2.5. PIXELS
In order for any digital computer processing to be carried out
on an image, it must first be stored within the computer in a suitable form
that can be manipulated by a computer program. The most practical way of doing
this is to divide the image up into a collection of discrete (and usually
small) cells, which are known as pixels. Most
commonly, the image is divided up into a rectangular grid of pixels, so that
each pixel is itself a small rectangle. Once this has been done, each pixel is
given a pixel value that represents the colour of that pixel. It is assumed
that the whole pixel is the same colour, and so any colour variation that did
exist within the area of the pixel before the image was discretized is lost.
However, if the area of each pixel is very small, then the discrete nature of
the image is often not visible to the human eye. Other pixel shapes and
formations can be used, most notably the hexagonal grid, in which each pixel is
a small hexagon. This has some advantages in image processing, including the
fact that pixel connectivity is less ambiguously defined than with a square
grid, but hexagonal grids are not widely used.
Part of the reason is that many image capture systems
(e.g. most CCD cameras and scanners) intrinsically discretize the
captured image into a rectangular grid in the first instance.[14]
Pixel Values
Each of the pixels that represent an image stored inside a
computer has a pixel value, which describes how
bright that pixel is, and/or what colour it should be. In the simplest case of
binary images, the pixel value is a 1-bit number indicating either foreground
or background. For a greyscale images, the pixel value is a single number that
represents the brightness of the pixel. Often this number is stored as an 8-bit
integer giving a range of possible values from 0 to 255. Typically zero is
taken to be black, and 255 is taken to be white. Values in between make up the
different shades of grey.
To represent colour images, separate red, green and blue
components must be specified for each pixel (assuming an RGB colourspace), and
so the pixel `value' is actually a vector of three numbers. Often the three
different components are stored as three separate `greyscale' images known as
colour planes (one for each of red, green and blue), which have to be
recombined when displaying or processing.
Multi-spectral images can contain even more than three
components for each pixel, and by extension these are stored in the same kind
of way, as a vector pixel value, or as separate colour planes.[13]
The actual greyscale or colour component intensities for each
pixel may not actually be stored explicitly. Often, all that is stored for each
pixel is an index into a colourmap in which the actual intensity or colours can
be looked up.
Although simple 8-bit integers or vectors of 8-bit integers
are the most common sorts of pixel values used, some image formats support
different types of value, for instance 32-bit signed integers or floating point
values. Such values are extremely useful in image processing as they allow
processing to be carried out on the image where the resulting pixel values are
not necessarily 8-bit integers. If this approach is used then it is usually
necessary to set up a colour map, which relates particular ranges of pixel
values to particular displayed colours.
PROPERTIES OF PIXEL
Brightness / Contrast: allows alteration of brightness and
contrast of selected pixels or over the entire RGB image if no pixels have been
selected.
Brightness: this is the amount of light intensity or received by
the eye regardless of color.
Luminance: is the quantity of light intensity emitted per
square centimetre of an illuminated area.
Hue: this is the predominant spectral purity of the colour
light.
Saturation: this indicates the amount of all colours present in
the given picture.
Brightness makes the image lighter or darker overall, while
Contrast either emphasizes or de-emphasizes the difference between lighter and
darker regions.
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Brightness
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Increase or decrease the brightness of pixels. Low brightness
will result in dark tones while high brightness will result in lighter, pastel
tones.
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Contrast
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Increase or decrease contrast. Increasing contrast increases the
apparent difference in lightness between lighter and darker pixels.
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Contrast
Contrast is easy to understand visually. Artistically,
contrasting colors are colors that are opposite on the color wheel colors
that are opposites. In a high contrast image, you can see definite
edges and the different elements of that image are accented. In a low
contrast image, all the colors are nearly the same and it's hard to make out
detail.
Contrasting colors in terms of a computer's representation of an
image, means the "primary colors" or the colors with color components of 0 or
255 (Min and Max). Black, White, Red, Green, Blue, Cyan, Magenta, and
Yellow are the high contrast colours. When all the colors in an image are
around one single color, that image has low contrast. Grey is the usual
color of choice because it rests exactly in between 0 and 255 (127 or 128).
Colors as Hue, Saturation and Brightness
Describing colors using hue, saturation and brightness is a
convenient way to organize differences in colors as perceived by humans. Even
though color images on computer monitors are made up of varying amounts of Red,
Green and Blue phosphor dots, it is at times more conceptually appropriate to
discuss colors as made up of hue, saturation and brightness than as varying
triplets of RGB numbers. This is because human perception sees colors in these
ways and not as triplets of numbers.
If we imagine the three primary colors red, green and blue
placed equally apart on a color wheel, all the other colors of the spectrum can
be created by mixes between any two of the primary colors. For example, the
printer's colors known as Magenta, Yellow, and Cyan are mid-way between Red and
Blue, Red and Green and Blue and Green respectively.
This diagram is called the color wheel, and any particular
spot on the wheel from 0 to 360 degrees is referred to as a hue, which
specifies the specific tone of color. "Hue" differsslightly from "color"
because a color can have saturation or brightness as well as a hue.
Saturation is the intensity of a hue from gray tone (no
saturation) to pure, vivid color (high saturation).
Brightness is the relative lightness or darkness of a
particular color, from black (no brightness) to white (full brightness).
Brightness is also called Lightness in some contexts.
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