On Sep 14, 2005, at 5:06 AM, Martin Trautmann wrote:
RAW *is* RAW sensor data. No interpolation or other processing
done. The "external programs" are called RAW converters and every
one of
them does the interpolation. Some better than others.
Thanks! Since I'm still analogue, I never checked RAW files up to now.
So I know very little about RAW:
- it's not a standard, but proprietary
(Sigma is open, Nikon is protected)
- it may include a jpeg preview
- it may be compressed
(while noise at higher ISO will reduce the
compressiability)
A RAW format image file is generally an enclosure file that contains
the following:
- Camera metadata: all the parameter for JPEG conversion that would
have been used in-camera for JPEG format image files plus bits like
time/date/manufacturer private data/etc.
- typically a JPEG thumbnail and JPEG preview rendering at low
resolution (maybe more)
- sensor data either uncompressed or losslessly compressed, a simple
2D matrix
RAW conversion software reads the metadata and takes the processing
parameters from it, reads it's own user settings, and performs
transformations on sensor data to transform the bayer matrix data
into RGB channel space. Those conversions include mapping the pixel
intensities for spatial resolution, interpolating the chroma
channels, and doing the linear to gamma-corrected rendering conversion.
So where does the masking of defective pixels, pixel calibration,
white
balancing or 'anti-vignetting' occur? Is RAW absolutely uncorrected?
All of that happens in the series of transformations performed by the
RAW converter, based on the camera metadata and user input it is
supplied with.
Is there some kind of 'golden raw' which may be used by the processing
software in order to compensate known errors? If it's not
preprocessed into
the raw output, is it included within every raw file?
Some of the parameters are constants fixed by the manufacturer for a
particular camera and built into the RAW converter at compile/link
time (that's why RAW conversion software usually needs to be revised
to accommodate new camera models), the rest are supplied in the
camera metadata.
There are two RAW standards efforts going on to normalize and
regularize the representation of RAW camera data: Adobe's Digital
Negative specification and the OpenRAW effort. Both of these propose
a standardized enclosure format and a way of representing metadata/
processing parameters such that a generic RAW conversion algorithm
can be devised independent of camera specific software development.
They are both still young efforts.
Godfrey
