A pixel is defined as: the smallest discrete
element of an image or picture on a computer screen; a single element of a
raster image.
Think of a tile mosaic where each individual colored tile is
placed in an exact order next to other colored tiles to produce a larger image
when viewed from a suitable distance. Each individual pixel represents one piece
of color – and when placed together with enough other pixels, an image is
formed, that to the human eye appears to be a continuous tone photograph.
Whether scanned from film or captured digitally, all digital imaging today is
reliant on these small pieces of color that come together to form a larger
image.
Side Bar:
Previous to modern digital imaging, photographic prints were
made optically, and were not based on pixels, but were made using enlargers.
On the publishing side, full color images were stripped in with color
separated negatives – which were then shot with a prepress camera to produce
metal plates. Today most digital photographic prints are produced by sending
digital images to various types of printers, the most common types being ink-jet
and dye-sublimination. Photos being published in books or magazines are
created by placing high-resolution image files in page layout applications,
which are then output to imagesetters or platesetters.
How deep are your pixels? Understanding bit depth
Bit depth describes the number of bits used to represent
each element in an image. A basic rule is that eight bits make up a byte,
which will be important later.
Lets start with the most basic of all image types: line
art. A true bitmap image of line-art is a single bit image, meaning
there is either 100 percent black or white. This is expressed by two values
only: 1 or 0, black or white, off or on. There are truly no shades of gray to a
single bit image.
So each bit has two possible values – black or white. Eight
(8) bits combine to make a byte – which is 2^8 (2x2x2x2x2x2x2x2), or 256
possible values or shades of gray. This is where the standard 256 level
grayscale comes from.
A standard 8-bit RGB color image consists of three
over-layed grayscale images, with one representing red, one green & one
blue in various levels for each channel. Each channel has 256 possible values x
3 channels (256x256x256) which equal 16.7 million possible values. This
is your normal RGB color image that is seen on websites or worked on in
Photoshop from a digital camera or scan.
The next step would be an image that is 12 or 16 bit,
meaning that instead of eight bits per channel, there are 16
(2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2), which would give you 65,536 possible values
for each channel. Instead of 16.7 million possible colors, 16-bit images can
contain up to 281,474,976,710,656 (65,536 x 65,536 x 65,536) colors in a single
exposure.
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