On 29 Dec 2012 Tim Hill <
t...@invalid.org.uk> wrote:
> Many people don't seem to understand that when a picture is
> lossy-compressed to a jpeg, the image is essentially reduced to be a
> complex mathematical formula, rather than the relatively simple x/y
> bitmap array of pixel information. It reduces a picture to an approximate
> algorithm. Editing that jpeg "formula" to make pixel-level changes to an
> image is impossible without staggering complexity and immense power.
That's not really the problem, even if you had the processing power
you would still lose more information every time a pixel is changed.
Indeed recalculating the coefficients every time a pixel changed would
be far far worse than one JPEG to bitmap operation, edit the
uncompressed bitmap and one convertion back to JPEG.
> consequently, as Druck describes, jpeg is always returned to be an 'array
> bitmap' so you can edit it. When re-saved it is re-interpreted from an
> array to be a jpeg 'formula' again. Each time you reload and save as jpeg
> you lose information to the approximations.
One conversion each way will lose very little further information
compared to what it already discarded by the JPEG process*, so is
acceptable for editing photos from a camera. What you want to avoid is
repeated conversion to and from JPEG when performing multiple editing
operations.
> Consequently, you should always work from a master 'array bitmap' and
> only publish in jpeg. Compression of 'array bitmaps' such as Sprites can
> still be achieved using non-lossy methods, such as storing them in a zip
> file.
It is far better to use a dedicated lossless compressed image format
such PNG, which can give much better results than zipping, and is also
immediately editable on a wide range of platforms.
* Regardless of the quality setting used, the JPEG conversion
immediately throws away half the colour information in the image. So
1000 pixel wide images can have 1000 changes of brightenss, but only
500 changes of colour on each line. The quality setting will determine
how much more information is discard from both sharpness of edges and
subtlety of graduations in both brightness and colour components.
---druck
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