File sizes after conversion

[Conway Upshur]

Hello,
I'm new to digikam and RAW processing.
I converted a RAW (CR2) file to DNG and it went from about 8MB to 17MB.
I edited and then saved it as PNG.  The PNG size is 40MB.
What am I doing wrong?   Thanks, Conway

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[Dr. Martin Senftleben]

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Hi,

Am 22.01.2012 21:50, schrieb Conway Upshur:
> Hello, I'm new to digikam and RAW processing. I converted a RAW
> (CR2) file to DNG and it went from about 8MB to 17MB. I edited and
> then saved it as PNG.  The PNG size is 40MB. What am I doing wrong?
> Thanks, Conway

we just had this discussion (for me the second time, and I still don't
understand it fully, yet I try): it has to do with colour depth and
other things, which are added to the files to make sure all data is
kept intact. Basically, there is more info about the image or rather
about each pixel stored in the files.

I'm sure someone else can explain it more detailed.

Cheers
Martin

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[Conway Upshur]

Thanks, I'll keep studying. 

2012/1/22 Dr. Martin Senftleben <[hidden email]>

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Hi,

Am 22.01.2012 21:50, schrieb Conway Upshur:

> Hello, I'm new to digikam and RAW processing. I converted a RAW
> (CR2) file to DNG and it went from about 8MB to 17MB. I edited and
> then saved it as PNG.  The PNG size is 40MB. What am I doing wrong?
> Thanks, Conway

we just had this discussion (for me the second time, and I still don't
understand it fully, yet I try): it has to do with colour depth and
other things, which are added to the files to make sure all data is
kept intact. Basically, there is more info about the image or rather
about each pixel stored in the files.

I'm sure someone else can explain it more detailed.

Cheers
Martin

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[Remco Viëtor]

On Sunday 22 January 2012 12:50:40 Conway Upshur wrote:
> Hello,
> I'm new to digikam and RAW processing.
> I converted a RAW (CR2) file to DNG and it went from about 8MB to 17MB.
> I edited and then saved it as PNG.  The PNG size is 40MB.
> What am I doing wrong?   Thanks, Conway

I can't give the exact details, but here are some indications:
- CR2 -> DNG: DNG has the possibility to embed the original file (thus adding
a few MB), further, teh compression of the RAW data might be different, and
certain metadat could be stored in a different format (text, instead of binary
codes, esp. for makerdata fields).
- PNG: forgetting compression for now, RAW (CR2) files store 12 or 14 bits per
pixel.  But those pixels have no inherent colour information (look up 'Bayer
matrix'). The PNG stores an image that has as many pixels as the RAW file, but
with colour information, at either 8 or 16 bits/colour, and possibly
transparency (also 8 or 16 bits). That gives you at least 24 bits/pixel (8
bits/colour, no alpha channel) and at most 64 bits/pixel. And you might have
added tags, caption, title, etc.

Given that you go from 8 MB for the raw to 40 MB for the png, I suspect you
use 16-bit colour depth

Remco

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[Martin (KDE)]

Am 22.01.2012 21:50, schrieb Conway Upshur:
> Hello,
> I'm new to digikam and RAW processing.
> I converted a RAW (CR2) file to DNG and it went from about 8MB to 17MB.
> I edited and then saved it as PNG.  The PNG size is 40MB.
> What am I doing wrong?   Thanks, Conway

Raw data does not contain any colour information. it is a
black/white/grey picture where the so called bayer-pattern is layed
over. The bayer-pattern takes four points from the raw greyscaled photo
and sets one to red, one to blue and two to green and then you have
coloured photos (this process is called demosaicing). With raw data you
have to compress the grey values only (but with 12 up to 16 bit). See
http://en.wikipedia.org/wiki/Bayer_pattern and
http://en.wikipedia.org/wiki/Demosaicing for more information.

For DNG conversion: This depends. You can embed the original raw data
together with the converted DNG data, so the result will have at least
double size.

PNG on the other had is a RGB colour format (with alpha channel). If you
encode it with 16bit per colour you increase the data again. If this can
have 40MB depends on the size and the data. Uniform coloured areas can
be compressed better, high iso noise is worse.

Regards
Martin
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