PC Plus HelpDesk - issue 259
||This month, Paul Grosse gives you more
insight into some of the topics dealt with in HelpDesk.
From the pages of HelpDesk, we look at:
- Awful clicking noise in Vista;
- Bring back Telnet on Vista;
- Ubuntu install screen;
- JPEG or GIF on the web;
- Optical mouse movements;
- Traceroute and LFT;
- Dvorak keyboard - making;
- Dvorak keyboard - using;
- Bogomips on scalable processors;
- Liquid nitrogen cooling;
- Linux elevator types;
- Bayer mask interpolation; and,
- Raw image format.
Awful clicking noise in Vista
Have you noticed something that Vista does that is
quite worrying? Or, perhaps more worrying that the other
things that it does? If you click on a new location in
Windows Explorer, there is, in the default configuration,
an awful clicking noise that sounds like the arm in a
hard drive clobbering some sort of stop inside the drive
itself. Its it?
Whilst we cannot be certain whether or not Vista is
slowly destroying your hardware (the index building
process certainly does hammer your disc drives) the most
likely source of this noise is actually quite innocent.
If you click on 'Start'/ 'Control Panel'/ 'Classic
View' and then double-click on 'Sound' (alternatively,
you can get to it via 'Appearance and Personalization'),
then, in the 'Sound' dialogue box, click on the 'Sounds'
tab and then, in the 'Program' list box, scroll down to
'Windows Explorer'/ 'Start Navigation' and click on it,
you are in a position to cure it.
In the 'Sounds' drop-down combo, 'Windows Navigation
Start.wav' (with spaces in the file name) should appear.
If you now click on the 'Test' button, you should be able
to hear the sound that seems like your hard drive being
Why they chose this sound or how they recorded it, we
don't know but you can switch it off by selecting
'(None)' as the sound and clicking on the 'OK' button.
It's as simple as that.
Bring back Telnet on Vista
know that telnet isn't secure but it has a lot of uses if
you know what you are doing. Normally, we would use SSH
for communicating with another machine's command line but
there is more than that to telnet. For instance, you can
change the port number that it uses so that you can check
that text-based protocol using servers are running all
right - examples include HTML, POP3 and SMTP.
So, as telnet isn't running in the default
configuration of Vista, is there any way of getting it
Click on 'Start', 'Control Panel' and then, in the
'Classic View', click on 'Programs and Features'.
In the left pane, click on 'Turn Windows features on
or off', type in your root/admin password and the Windows
Features dialogue box opens up.
This takes ages for the box to fill - even on a fast
So, after a short wait, you see a listbox full of
|Scroll down to 'Telnet client' and check the
Click on 'OK' and then another little window opens
up telling you that it might take several minutes - and
Now, you can use telnet from the command line just
like you used to on XP.
No SSH yet though - maybe on SP1.
Ubuntu install screen
tried to install Ubuntu 7.04 and had the problem that the
dialogue box is too large because the screen resolution
is too small - a maximum of 800x600 - you'll find that
you can't get to the buttons at the bottom of the
Pressing the [Enter] key will work to a certain
extent, until it comes to having a choice - such as when
you need to choose a partition to install it on.
So, there are several ways around this:
- At the beginning of the boot procedure, you have
a menu and at the bottom, a chance to pick the
resolution to use. This is probably set at VGA.
Change this to whatever you need it to be; or,
- If you realise too late but you don't want to go
back to the beginning again, you can still start
the install but the buttons are not visible.
Moving the dialogue box upwards won't work and
800x600 is the highest resolution for the VGA
The solution is to drag the upper and lower panels to
the sides of the screen as in the screenshot. Simply
press the mouse button on a clear part of the panel and
drag the whole thing to the side of the screen. You can
now move the panel upwards. You still won't see all of
the dialogue box but you will be able to see enough of
the writing on the buttons to know what they say.
JPEG or GIF on the web
Even though PNG image files (Portable Network
Graphics) have been around for years, it has taken that
long for Internet Explorer to catch up with the rest of
the world and support intermediate levels of
transparency. However, the other main image file types,
JPEG and GIF are still in abundance.
PNG is what is called a lossless file compression
format and will preserve the RGB and transparency values,
pixel for pixel. However JPEG and GIF are not so which
one do you choose?
The answer is that they are both fairly good at their
GIF uses a palette of up to 256 colours but has built
in, an algorithm that allows it to display them
perfectly. The problem with GIF is that if your image has
more than 256 colours in it, your image processor has to
decide which ones to throw away and that will degrade the
image - look at the rose in the photograph to see what
happens. Of course, if you don't use transparency and you
have 256 colours or less in your image, GIF will preserve
all of the image. This is all right for diagrams but you
will be lucky to find a photograph that fits the bill.
|The problem with JPEG (files saved with
'.JPG' as the extension) is that it is not very good at
keeping local detail. On the right, there is a photograph
of a tower crane in the middle of Derby (UK). It has a
nice, steady transition from darker blue to lighter blue
and some fine details in the ironwork.
|This is the GIF version using a standard
palette (not a customised palette) and you can see that
the algorithm has chosen the closest colour that is
available from the palette.
