The consolidated CRAW compression thread |
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springtide ![]() Senior Member ![]() Joined: 29 October 2007 Location: United Kingdom Status: Offline Posts: 252 |
![]() ![]() ![]() ![]() Posted: 09 November 2007 at 10:29 |
Has everybody read this... http://photoclubalpha.com/2007/11/08/nikon-d300-and-sony-a700-sensor-similarity/ This was the bit that caught my eye.... "Sony’s .ARW2 file format, uncompressed, is 12-bit. Their cRAW compressed format uses 8-bit depth......" This seems to be saying that the dynamic range is reduced for cRAW. Is this right? {edited by ab012 - merged with the other ongoing CRAW compression thread) Edited by ab012 - 01 May 2008 at 14:25 |
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Simon
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Gabriel ![]() Senior Member ![]() Emeritus Member Joined: 05 December 2006 Location: France Status: Offline Posts: 1931 |
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I'm a bit surprised.
I thought that the only difference between Sony's uncompressed raw and compressed raw was the filesize and processing speed. In other words, I thought that cRaw was using a lossless compression. After all, a100 is also using a lossless compression for its raw files. Anyhow, I'd be interested by a test of this. |
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gm4jjj ![]() Senior Member ![]() Joined: 20 August 2005 Location: Scotland Status: Offline Posts: 660 |
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I don't think that is true at all, certainly it isn't even hinted at in the Sony FAQ on cRAW v RAW.
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-- David
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vnatchu ![]() Newbie ![]() Joined: 02 March 2007 Location: United States Status: Offline Posts: 13 |
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cRAW is most certainly lossy.
it uses a fixed length 8-bit compression of the 12-bit data. "most" data in "most" cases should be preserved, but the compression scheme is complicated and can be lossy in certain cases. If you are looking for proof that the scheme uses a fixed 8-bit compression or the scheme itself look at Dave Coffins dcraw.c it has the code for this and will likely be used in a variety of converters. vishnu |
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springtide ![]() Senior Member ![]() Joined: 29 October 2007 Location: United Kingdom Status: Offline Posts: 252 |
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OK, no more cRAW then! |
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gm4jjj ![]() Senior Member ![]() Joined: 20 August 2005 Location: Scotland Status: Offline Posts: 660 |
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That's most interesting, so do we still get 12 bit data when it is uncompressed from cRAW?
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-- David
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springtide ![]() Senior Member ![]() Joined: 29 October 2007 Location: United Kingdom Status: Offline Posts: 252 |
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I assume once you've thrown away that data, it's gone. |
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Sanjuro ![]() Emeritus group ![]() Administrator emeritus Joined: 19 September 2005 Country: United Kingdom Location: Sweden Status: Offline Posts: 5849 |
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I always wondered why Sony offers these 2 formats, and If I would have a A700 I would NOT shoot in cRAW.
It has to be something that is why it is called compressed and compressed for me doesn't sound good. So this finally has given an answer. Sounds very logical to me, so the Dynamic range in certain kind of images could be reduced even more in cRAW. |
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Rgds
Sanjuro "I paint objects as I think them, not as I see them." --Pablo Picasso |
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gm4jjj ![]() Senior Member ![]() Joined: 20 August 2005 Location: Scotland Status: Offline Posts: 660 |
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Be careful - bit depth and dynamic range are not the same thing.
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-- David
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pegelli ![]() Admin Group ![]() Dyxum Administrator Joined: 02 June 2007 Country: Belgium Location: Schilde Status: Offline Posts: 39026 |
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Exactly !: Dynamic range is the difference between the darkest part that gives the smallest signal and the brightest part before it "blows out". Bit depth is how many levels this range between darkest and lightest is devided into. Also be carefull to say that compression allways throws away data. There are compression algorithms which are scientifically proven to be lossless (eg. LZW compression of Tiff files). Edited by pegelli - 09 November 2007 at 14:01 |
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gm4jjj ![]() Senior Member ![]() Joined: 20 August 2005 Location: Scotland Status: Offline Posts: 660 |
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In another thread on Dyxum Frank summed this up:
see this page
I will stick to cRaw as I have never seen any difference in output between the it and RAW and it has the advantage that it takes less memory card space and it allows faster frame rates. Sony's FAQ is very well crafted to avoid the answer as to whether it is lossy or not. ![]() |
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-- David
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pegelli ![]() Admin Group ![]() Dyxum Administrator Joined: 02 June 2007 Country: Belgium Location: Schilde Status: Offline Posts: 39026 |
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I can't be angry about this. Probably if they would word it differently a lot of people will discard it for the theoretical reasons mentioned in this and other threads while as you say in practice you cannot tell the difference. Probably even pixel peeping at high magnification won't show you what it does, so no way a print will be influenced |
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gm4jjj ![]() Senior Member ![]() Joined: 20 August 2005 Location: Scotland Status: Offline Posts: 660 |
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Yes perhaps angry was a bit strong, just perplexed by their evasion.
I think the main thing to appreciate is that Camera dynamic range is not compromised by cRAW and it does not produce 8 bit depth output when uncompressed. |
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-- David
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Gabriel ![]() Senior Member ![]() Emeritus Member Joined: 05 December 2006 Location: France Status: Offline Posts: 1931 |
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After a quick look at dcraw, cRaw is indeed lossy, and using 8 bits. It seems that at each group of 16 bayer pixels, it stores the max value, and then the following 16 pixels will be stored using 8 bits with values that can go up to this max value. So overall dynamic range is preserved, but you only have 8bits of dynamic per 16 pixels. This is a very simple scheme, and for sure it could have been a bit more efficient, but overall it's not that bad. |
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