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 2ยข
Alex
______________________________________
Alexander Forsythe
Senior Imaging Engineer
Academy of Motion Picture Arts and Sciences
Science and Technology Council
email - [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
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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