|
BOOT CAMP 188 (16/08//01)
PICTURES AND PIXELS part 2
In part one of this short series on computer imaging we
looked at the rudiments of resolution with special reference to monitor screens.
This week we'll tackle resolution in the context of digital cameras, scanners
and printers and make a start on how imaging devices process colours.
Broadly speaking resolution or the ability of a digital
camera to capture fine detail is determined by the number of pixels on the
surface of the image sensor chip inside the camera. (Each pixel is made up of
three light sensitive elements, sensitive to red, green and blue light). The
sensor is the equivalent of a frame of photographic film so it's tempting to
compare the two in terms of the number of pixels. However, film is an analogue
medium and doesn't easily lend itself to that kind of comparison, but we'll have
a go anyway. In the case of a top quality camera and lens shooting a static
scene in good light a frame we can say that a frame of 35mm film contains the
equivalent of 20 million or so pixels rising to more than 30-million pixels on
the finest grain professional films.
That figure falls to between 9 and 12 million pixels in the
case of a mid-range camera loaded with ordinary film. The best of today's
high-end digital cameras have sensors with between 4 and 6 million pixels (4 and
6 'megapixels') so you can see that they still have a way to go to catch up with
film.
Many other factors are involved in determining the quality of
a picture shot on a digital still camera, including the way the camera and PC
processes the data but all things being equal and assuming a decent printer, a
mid-market digital still camera with a 2.1 or 3-megapixel sensor can produce
very acceptable looking 4 x 6-inch prints that stand comparison with photos shot
on a 35mm compact camera.
Scanner resolution is measured in a slightly different way to
cameras, not by pixels but in dots per inch or 'dpi'. A dot as we explained last
week is a pixel by any other name. Inside a flatbed scanner, on the scan 'head'
that moves under the glass 'platen' on which the image is placed, there is a
strip of light sensitive elements -- the dots or pixels --- and how many of them
there are to the inch is the scanners 'optical' resolution. This is typically
600 to 800dpi on budget models and 1200 to 2000dpi on more advanced types. A
scanner's optical resolution is usually quoted as two figures (i.e. 600 x
1200dpi), the second number '1200' denotes the number of 'steps' the scanner
head makes per inch as it travels down the platen
Note that optical resolution is the true measure of a
scanners performance and it is not the same as the larger and more
impressive-looking 'interpolated'
resolution figure that many scanner manufacturers are fond of quoting. This is
essentially a trick whereby the scanner software takes an educated guess and
fills in the missing detail between each pixel.
How much resolution you need from a scanner depends on what
you are using it for. If you only want to scan images that will appear on a
video monitor – i.e. pictures for web pages, multimedia presentations etc. --
then you may be surprised to know that the resolution of most PC screens is
between 75 and 100dpi; scanning at higher resolutions is basically a waste of
time, effort and disc space as all of the extra detail is lost. Scanning text
and documents for faxing, copying or optical character recognition (OCR) is
another relatively undemanding application and a resolution of 300dpi is usually
more than sufficient. Higher resolutions start to make sense when the aim is to
print out the results. Most laser printers and colour inkjets operate in the
range 300 to 800dpi. Imagesetters, used in the production of books and magazines
normally require images to be scanned at resolutions of at least 1200dpi but for
high quality work it can rise to 3500dpi and above.
We've already touched briefly on printers and the sort of
resolutions they can achieve and like scanners the ability to reproduce fine
detail is measured in dots per inch (dpi). However, the numbers are not so
clear-cut. There are many different printer technologies and numerous techniques
for making sharper images and more natural-looking colours. The type and quality
of paper can also have a big impact on the finished results so it's a bit of a
minefield. The simple rule of thumb is that the more dpi's a printer can manage
the better and 'photorealistic' models with multi-colour (i.e. 4, 5 or 6 colour
inks) printing systems produce the best results on photographs shot on a digital
still camera.
Colour plays a very important role in image quality, in
particular something called colour depth. In keeping with tradition the PC
industry has managed to make the whole business seem a lot more complicated that
it actually is but we'll try and make some sense of it.
The human eye is very sensitive to colour and we have the
ability to distinguish up to 10-million shades. Digital imaging systems are even
more adept at processing colour and can detect and reproduce millions more
colours by assigning each colour a number, and as you know computers are very
good with numbers.
Most PCs and peripherals use four basic colour formats. The
first is the VGA standard of 16-colors, also referred to as 4-bit color (excuse
the American spelling), which is the number of binary digits (bits) used to
identify each of the colours. 4-bit color is okay for displaying simple graphics
and icons but it can't handle the shades in photographic images and they look
really coarse and blotchy. Next is 8-bit or 256-colors. This is just enough for
a photographic image though variations in colour and shade tend to look very
patchy. Picture quality takes a big leap with 16-bit color, also known as 'High
Color' which can resolve 65,536 colours. Finally there's 24-bit and 32-bit 'True
Color' that describes more than 16 million (16,777, 216 to be precise) colours.
32-bit True Colour is a special format used mainly for video games and high-end
graphics applications where the extra 8-bits of information, known as the 'Alpha
Channel' is used for creating special transparency and texture effects.
Next week – Pictures and pixels part 3 – image sizing
JARGON FILTER
IMAGESETTER
Device used to convert image data produced on a PC into
photographic film used for making lithographic printing plates
MEGAPIXEL
As near as makes no difference one million pixels or picture
elements
OCR
Optical Character Recognition – converting the scanned image
of a document into a text file that can be read by a word processor
TOP TIP
It's all very well your PC being able to process over 16
million colours but can you see them all on your monitor screen? This simple
little freeware monitor test program will help you find out and adjust your
settings to produce the best possible picture. The self-extracting 'zip' file is
only 278Kb and should only take a couple of minutes to download from:
http://www.monitortest.net/monitortest.html
NEXT
|