If
java disabled or no main menu showing above - use links
below.
|
|
Moving
on a step further from the description of such things
as color models and color attributes - let's take a
look briefly at the factors underlying the image itself.
Resolution and DPI - and how they affect image construction
and properties.
Lot of text - Sorry! |
Resolution
-
Very briefly, your machine and graphics card performance
along with your monitor screen size, will determine
at what resolution level you can operate. Although an
old standard was 640 pixels (dots) x 480 pixels (dots)
on a 14" screen, many of us now are lucky enough
to have a 15" monitor or larger, and a graphics
capability of at least 800 pixels x 600 pixels. More
likely now 1024 x 768 and upwards! I am now running
1600 x 1200 on a 21".
To gauge what this does for you - let us say that with
the lowest figure, you may be hard pressed to see all
of a page in your word processor, without zooming out.
At 800 x 600, you will see more of it (and your desktop
icons will look smaller, and take up less space), and
if then you can use 1024 x 768 pixel resolution, you
will have much more "space" on your screen
to see everything and, feel you have "space to
spare"!
As a compromise, it is best we assume (worst case scenario
maybe) that many people are for now most likely to still
be on 800 x 600 resolution and so construct web material
to be displayed at this level. If you use 1024 x 768,
then you can use your browser at a reduced size and
still see it at effectively 800 x 600, without filling
the whole screen. Remember too ... even on newer laptops
.. 1024 x 768 is often a practical max.
It is possible of course to reset your resolution in
the Windows 9x and up operating system - the video card
is what ultimately determines how high you can adjust
your screen resolution - and with 68 Mb and 128 Mb 3D
cards more common now, higher resolutions are achievable
more easily. Many earlier "Mac" users may
still be working at 800 x 600 but probably much better
too these days.
Depending too on your monitor's intrinsic resolution,
you may have a screen ''DPI'' of 75, although some can
achieve 96 ''DPI'' .... see below. |
DPI
-
This is a way of expressing the resolution in an
image, by referring to how many  ''dots
per inch'' there are. Let's show this graphically
.......... here we have a screen grab from this
document .... of part of this page above - albeit
with background color brightened for visibility.
It was sized at exactly 1" x 1". So at
75 DPI resolution, that means there are 75 dots
across and 75 dots down (total ....... 5625). If
you were to place the dot from a letter ''i'' 75
times across or down ... it would be a one inch
straight line! Of course, on the screen - for dots
read pixels. Your own monitor resolution and screen
settings will influence just how big this appears.
These figures are not so important when displaying
pics on your computer (except for the effects on
filesize - see below) but become very important
when having pic's for printing. What can look large
on a PC screen may only print out at something very
small in dimension if a high DPI image.
Consider .... an image scanned at 600DPI ........
on your screen, that will be 600 pixels for every
inch of picture.!! Of course, at screen res' of
75 DPI, that one inch of the pic will 'seem' like
8 inches across, when viewed at 100%.
To see graphic representations of the pixels in
a photograph, look at the page on Image
Basics, if you wish, and have not already
done so.
|
Resolutions
and file sizes -
The resolution of an image will affect the size
of file. So also will factors like color settings
and of course compression - if any. See some extra
details
on compression.
Suffice for now and simplicity's sake ..... let
us look at two examples of image specifications
and how they affect file size --
1]
- At one extreme (lower) consider a 256 color
(8 bit) BMP ....... of 291 x 294 pixels.
Nominal size on disk (and in memory) is 85k. If
we multiply the pixels ....... we get a total
pixel count of 85554 .... in fact the size of
the image in bytes, because each pixel information
is carried by just 8 bits (one byte) ....... value
between 0 to 255 (256 total states).
2] - A 16 million color BMP image (24
bit) ....... of 266 x 319 pixels. Nominal size
on disk (and in memory) is 250k. If we multiply
up the pixel count here, we get a total of 84854.
This though is about one third the file size.
Why? Because this is 24 bit (3 bytes) and so three
times bigger than the last image (#1 above). Of
course if this was compressed to a JPG
at say, 10:1 compression, then it would store
as only 25k nominal file size.
If
we take this one stage further relative to the
DPI matters discussed above, then if we
scanned the same picture, of say 6" x 4"
(a common size print) at 75 and 200 DPI resolution
and compare, you may begin to see how file sizes
potentially go way up as resolution increases.
Taking these dimensions, 6" x 4" = 24
square inches ......... each square inch scanned
at 75 DPI will therefore contain 75x75 pixels
(5625), so a total image pixel count of 135,000.
For an 8 bit color set (256 color) this would
be the actual max file size in bytes and size
in memory when displayed. Go up to 24 bit color
(16 mil colors) and we have to multiply this by
three ... then a max file size of just over 400k
and same in memory when displayed. This will display
on the computer also as 6" x 4" but
will have a fuller and smoother color rendition
than 256 color.
Now - scan at 200 DPI, and we have a total pixel
count of 960,000!! (200 x 200 x 24). Thus for
an 8 bit (256 color) print at this size we already
have a very large file ........ use that however
as a 24 bit color image (16 million colors) and
we again multiply by three .... giving a large
2.9Mb uncompressed file size, and size in memory.!!
Of course, 10:1 compression to JPG will make that
latter more bearable - at 290k. This image on
the computer would, if shown in its entirety,
be effectively 16" x 10.6" (because
we will probably view the 200 DPI at screen 75
DPI). Usually, to see this as a full image on
the screen it must be ''zoomed out'' somewhat
and viewed at a lower percentage...... when not
viewed at 100% real size, usually some perceived
degredation can result, depending on the sizing
algorithm - it is only temporary.
In all these cases we are looking to be able to
print at a physical size of 6" x 4"
... but if we required an image for web use then,
possibly a 500 pixel (or less) square image at
75 DPI might suffice ....... and at 24 bit this
would give a max file size of 750k. JPG compression
here though, of say 15:1 (as high as I go before
losses start to really creep in) .. then that
would reduce to a file size of 50k ...........
very manageable and reasonably fast to download.
On screen it would appear as a 5" square,
dependent on screen resolution. Even at 1600 x
1200 96 DPI in fact - it will be very near to
physical size - around 80%.
|
|
Back to Top
|
