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White Balance
Readout
Electronics Sontrast JPEG = JPEG File |
aturation Compression
Interpolation
'so pose Sharpness
setting etc
I RAW file
(a)
DODZMOA
8 bit data
12 bit data White Balance
Contrast JPEG
Bayer JPEG File
( b ) RAW file Saturation - | Compression
Sharpness
etc
[ ] TIFF File ]
8 or 16 bit data
Figure 3-1. (a) Processes carried out by the camera to the data from the sensor to produce |] PEG
and/or RAW files and (b) Processes carried out to the RAW file by the RAW image conversion
software to produce J PEG and/or TIFF files. Adapted from °
The output from each of the original red, green and blue sensitive pixels of the image sensor,
are read out of the array by the array electronics and pass through an analog to digital
converter. The readout electronics collect and amplify the sensor data according to the ISO
setting (little amplification for a low ISO setting and greater amplification for a high ISO
setting). At this point the data can be saved into a RAW file on the memory card, or it can be
further processed to yield a JPEG image file by the camera (Figure 3-1(a)) or the RAW
image conversion software (Figure 3-1(b)). Note that some cameras can store a JPEG
image along with the RAW file.
The RAW file contains two different types of information: the image pixels themselves, and
the image metadata that RAW converters need in order to process the sensor data into an
RGB image. This metadata tells RAW converters which colour each pixel represents.’ The
RAW converter then uses this to convert the greyscale RAW data into a colour image by
interpolating the “missing” colour information for each pixel from its neighbours. This process
is known as Bayer interpolation or demosaicing. However, in addition to demosaicing RAW
conversion typically also involves the following steps:
1) White balance. The white balance setting on the camera is recorded as a metadata tag in
the RAW file. Some RAW converters can read this tag and apply it as the default white
balance (which the user can then override if desired), while others may ignore it
completely and analyse the image to determine white balance.
2) Colorimetric interpretation. Each pixel in the RAW file records a luminance value for either
red, green, or blue. The RAW converter has to assign the correct, specific colour
meanings to the “red,” “green,” and “blue” pixels, usually in a colorimetrically defined
colour space such as CIE XYZ, which is based directly on human colour perception.
3) Gamma correction. Digital RAW captures have linear gamma (gamma 1.0), a very
different tonal response from that of either film or the human eye. The RAW converter
applies a gamma correction curve to redistribute the tonal information so that it
corresponds more closely to the way our eyes see light and shade. Most converters use
an encoding gamma of 1/2.2 but this may vary.
Version No. 1.0 104 Date : 14/10/2013
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