Technical information

© CLEAPSS 2005 1619 ICT in the Science Department
Even if the image can be enlarged (and this is not possible with some software),
the information from each single pixel of the image will be displayed on a block of
pixels of the display, all displaying identical information, and the image will
appear less sharp. This is illustrated for a triangular object in the box overleaf. It
is, however, generally possible to decrease the resolution of displays to match that
of the camera; the image would then be larger and fill more of the screen.
Interpolation
The process of interpolation can compensate for this problem to some extent.
Interpolation (resampling) of digital images is a technique for increasing (or
decreasing) the number of pixels in an image [and hence increase (or decrease) the
size of the displayed image]. Interpolation may be employed within the camera in
order to produce larger images than those actually captured by the sensor or to
provide a digital zoom facility. Almost all image-editing software packages also
support some form interpolation. To enlarge an image, information from adjacent
pixels is used to estimate what the information between them might have been, so
that rough edges produced by enlargement are ‘smoothed out’ and gradations of
shade are also improved; see the box overleaf. Interpolation may therefore be used
to match the image resolution to the display resolution.
Anti-aliasing
Anti-aliasing is a similar technique that is utilised to produce a better-defined
image and smooth out jagged edges when a high-resolution image is displayed on
a lower-resolution display.
Moving images
Most digital cameras which allow short recordings of moving images have a lower
resolution for such video clips and a lower frame rate (see below) than that of
video cameras. The low resolution might be a problem, causing the smooth move-
ment of objects to appear quite jerky.
Frame rate
This specification determines the speed with which a camera sends images to the
processor/software and ultimately to the display or computer video file.
Clearly, the faster this occurs, the more closely the image of a moving object
mirrors the actual movement or position of the object (the image appears in ‘real
time’ and does not move artificially jerkily). It is also more difficult to manipulate
a slide on the stage of a microscope if there is a noticeable delay in displaying its
image. Typical frame rates for lower-priced cameras are currently 15 fps (frames
per second); a rate of 30 fps produces noticeably better moving images. A USB 2
digital connection might provide noticeably smoother moving images with cam-
eras offering higher frame rates.
Other factors in image display
Colour palette
The colour palette is the range of colours within an image or that a device is
capable of displaying. If a display cannot match the exact colour in an image,
‘dithering’ is a technique that produces an approximate colour match by substitut-
ing a ‘blend’ or pattern of similar colours that can be rendered by the display.
Refresh rate and
response time
The refresh rate of a display device is the frequency at which the display image is
redrawn. The rate is measured in Hertz (Hz) and corresponds to the number of
times the image is redrawn per second. A CRT monitor set to a slow refresh rate
may appear to flicker. With flat-panel TFT and LCD displays the value is quoted
as the response time in milliseconds and refers to the rate at which the display
image is updated since these displays do not so much ‘redraw’ the whole screen as
update the pixels that have changed. Slow refresh rates or response times will
result in blurring of fast movements when playing back video but since most
digital cameras have relatively low frame rates this should not be an issue.