[정보] D70 의 NEF는 손실이 있는 포멧 이랍니다.

http://www.majid.info/mylos/weblog/2004/05/02-1.html

전략....

To find out which one is correct, I read the C language source code for Dave Coffin's excellent reverse-engineered, open-source RAW converter, dcraw, which supports the D70. It appears the sensor data is first quantized using a curve that maps the 4096 linear values returned by the sensor into 683 discrete values, that are then encoded using a variable number of bits (1 to 10) with a tree structure similar to the lossless Huffmann or Lempel-Ziv compression schemes used by programs like ZIP.

The decoding curve is embedded in the NEF file (and could thus be changed by a firmware upgrade without having to change NEF converters), I used a D70 NEF file made available by Uwe Steinmuller of Digital Outback Photo.

The quantization is a lossy operation, and converts 12 bits into 9.4 bits' worth of resolution (dynamic range is unchanged). This is a fairly common technique - digital telephony encodes 12 bits' worth of dynamic range in 8 bits using the so-alled A-law and mu-law codecs. I modified the program to output the data for the decoding curve (Excel-compatible CSV format), and plotted the curve (PDF) using linear and log-log scales, along with a quadratic regression fit (courtesy of R). The curve is a gamma correction curve, linear for values up to 215, then quadratic.

In conclusion, Thom is right - there is some loss of data, mostly in the form of lowered resolution in the highlights.


위 글에 링크된 http://www.astrosurf.com/buil/d70v10d/eval.htm  에서도 같은 결론을 내리고 있습니다.

Nikon D70 seems to use a lossy compression algorithm through a transcoding table for the NEF files. The result is an average coding of about 9.5-bit per data point, and not the original 12-bit. It is possible that the codes missing ("gaps") in image RAW is the result of the decompression process (9 to12 bits coding), which created a phenomenon of posterization. Once image RAW processed (registration, stacking, ...) and converted into colors, the phenomenon practically disappears and the visual aspect of the image is preserved. See also this article. This interpretation of the aspect of the histogram remains to be confirmed.


한 채널에 주어진 12 bit을 9.4 bit로 변환한다고 하는군요. 이 두 글의 내용이 맞다면 (저는 맞다고 생각합니다만) 12 bit Lossless라 광고했던 Nikon은 명백한 거짓말을 했다고 볼 수 밖에 없습니다.

by shotgunlee | 2009/06/14 11:28 | 디지털카메라의 기술적인 면 | 트랙백 | 덧글(0)

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