Digikam image re-compression - is it reliable?

[Shachar Shemesh]

On 06/20/2012 06:13 AM, Amos Shapira wrote:
> Hi,
>
> I'm preparing a disk-on-key with family photos to send to my mum and
> noticed something a bit unexpected.
> Most of the photos were taken with a Canon EOS 300D, maximum
> resolution and minimum compression.
> Some were taken with Android phone and iPhone 4.
> I use Digikam on Debian to manage my photos.
> The total space of the original images (including movies, which
> weren't touched) was ~7.6Gb.
> The total space after re-compression using default parameters (75%,
> JPEG, no resizing) - < 1Gb.
>
Here's a brief, and probably completely incorrect on several counts,
explanation of what JPEG compression does (and, for that matter, also
the single frame compression element of MPEG, MPEG2, MPEG4, H264 and
just about any lossy picture compression).

The picture is divided into squares. Each square is processed with an
algorithm called Discrete Cosine Transform (or DCT). If you know Fourier
transform, this is essentially the same thing, only in 2D. The resulting
is a square of the same size, but with each component in it representing
some "frequency", rather than a single pixel.

And here's the thing. Some positions in this square are more important
than others. The practical upshot is that getting the value for some of
the positions in this square will result in errors in the picture that
are more visible to the human eye than others. Coincidentally, some
positions in this square also tend to have lower values (formally, the
waves these positions represent have a lower energy in the actual
picture). The encoding allows the final image format to not contain the
full square, but leave out a certain part of it.

So, for lossless JPEG, all you do is take those components that have
energy, and use those. This still provides a considerable saving on the
uncompressed size. You didn't say how much each picture took, but an
uncompressed 24bits/pixel 1920x1280 image will take a little over 7MB.
Lossless compression should save about half of that. Lossless JPEG can,
depending on the actual picture, be about 3MB. Allowing even a small
amount of lossiness (say, JPEG 95%) should bring you down to about 2MB,
depending on the actual picture. As usual, the law of diminishing
returns is in effect. You pay little visual artifacts for the initial
reduction of size, and much more later.

I hope this enhances your understanding, and therefor your ability to
rely on the compression.

Shachar


-- 
Shachar Shemesh
Lingnu Open Source Consulting Ltd.
http://www.lingnu.com

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