Digital Image Characteristics;
Understanding Pixels, Dots,
Samples & Viewing
Michael J. Glagola
mglagola@cox.net
703-830-6860 1-15-05 ©Michael J. Glagola 2005 2
Preliminary Comments
1. This presentation will be contrary to many
individual’s long-held beliefs about digital
imaging.
2. Everyone (especially myself) has been and
will be careless in their use of digital imaging
terminology.
3. Photoshop’s focus is desk top publishing and
as such it is oriented on the production of the
printed final product.
4. It is very, very difficult to keep track of all these
concepts; do not be surprised if you are
confused.
1-15-05 ©Michael J. Glagola 2005 3
Session Goals
To provide an understanding of:
• What is a digital image and how is it
stored
• What is a pixel, dot, and sample
• What does PPI, DPI, SPI, & image
resolution really mean
• Considerations one must take into
account when viewing and
manipulating digital images
1-15-05 ©Michael J. Glagola 2005 4
Basic Definitions
• Pixel - An abbreviation of the term 'picture element.' A
pixel is the smallest picture element of a digital image. A
monochrome pixel can have two values, black or white/0
or 1. Color and gray scale require more bits; true color,
displaying approximately 16.7 million colors, requires 24
bits for each pixel. A pixel may have more data than the
eye can perceive at one time.
• Dot – The smallest “unit” that a printer can print.
• Voxel – An abbreviation of the term ‘volume element.’ The
smallest distinguishable box-shaped part of a threedimensional space. A particular voxel will be identified by
the x, y and z coordinates of one of its eight corners, or
perhaps its centre. The term is used in three dimensional
modeling. Voxels need not have uniform dimensions in
all three coordinate planes.
1-15-05 ©Michael J. Glagola 2005 5
What is a Digital
Image?
• Collection of pixels laid out in a specific order with
width (x) and height (y) in pixels
• Each pixel has a numerical value which
correspond to a colour or gray scale value
• A Pixel has no absolute size; pixels MAY
(sometimes NOT always) have a spatial value
(Spatial data is data associated with the pixels