1, []);
end

Here is also the code for jpeg_read.m

output = jpeg_read(imIn);
% JPEG_READ  Read a JPEG file into a JPEG object struct
%
%    JPEGOBJ = JPEG_READ(FILENAME) Returns a Matlab struct containing the
%    contents of the JPEG file FILENAME, including JPEG header information,
%    the quantization tables, Huffman tables, and the DCT coefficients.
%
%    This software is based in part on the work of the Independent JPEG Group.
%
%    See also JPEG_WRITE.

% Modified by Markos Zampoglou ([e-mail]), ITI-CERTH 2016.
% It now operates as a function

% Phil Sallee 6/2003

error("Mex routine jpeg_read.c not compiled\n");

I don't really understand what this last jpeg_read.m does... it is just calling jpeg_read(); which... is... what? is it a .c function? (I never get the error Mex routine jpeg_read.c not compiled so I am confused what is the point.

[Jeffrey Clark]

It looks like this is what you need to do to compile the code for your platform: what are the steps to use Phil Sallee's JPEG toolbox for MATLAB? - (mathworks.com)

[dpb]

What does

which -all jpeg_read

return? There should be a .mex file first; if all you get is the m-file, then you don't have all you need...

If it shows that there is a mex file, it would seem the c code that was compiled wasn't up to date in being a function.

What happens if you just type

jpeg_read

at command line?

[Bojan Poletan]

thanks for your reply,

here is the output of the command you asked:

>> which -all jpeg_read
/Users/irene/Desktop/sharing-chains-reconstruction-main/src/jpegtbx_1.4/jpeg_read.m

and the second one:

>> jpeg_read
Unrecognized function or variable 'imIn'.

Error in jpeg_read (line 1)
output = jpeg_read(imIn);

here is my working directory, so there is no .mex file as you can but there is a mexa64 in jpegtbx_1.4see in the pic:

I wrote my prof saying that I am facing a lot of troubles but he replied: that it should work without writting code and it works well on his pc in uni.

[Bojan Poletan]

@Jeffrey Clark I should not write any code, that is what my teacher sait. :) Thanks for the link, will take a look :)

[dpb]

You don't have to write the code, but you will have to install all the necessary pieces to be able to use it on that machine, or your prof will have to give you access to a machine on which it is installed.

He's probably used it long enough and/or a grad student set it up for him so he(she) is totally unaware of the actual implementation.

[Walter Roberson]

output = jpeg_read(imIn);

That file is missing the keyword "function"

[dpb]

>> jpeg_read
Unrecognized function or variable 'imIn'.

Error in jpeg_read (line 1)
output = jpeg_read(imIn);

Good eyes by @Walter Roberson!!! The m-file isn't a valid m-file function because it is missing the function keyword as noted. That's why the error about executing a script file as a function. The intent was that if the mex file wasn't installed you'd get an error message but apparently wasn't tested -- so when it was needed, it failed with a confusing message -- correct, but not one expected.

Change that function first line to read as

function output = jpeg_read(imIn)

and then you'll get the error in use if the mex file isn't found as was intended.

[Bojan Poletan]

Thanks, ok, now I get the error

jpeg_read() error: Mex routine jpeg_read.c not compiled\n
Error opening file: Dot indexing is not supported for variables of this type.

So I googled and tried to compile the file as follow:

mex src/jpegtbx_1.4/jpeg_read.c

Which is the path from my current position (see the pic below):

and got the following error:

22-05-21 21:07:31.020 xcodebuild[3265:130957] Requested but did not find extension point with identifier Xcode.IDEKit.ExtensionSentinelHostApplications for extension Xcode.DebuggerFoundation.AppExtensionHosts.watchOS of plug-in com.apple.dt.IDEWatchSupportCore
2022-05-21 21:07:31.021 xcodebuild[3265:130957] Requested but did not find extension point with identifier Xcode.IDEKit.ExtensionPointIdentifierToBundleIdentifier for extension Xcode.DebuggerFoundation.AppExtensionToBundleIdentifierMap.watchOS of plug-in com.apple.dt.IDEWatchSupportCore
2022-05-21 21:07:31.613 xcodebuild[3270:130976] Requested but did not find extension point with identifier Xcode.IDEKit.ExtensionSentinelHostApplications for extension Xcode.DebuggerFoundation.AppExtensionHosts.watchOS of plug-in com.apple.dt.IDEWatchSupportCore
2022-05-21 21:07:31.614 xcodebuild[3270:130976] Requested but did not find extension point with identifier Xcode.IDEKit.ExtensionPointIdentifierToBundleIdentifier for extension Xcode.DebuggerFoundation.AppExtensionToBundleIdentifierMap.watchOS of plug-in com.apple.dt.IDEWatchSupportCore
Building with 'Xcode with Clang'.
Error using mex
2022-05-21 21:07:34.834 xcodebuild[3275:131019] Requested but did
not find extension point with identifier
Xcode.IDEKit.ExtensionSentinelHostApplications for extension
    Xcode.DebuggerFoundation.AppExtensionHosts.watchOS of plug-in
    com.apple.dt.IDEWatchSupportCore
    2022-05-21 21:07:34.835 xcodebuild[3275:131019] Requested but did
    not find extension point with identifier
    Xcode.IDEKit.ExtensionPointIdentifierToBundleIdentifier for
        extension
        Xcode.DebuggerFoundation.AppExtensionToBundleIdentifierMap.watchOS
        of plug-in com.apple.dt.IDEWatchSupportCore
        /Users/irene/Desktop/sharing-chains-reconstruction-main/src/jpegtbx_1.4/jpeg_read.c:52:10:
        fatal error: 'jerror.h' file not found
        #include <jerror.h>
        ^~~~~~~~~~
        1 error generated.

I do not understand the message. When it says xcodebuild not found... it means I miss xcode on MacOS? but... I am pretty sure I have it.

[dpb]

I "know nuthink!" about MacOS, sorry.

Make sure first you have mex set up -- try

mex -setup

to see if it finds the compiler successfully; often IME the automated search tool fails to find an installed and operating compiler because it makes too many assumptions about which releases are allowed and where it might be installed if anything is out of the "plain vanilla, accept all defaults" install.

If that succeeds in showing a C compiler is installed or you can run it with the "C" language parameter and it then finds one, then ensure you can compile/run one or two of the trivial examples -- timestwo.c and the like.

After that, you're ready to try to compile a real mex file in anger...

Good luck...

[Walter Roberson]

See https://osupdocs.forestry.oregonstate.edu/index.php/Compiling_in_MacOS_X#Installing_XCode for instructions on installing the MacOS Command Line compiler tools.

Answer 1

[Walter Roberson]

The library contains executables for Windows and Linux but not for Mac. You need to cd into the library and use mex to compile jpeg_read

[Bojan Poletan]

I wasn't really able to do that because I got many errors while compiling the C files. Missing includes, syntax errors and so on. So I contacted my prof and he provided me with a VPN access to the uni pc

Answer 2

[Bojan Poletan]

My teacher provided me with a VPN acces to the university server they use for simulations so it should be fine. I don't think this answers to the question but I am not proceeding further in trying to solve it.