Re: Gimp hangs when saving as png

Per Larsen wrote:
> Amedee Van Gasse wrote:
>
>> Wolfgang Fellger schreef:
>>
>>
>>> Amedee Van Gasse wrote:
>>>
>>> Hi Amedee!
>>>
>>>
>>>> Gimp 2.2.8 on Windows 2000.
>>>>
>>>> I have never been able to save images as png in the Gimp.
>>>
>>>
>>> Only a dumb guess, but: Have you waited long enough? I've seen Gimp
>>> sometimes
>>> needs ages to open a new dialog, or alternatively places the new
>>> window behind
>>> existing ones.
>>
>>
>>
>> 5 minutes for a 22K file is too long in any circumstance, wouldn't you
>> agree?
>> But if you say so: I'll leave it running for an hour or so and see what
>> happens.

Yikes. Hmm. Interesting. No, that sounds like a crash to me. My biggest
(5-20M) files are usually saved in under 2 minutes.

>
> My Gimp 2.2.8 saved a 1.012 kB jpg-file (2400x2400 dpi) as png
> (Compression level 9, Interlacing (Adam7)) in about 20 secs. It ended up
> as 10.534 kB.
>
> I've never used png-format before and I'm a little surprised about the
> compression levels and file sizes:
>
> Level Size
> 0 22.334 kB
> 1 11.701 kB
> 2 11.572 kB
> 4 10.812 kB
> 9 10.534 kB
>
> Obviously no big difference between 1 and 9
>
> PerL

The difference being that JPEG is a lossy format (Throws information
away, so it's easy for it to be smal), while PNG is a lossless format
(All information is kept.). Level 0 is no compression at all, while
level 9 is trying to get the maximum compression. As you can see, higher
and higher compressions start to suffer from the Law of Diminishing
Returns. In my experience, lossless compression of any file (zip, bzip2,
png) rarely gets the file to less than 50% of the uncompressed size.
(Exception being raw text of course. Generally you can get that to 5-10%
of original size.)

While there isn't much point converting a jpeg to a png if you're just
going to put it up on the web (information is already gone, so you may
as well go for the smaller jpeg), if you want to edit and alter the
image, saving in jpeg between each edit will eventually leave you with a
very damaged image. (Information lost at each save.)

That's why I use my digital camera in RAW mode rather than JPEG mode,
because I get all the information that the camera saw. *sighs* Including
the sensor noise when I try long exposure night shots. *sighs again*

Personally, after editing any photo, I tend to save the result as an
XCF, and then flatten and save to a PNG. While a PNG is bigger than a
JPEG, often a few weeks after "finishing" an image, I'll see some error
that needs fixing. I could do it in the XCF, but often the error is a
colour cast, which means I'd need to flatten the image anycase - may as
well work on a pre-flattened PNG copy. If it gets too complex and starts
to involve layers again, I then save the image as <filename>-2.xcf/.png,
<filename>-3.xcf/.png, etc.

Joal Heagney
.

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