Re: [Openexr-devel] RAW images in OpenEXR?

[Florian Kainz]

In the proposed format the raw image is split into channels, one
per filter color.  The individual channels are not fully populated;
for example, with an RGB Bayer-pattern CFA, only half of the pixels
in the green channel, and a quarter of the pixels in the red and
blue channels have data.  My original email describes how this can
be stored efficiently in an OpenEXR file.

As I mentioned earlier, I have not done enough testing to know how
algorithms such as B44 affect RAW images.  On the other hand, looking
closely at the RAW images output by some cameras, I suspect that the
files have been compressed with a lossy method (or maybe the camera
sensors produce rather peculiar block-like artifacts).



Alexander Forsythe wrote:
> Thanks Florian
>
> That's good news that the channels are compressed individually.
> How exactly what this achieved given there's essentially only 
> one channel in a RAW image?
> Regardless, it's always been my impression that lossy compression, via 
> either a lossy compression algorithm or quantization, of RAW images is 
> frowned upon, but maybe options are good ... Just my 2¢
>
> Alex
>
> ______________________________________
> Alexander Forsythe
> Senior Imaging Engineer
> Academy of Motion Picture Arts and Sciences
> Science and Technology Council
> email -   address@hidden <mailto:address@hidden>
> address - 1313 Vine Street
>                 Hollywood, CA 90028
> phone -    310-247-3000 x310
>
>
>
>
>
> On Jun 18, 2008, at 10:40 AM, Florian Kainz wrote:
>
> > Alexander Forsythe wrote:
> > > Florian, B44 and B44A are wonderful algorithms for traditional uses 
> > > of OpenEXR, but you correctly pointed out that Lossy compression 
> > > methods (B44, B44A) would introduce crosstalk between the channels. 
> > >  This is highly undesirable and exactly what the Red camera folks are 
> > > now taking a lot of heat for.  I'd suggest limiting compression to 
> > > lossless compression methods for RAW data storage.
> >
> > Hi Alex,
> >
> > the proposed RAW storage format eliminates crosstalk between channels.
> > The channels are compressed individually, and no information leaks from
> > one channel into another.
> >
> > Artifacts caused by lossy compression still introduce color and
> > luminance noise, just as with RGB or luminance/chroma images. 
> >  Sharpening peformed during conversion to RGB may amplifiy noise in 
> > luminance/chroma
> > images.  So far we haven't done any tests to determine which compression
> > methods would work well for RAW images.
> >
> > Florian
> >
> >
> >
> >
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