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Bits
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In computer terms, a "bit"
(binary digit) is the smallest piece of information and has a value
of either "0" or "1" which actually corresponds to one of the
millions of "switches" inside the computer being "ON" or "OFF".
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In a 1 bit image we can
assign the binary value "0" to black and "1" to white.
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A
2 bit
image can have 2^2
= 4 tones: 00 (black), 01 (gray), 10 (gray), and 11 (white).
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8 bit
image can have 2^8
= 256 tones ranging from 00000000 (0) to 11111111 (255). |
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JPEG
images are often referred to as 24 bit images because they can store
up to 8 bits in each of the 3 color channels and therefore allow for
256 x 256 x 256 = 16.7 million colors.
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| 32 bit Floating Point Format (for advanced
users) |
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In the topic about sensor linearity we saw
that the upper half of the tones is used to describe the brightest
stop. As a consequence, even a 16 bit INTEGER image only has 16
tones to describe the darkest stop in a 12 stop image, while for the
brightest stop, 32,768 tones are available. This is the opposite of
human vision which is more sensitive to shadow detail than to
highlight detail. A 32 bit INTEGER image provides more tones but has
the same limitation of having a disproportionate amount of tones for
the highlights. 32 bit FLOATING POINT images address this issue by
making more efficient use of the 32 bits. Instead of using 32 bits
to describe 4,294,967,296 integer numbers, 23 bits are allocated to
a fraction, 8 bits to an exponent, and 1 bit to a sign, as follows:
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V = (-1)^S
* 1.F
* 2 ^ (E-127),
whereby:
S
= Sign, uses 1 bit and can have 2 possible values
F
= Fraction, uses 23 bits and can have 8,388,608 possible values
E
= Exponent, uses 8 bits and can have 256 possible values
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Practically speaking, this allows for an
almost infinite number of tones between level "0" and "1", more than
8 million tones between level "1" and "2" and 128 tones between
level "65,534" and "65,535", much more in line with our human vision
than a 32 bit integer image (*).
Because of the infinitesimally small numbers that can be stored, the
32 bit floating point format allows to store a virtually unlimited
dynamic range. In other words, 32 bit floating point images can
store a virtually unlimited dynamic range in a relatively compact
way with more detail in the shadows than in the highlights and take
up only twice the size of 16 bits per channel images, saving memory
and processing power. A higher accuracy format allows for smoother
dynamic and tonal range compression. This format is important in the
computer graphics industry (gaming and animation) and supported by
Adobe Photoshop CS2.
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(*) Which
would have the same number of tones (65,536) between level "0" &
"1", level "1" &"2", and level "65,534" &"65,535"
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