How do I encode a jpg to a set quality level in order to calculate it's ela without first writing it to a temp file?

Question

Thomas 2013-6-14

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https://ww2.mathworks.cn/matlabcentral/answers/79108-how-do-i-encode-a-jpg-to-a-set-quality-level-in-order-to-calculate-it-s-ela-without-first-writing-it

I have tried looking at using a encoder some exist in the File Exchange, but are not written to allow adjustable quality levels or are faithful adaptation of the original jpeg encoder!

I'm now using a virtual ram disk to hold the temp file, which is then deleted!

 function [] = generate_ela(filename,quality)
    if nargin > 2 | nargin < 2
        error('usage: generate_ela(filename,quality)');
    end;
    img_orig = imread(filename);
    tempfile = 'z:\temp.jpg';
    imwrite(img_orig,tempfile,'jpeg','Quality',quality);
    temp = imread(tempfile);
    delete(tempfile);
    idiff = uint8(abs(double(img_orig)-double(temp))*30);
    me = max(max(max(idiff)));
    idiff = uint8(double(idiff)*255/double(me));
    %imwrite(idiff,'ela.jpg');
    figure; imshow(idiff);
end

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Thomas 2013-6-14

编辑：Thomas 2013-6-15

I was asking how to convert the code so that it does not need to actually write to disk.

Image Analyst 2013-6-15

I don't believe there is a function to generate the jpeg artifacts other than by calling imwrite followed by imread.

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Answer 1

Walter Roberson 2013-6-14

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https://ww2.mathworks.cn/matlabcentral/answers/79108-how-do-i-encode-a-jpg-to-a-set-quality-level-in-order-to-calculate-it-s-ela-without-first-writing-it#answer_88842

编辑：Walter Roberson 2013-6-14

Writing to file is by far the easiest method. If you had a RAMDISK or solid state drive then the writing (and reading) would be quite fast.

Otherwise...

Have a look at this which shows how to do JPEG compression in Java. There is a point where the code creates an output stream associated with a disk file. I suspect that it is potentially possible to create an output stream into memory instead of into a file. Or you could probably connect the output stream from the compressor as the input stream to the decompresser. I have not worked with Java more than a little, so I am not certain that these can be done.