title: How to save quantized DCT coefficients as a JPEG image with Python?
body: <p>I am creating a Python 3 application for steganography, specifically JPEG steganography. For this reason, I have to implement some basic JPEG compression to access the quantized DCT coefficients and embed bits into the LSBs. I have implemented all of this, but now I have a problem with saving the coefficients as a JPEG image.</p>
  
  <p>All of the libraries I have found will perform full JPEG compression when saving an image with the .jpg extension. But I don't want it, as I have already done the lossy parts of the compression myself. I want it to only perform the lossless parts and save the image without performing DCT transformation and quantization on it again.</p>
  
  <p>Has anyone tried to do this before? Are there any libraries out there that let you essentially save a 2 or 3 dimensional numpy.ndarray as a JPEG image without performing lossy compression on it again?</p>
  
  <p>Here is an example of how the transformed, quantized, and embedded coefficients look:</p>
  
  <pre><code>[[[-48.  -1.  -9.]
    [ -3.   0.   1.]
    [  0.  -0.  -0.]
    ...
    [  0.  -0.   0.]
    [ -0.  -0.   0.]
    [ -0.  -0.  -0.]]
   [[  3.   0.  -2.]
    [ -0.  -0.   0.]
    [  0.   0.   0.]
    ...
    [ -0.   0.  -0.]
    [  0.   0.  -0.]
    [ -0.  -0.   0.]]
   [[ -0.   0.   0.]
    [ -0.   0.   0.]
    [  0.   0.   0.]
    ...
    [  0.   0.  -0.]
    [ -0.   0.   0.]
    [  0.   0.  -0.]]]
  </code></pre>