Transcript Digital RAW Photography – Introduction September 2006

Digital RAW Photography –
Introduction
September 2006
• What is RAW and should I care?
• REVIEW - How most still camera sensors
capture a photo.
• (Sigma’s Foveon sensor works differently, but
we’ll ignore it in this discussion)
REVIEW
• The sensor is capable of only seeing gray values.
It’s an analog device that records light (photons)
as an electrical charge. The charge from each
picture element (pixel) is amplified and converted
to a 10, 12 or 14 bit digital signal via the ADC
(analog to digital converter) with up to 1,024,
4,096 or 16,384 levels, or shades of gray.
REVIEW
• The RAW file is a dump of the gray scale data
that the CCD captures. It is a relatively compact
file with far more color information than the
JPG/TIFF files produced inside the camera. We
will see why this is so a bit later
• Cameras that actually write the raw format to a
data file use 12 or 14 bit ADC. Cameras that only
record TIFF and/or JPG files use a 10 or 12 bit
ADC.
REVIEW
• How does the Raw data translate into an RGB
image?
• On top of the CCD's array of pixels (also called
photo sites) is fixed a colored filter with millions
of little colored cells, each aligned with a single
CCD pixel. The filters are arranged in alternating
rows, every other row with the filter colors Red
and Green, and the remaining rows as Blue and
Green. Green appears twice as often because our
eyes are most sensitive to differences in green for
the detail in images. This is the Bayer mosaic or
array.
REVIEW
• Still we don't have a RGB image at this point. To
get this we need to interpolate the RGB values
from the information we have in the neighboring
pixels. This interpolation, or de-mosaic process
creates a file with RGB values for each pixel. The
de-mosaic logic ranges from complex to
extremely complex and constitutes the lion’s
share of the computation work done by the Raw
converter
Digital Camera Imaging
Process
Raw Converter
• De-mosaicing, is the key role a raw converter
plays, but it’s not the only one. Raw conversion
requires the following additional steps:
– Colorimetric interpretation
– White Balance
– Gamma Correction
– Anti-aliasing & Sharpening
– Noise Reduction,
Colorimetric interpretation
• Many different filter sets are in use with digital
cameras. So the raw converter has to assign the
correct, specific color meanings to the red, green,
and blue pixels, usually in a colorimetrically
defined color space such as CIE XYZ, which is
based directly on human color perception.
– Filter set information must be available from the sensor
manufacturer to “support” a camera
White balance
• The white balance setting on the camera has no
effect whatsoever on the captured pixels when you
shoot raw—it’s simply 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 analyze
the image to determine white balance. Some
allow “pointing” at a neutral feature in the photo,
and / or specifying color temperature.
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. So the raw converter
applies gamma correction to redistribute the tonal
information so that it corresponds more closely to
the way our eyes see light and shade. (More
discussion later)
Anti-aliasing and Sharpening
• Problems can arise with very small details in an
image. If the detail is only captured on a redsensing pixel or a blue-sensing pixel, the raw
converter may have a hard time figuring out what
color that pixel should really be. Simple demosaicing methods struggle to maintain edge detail
so raw converters employ edge-detection and
anti-aliasing to compensate + sharpening to
restore edge acutance.
Noise reduction
• High ISO settings increase noise when the analog
signal is amplified before being processed by the
ADC. Noise reduction may be applied before demosaic (color noise suppression) and / or after to
the pixel luminance. Some converters also
suppress “hot pixels” that show up in long
exposures.
• It’s worth noting that ISO setting occurs before
the ADC step, so it can’t be changed by the Raw
converter.
Converter Summary
• All raw converters perform these tasks, but they
may use very different algorithms to do so, which
is why the same image may look quite different
when processed through different raw converters
(or cameras).
• 3rd party converters have a UI (user interface) that
allows a degree of control for some or all the
functions.
• Many converters add controls for saturation,
contrast, brightness and more.
All digital cameras have a raw
converter
Vendors make a subjective determination of what
the best “look” is, and then adjust their converter to
produce that result. (In-camera process variation is
limited to the camera’s preset options). Also to
save processing time, given the limitations in the
camera’s small CPU, the converter uses only 8 bits
of the available 10 or more bits of pixel
information. The unused bits are discarded. When
the RBG file is encoded and written, either as a
TIFF or JPG, the original raw file is discarded.
In-camera Image Quality
• Despite the limitations of the in-camera
converter, the photo quality can be
excellent. However, the quality may not
hold up if it’s extensively edited, so getting
exposure and white balance “right” are
critical.
Raw Inconveniences
• Raw files can’t be directly viewed (The EXIF file
contains a low res thumbnail for camera LCD display)
• Raw shots are SLOW - A 12-bit raw file contains 5.3X
more data vs. an 8 bit TIFF; 20X more than a high quality
JPG, so writing time to memory suffers.
• Investment of your time - Converters will do a “default”
conversion, with results not much (or no) better than incamera (but 12-bit file is retained).
