Image Storage and Compression
As noted above, each RGB pixel
in the image records 24 bits of data, which requires the same amount of
storage space needed for three characters in this document (i.e., 3
"bytes"). Thus, when you take a picture with a 1.6 megapixel
camera, it must process and store 1600 x 1000 x 3 = 4,800,000 bytes
(4.8 MB) of data. A camera with 3.45 megapixel resolution will
generate and must process 2500 x 1500 x 3 = 11,250, 000 bytes (11.25 MB) of
data for each image!
With some cameras it is possible to record all of the image data using
TIFF or other "lossless"file formats which retain every bit recorded.
Digital digital cameras also use a file compression algorithm known as JPEG to
reduce the file size. JPEG data compression is similar to how a cashier in a
supermarket with a price scanner operates. If you buy a case of soft drinks the
cashier scan one can to get its value then enter the quantity "24" to
record the fact there are 24 cans with that value. The JPEG software scans the
image row-by-row looking for areas were numerous pixels have the same or similar
tone, and records 100 similar pixels with the color of the first, plus a
notation to make the next 99 the same when the file is displayed. The net effect
is a significant reduction or compression in file size without a significant
loss of actual image data.
Because the value of each color is stored as a finite value
consisting of 256 levels each of Red, Green, and Blue (256 x 256 x
256 = 16,777,216 discrete colors) it is possible for the JPEG
algorithm to further compress the image by encoding pixels within a
similar range of color as if they were the same color. In other
words, if ten adjacent pixels in the sky of a photograph differed by
only slightly the JPEG algorithm will average them and encode them as
that color x 10, reducing the data storage required by a factor of
almost 10 to 1 (i.e., 10:1). The downside of this type of JPEG
compression is that subtle details which were recorded by the CCD array are
irrevocably lost.
Compression in the camera
Because digital cameras typically offer several different
"quality" levels for image capture using generic terms such
as "uncomp", "best", "better", and
"good" it is impossible to explain here what they represent.
In some cameras such as the Kodak DC290 I use, "uncomp"
quality stores the file in uncompressed TIFF format using 6.6 MB to
record each 1792 x 1200 pixel image (1792 x 1200 x 3 = 6,451,200)
while "best", "better", and "good" apply
progressively more JPEG compression and smaller files, but at the
expense of lost image detail.
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