V1.02
Thom Hogan’s Complete Guide to the Nikon D300
Page 132
(compressed)
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. Essentially, pixels are converted from
numbers into equations (the calculation used is called
a Discrete Cosine Transform). Blocks are operated on
from top left to bottom right. Essentially, detail within
each 8 x 8 pixel block is reduced, the amount of
reduction determined by the amount of quantization
(compression) applied.
3. The quantized (compressed) results for each block are
gathered into a single binary sequence, and this block
sequence is further encoded in a scheme called
modified run-length algorithm, which generally
produces further compression of the information (run-
length encoding assigns the shortest bit sequence to
the most-often-used pixel value, and the longest bit
sequence to the least-used pixel value).
Note that compression happens twice when a JPEG file is
created. The first compression is variable in level, but results
in
permanent
loss of information. Generally, it takes a JPEG
compression ratio of 10:1 or more to produce annoying
artifacts (see “JPEG Artifacts” below). JPEG compression ratios
of 4:1 or lower produce virtually imperceptible artifacts in
most common photo scenes. The second compression step
(run-length encoding) is lossless, meaning that the original
information—in this case, the discrete cosine transform
formula for each block—can be fully retrieved.
The D300 can produce photos encoded in JPEG format. The
D300 has two settings that determine how much compression
is used. The first, consists of the type of JPEG image:
fine
,
normal
, and
basic
, which determines the gross level of
compression. The second consists of the optimization of the
JPEG compression, with your choices being
Size priority
or
Optimal quality
. In terms of the visual quality of the final
image, your choices array something like this:
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An aside: which set of words you use (transforms, coefficients, quantized or
calculations, results, compressed) depends upon whether you’re a mathematician or
a layperson. A nerdy party trick is to use the vocabulary of the one you aren’t.
