[digiKam-users] BMP Vs PGN

[Remco Viëtor]

On vendredi 25 novembre 2022 16:17:26 CET Alberto Guardia wrote:
> Hi,
> 
> I shot in RAW (actually Fuji's RAF) and get standard 26.1 Mb files.
> 
> After processing them I get respectively :
> 
> PNG 79,8 Mb  ==> 3 times !!
> 
> BMP 48,8 Mb  ==> 1,87 times !
> 
> JPEG 14,2 ==> 0.54 times.
> 
> 
> Do Png files really need to be so large or is there something wrong in
> my settings?

You can expect an uncompressed image file to be larger than the corresponding 
raw file, as you go from 12 or 14 bits/pixel to at least 3×8=24 bits/pixel (or 
even 48 bits/pixel, when you use 16 bit/channel). How much larger exactly will 
depend on compression settings (both raw and image files can be compressed).
24 bit/pixel already gives an increase of ~1.7× the raw file, more if the 
latter is compressed.

Difficult to say anything about the relative sizes of the .BMP and .PNG files 
without knowing which bit depth and compression settings you used.

PNG files can be 16 bit/channel. The format also supports (lossless) 
compression. Compared to an 8-bit/channel format, that alone would double the 
file size (but may leave more room for further processing).
 ~80Mb for a 16-bit PNG sounds reasonable.

.BMP files can have quite a few encodings, but no 16 bits/channel. As I'm not 
using MS-Windows, I rarely ever use .BMP

JPEG is expected to be much smaller, as it is a (possibly very) lossy 
compressed format with 8 bits/channel. The lossy compression allows much 
smaller files than a lossless compression.

Remco