• Raw file formats are proprietary - Not all converters can
convert every format. Adobe’s DNG open format hasn’t
been accepted by the camera manufacturers
Inconveniences Cont’d
• 12-bit color information is only available with
RAW files
• Full power of a computer to perform more
complex conversion algorithms utilizing up to 16
bit files yields enormous control over image
tonality
• Post-capture processing flexibility - many more
options and tools to adjust contrast, saturation,
white balance, sharpening, and exposure.
So why shoot Raw?
• 12-bit color information is only available with Raw files
• Full power of a computer to use more complex
algorithms utilizing up to 16 bit files yields enormous
control over image tonality
• Post-capture processing flexibility - many more options
and more precise tools to adjust contrast, saturation, white
balance, sharpening, and exposure
• The Raw file can be processed again and again as better
converters becomes available
Our eyes and film photography
• Our eye’s response to light is fundamentally
different compared to the digital camera’s sensor
• It’s non-linear –½ the light looks about 30%
dimmer, 10X the light looks about 2X as bright.
The human visual system easily comprehends
10,000:1 illumination range.
• Coincidentally, silver halide photography responds
in a manner very similar to the human eye. We
measure exposure in zones or f-stops; i.e. doubling
(or halving) exposure yields an exposure shift of
one zone or f-stop.
Digital Camera Sensor
• A linear device -1/2 the light is 1/2 as bright, 1/10
the light is 1/10 as bright
• A linear device is said to have a “Gamma” = 1
• An RBG image, at Gamma =1, will look very
dark (The sensor will only capture a brightness
range about 100:1 to 250:1, depending on the
sensor noise & pixel size, but the scene
illumination may vary by 10,000:1 or more)
Gamma Correction
• Gamma correction applied by the converter
redistributes the pixel luminance values so that
limited brightness range captured by the sensor is
“mapped” to match our eye’s sensitivity. Gamma
= 2.2 is a good match to distribute relative
brightness in a print or in a video display.
Gamma =1 vs. Gamma = 2.2
Tonality & Dynamic Range
the 12-bit advantage
Conclusions from Table
• Levels are lost when converting from a 12-bit RAW
format (4096 levels total) to an 8-bit file (8-bit B&W or
24-bit color). A print has an effective contrast range of
about 100:1 (6.6 f-stops), so image quality in an 8 bit
converted file will be adequate after gamma transform if
the exposure is correct and little editing is required.
• This is achievable in studio environments, but may be a
problem when using "natural" (i.e., uncontrolled) light.
The result will be “blocked out” shadows and/or “blown”
highlights, which have no tonal detail. 12-bit files have
plenty of levels - 4096 total. (Files are processed as 16-bit
in the computer, but 4 bits are unused)
Conclusions Cont’d
• You can edit to your heart's content without fear of
banding or other artifacts arising from limitations
of 8-bit files. More important, the extra levels
allow exposure adjustment “after the fact”.
• White Balance adjustment is more accurate
because the extra levels allow smaller, more
accurate shifts in the RBG channels.
What is NOT evident!
• Gamma transformed files are Non-linear
• Adjusting any of the converter functions discussed
earlier should be done before changing gamma to
avoid unexpected RBG color shifts and/or color
artifacts.
• Not all converters operate in this manner, nor does
the software documentation disclose it. The best
converters make adjustments in Gamma=1 space,
but display the results in Gamma=2.2 space after
adjusting.
NOT evident - Cont’d
• Photo Editors operate on Gamma transformed
files, so even though similar actions are available
in the editor and the raw converter, the results may
not be the same, especially for colorimetric
adjustments (levels, saturation, white balance).
Geometric operations (resize, crop, perspective,
etc) are generally not affected by gamma.
Guidelines for Raw workflow
• Save the original raw file. It’s a digital negative
that can be converted again as new and more
sophisticated raw converters are developed.
– Use a converter that doesn’t change the raw file.
Instead, it creates a “transform instruction set” that
outputs a standard format RBG. The converter
software retains the instruction set, so it can be used
again.
Guidelines Cont’d
• Use the converter for all colorimetric
adjustments. If local adjustments are needed,
create additional converted files with appropriate
adjustments, then use selections to build a
composite via layers in the photo editor.
• Limit sharpening to the minimum required to
compensate for anti-alias filter effects during the
de-mosaic process.
– Selective sharpening can’t be done in the raw converter anyway
and output specific sharpening (web or print) should be the last
step in the photo editor.
Guidelines Cont’d
• Apply noise reduction cautiously- it comes at the
cost of detail, which can’t be recovered later.
• Avoid cropping in the raw converter to reduce the
file size – Many of the geometric adjustments
available in the photo editor are lens-centric.
Perform them first, then crop.
– Some photo editors allow editing in 16-bit files.
However, if rule #1 is followed, there is little advantage
unless very extensive geometric editing is required,
where accumulated 8 bit quantizing errors might be
visible. In the end, an 8-bit file is all that can be viewed
or printed with today’s technology anyway.
Raw Conversion Software
• New 3rd party software seems to appear monthly
and revisions to existing are nearly as frequent.
In addition every camera manufacturer who offers
a raw format also offers a converter software
package(s), both free and/or purchased. At last
count there are 40+ converters available on the
web. However, many of them use the same raw
converter engine, DC RAW, supplied by David
Coffin
http://www.cybercom.net/~dcoffin/