Well, OK, some interpretation: that page is about as it says in
the subtitle, "A discussion of the ambiguous meanings of pixel and
megapixel," not a justification for a uniquely correct answer. If
I can ever get a clear definition of "pixel" from you, I'd like to
compare and contrast it to these, and see what shakes out.
Hey j, I think I've finally got the clear definition of "pixel".
And it doesn't even involve buyng a $220 book (or getting my copy
back from the guy who borrowed it).
It's from RLG, the "Research Libraries Group", and they seem to be
a pretty authorative outfit, founded by Columbia, Harvard, and Yale
Universities and The New York Public Library and claiming a
membership of "over 160 universities, national libraries, archives,
historical societies, and other institutions".
http://www.rlg.org/rlg.html
This is part of their series of "Guides to Quality in Visual
Resource Imaging". Those are frighteningly well researched, quote
ISO standards, technical papers, you name it.
http://www.rlg.org/visguides/visguide3.html
I've quoted the entire section 2 "Basic Terminology". It pretty
clearly defines a "pixel" as a spatial entity, and color as an
attribute of a pixel.
First, three sentences, out of context
"The pixel dimensions of the image are its width and height in
pixels. The density of the pixels, i.e., the number of pixels per
unit length on the document, is the spatial sampling frequency, and
it may differ for each axis".
"The value of each pixel represents the brightness or color of the
original object..."
Then the entire "Basic Terminology" section, with those three
sentences in context.
--- begin quote ---
2.0 Some Basic Terminology
Digital images are composed of discrete picture elements, or
pixels, that are usually arranged in a rectangular matrix or array.
Each pixel represents a sample of the intensity of light reflected
or transmitted by a small region of the original object. The
location of each pixel is described by a rectangular coordinate
system in which the origin is normally chosen as the upper left
corner of the array and the pixels are numbered left-to-right and
top-to-bottom, with the upper left pixel numbered (0,0).
It is convenient to think of each pixel as being rectangular and as
representing an average value of the original object's reflected or
transmitted light intensity within that rectangle. In actuality,
the sensors in most digital image capture devices do not "see"
small rectangular regions of an object, but rather convert light
from overlapping nonrectangular regions to create an output image.
A document or another object is converted into a digital image
through a periodic sampling process. The pixel dimensions of the
image are its width and height in pixels. The density of the
pixels, i.e., the number of pixels per unit length on the document,
is the spatial sampling frequency, and it may differ for each axis.
The value of each pixel represents the brightness or color of the
original object, and the number of values that a pixel may assume
is the number of quantization levels. If the illumination is
uniform, the values of the pixels in a gray-scale image correspond
to reflectance or transmittance values of the original. The values
of the pixels in a color image correspond to the relative values of
reflectance or transmittance in differing regions of the spectrum,
normally in the red, green, and blue regions. A gray-scale image
may be thought of as occupying a single plane, while a color image
may be thought of as occupying three or more parallel planes.
A bitonal image is an image with a single bit devoted to each pixel
and, therefore, only two levels—black and white. A gray-scale image
may be converted to a bitonal image through a thresholding process
in which all gray levels at or below a threshold value are
converted to 0-black—and all levels above the threshold are
converted to 1-white. The threshold value may be chosen to be
uniform throughout the image, that is, "global thresholding" or it
may be regionally adapted based on local features, or "adaptive
thresholding." Although many high-contrast documents may be
converted to bitonal image form and remain useful for general
reading, most other objects of value to historians and researchers
should probably not be. Too much information is lost during
thresholding.
These concepts are illustrated in fig.1, which contains a
gray-scale image of the printed word "Gray" and a color image of
the printed word "Color." A portion of the gray-scale image has
been enlarged to display the individual pixels. A bitonal image has
been created from the enlarged section to illustrate the consequent
loss of information caused by thresholding. As may be observed, the
darker "r" remains recognizable, but the lighter "a" is rendered
poorly. The color image is also displayed as three gray-scale
images in its red, green, and blue planes. Note that the pixels
corresponding to the color red are lighter in the red plane image
and similarly for the green and blue plane images.
Figure 1. A gray-scale image with a portion enlarged, a bitonal
image of the same portion, and a color image with its three color
planes shown separately.
--- end quote ---
--
Ciao!
Joe
http://www.swissarmyfork.com
