Thanks. On 21 June 2012 04:53, Shachar Shemesh <shac...@shemesh.biz> wrote:
> 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 > > -- [image: View my profile on LinkedIn] <http://www.linkedin.com/in/gliderflyer>
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