In the case of the blue
sky, this has ended up as bands of colour - almost as
though it has been posterised.
By choosing a custom palette (most image processors
will make you one up on the fly and it will usually be a
pretty good choice of colours), you can make this look a
|This is the same image but preserved as a
JPEG. You can see that there is a peculiar fuzziness
around the fine detail of the iron work
If you have a
diagram with writing on it and blocks of colour, the
boundary between the background and the foreground will
be poorly defined, leading to smudged writing and loss of
saturation at the border of coloured blocks. Also, there
is no support for transparency.
|This is how jpeg does with text (ie, high
contrast borders). The Hello World on the top is how it
was at the beginning and the one at the bottom is how it
looked after being saved as JPEG. (This image and the one
below are actually saved as GIF because there are less
than 256 colours.)
|Just in case you cannot see the aberrations
above, here they are blown up five times. You can see how
the algorithm has to make guesses to save on file size
and correct for them as well, leading to the image on the
|So, if you have photographs, use JPEG
compression and for diagrams, use GIF.
Optical mouse movements
Are your Optical mouse movements a bit jumpy? It could
be your mouse mat. Optical mice are quite sensitive but
they can be fooled by some surfaces - sometimes going
dead, sometimes coming to life again but always on a
particular piece of mat or other surface.
The beauty of an optical mouse is that it doesn't
really matter how many bags of crisps your children/work
colleagues eat before they use it, it isn't going to stop
working because of a bit of grease. In the image on the
right, you can see that the underside of the mouse is
quite smooth and, perhaps most importantly, there is not
one of those horrid little rubber balls with the rollers
that refuse to go around smoothly.
|Many optical mice run all right on many
surfaces - but not all of them. The image on the right is
of the sensor. It is only small and therefore can only
see a small part of your mouse mat. If there are areas of
your mouse mat where the surface is smooth and there is
little pattern, then you can encounter dead spots.
trick is to make the mouse mat very even.
|So, try printing out the file for the
mouse mat (standard size - it is just on its end so that
it will fit onto a piece of A4 paper) - click on the
mouse mat image on the right to open the PDF file in a
new window. Cut it out and laminate it. You should find
that it works just fine.
Traceroute and LFT
Some people have noted that pathping seems to take
ages to work However, there are alternatives and if you
use an OS that was designed as a networking OS right from
the very outset, you can get accuracies that are
substantially better than 1ms.
and tracert - the DOS versions of Van Jacobson's
traceroute - work in pretty much the same way as each
other. They both send to the destination, type eight
(Echo request) ICMP (Internet Control Message Protocol)
packets with a TTL (Time To Live) of one to start with.
Each router that the packets go through decrements the
TTL and the router where the TTL becomes zero sends back
a type 11 (TTL exceeded) ICMP packet that encapsulates
the header of the original packet (containing the
sequence number and so on). In this way, the first router
that is encountered is identified, along with the time
delay of the round-trip.
The program that is sending out the packets increments
the TTL and sends it out again. This time, the next
router sends back a type 11 ICMP packet and so on.
From the results, a map is built along with the delays
The difference is that pathping does a more detailed
analysis although times are limited to whole milliseconds
because of the way that Windows works.
Traceroute works slightly differently in that it sends
out UDP (User Datagram Protocol) packets instead of ICMP
- although the routers still send back type 11 ICMP
packets when the TTL is exceeded. Traceroute's output
also includes any alternative routers, along with their
times. Traceroute runs on UNIX variants so its times are
limited to microseconds.
There is another program (UNIX-like OSes) called 'lft'
(Level Four Trace). It can be made to run using ICMP
(like tracert or pathping), UDP (like traceroute) or TCP.
It is highly configurable and can be made to run
adaptively to go through some firewalls.
You can download LFT from http://pwhois.org/lft/index.who
and compile it for your system using by typing the
One thing that is worthy of consideration is the time
it takes to perform the analysis. Here is a table of how
long it took (figures taken from the packet stream
sniffed by Ethereal/Wireshark) for a subsequent lookup
(ie, this is not the first time the program was run
therefore the DNS caches in the servers along the way
already had the appropriate information).
It must be said that
judging where exploration finishes and verification
starts is difficult (they seem to happen pretty much at
the same time on traceroute and tracert and there is less
than a second's difference with LFT) although the
important figure is the total time for the program to
One thing to remember is that if this is
the first time that a particular route has been used,
there may be delays whilst other machines en route
perform DNS lookups. I found that this was the case with
Traceroute where a later (several days) traceroute
command to the same address (so the en route DNS caches
did not contain the required information) took around 10
seconds and a subsequent traceroute command using the
same address took less than a second. The first of these
two took only around 10 seconds which is still
substantially less than all of the others which were all
performed as subsequent runs - ie, the figures above are
what you would get on a second or subsequent run so that
en route DNS caching is not an issue.
For a look at screenshots of the command
line output, click here and for a look at the Wireshark
files, for each of the above runs, click here.
Dvorak keyboard - making
QWERTY keyboards are not the only type of keyboard
around. There have been a number of other layouts that
have existed over the years for a number of purposes. One
was the Blickensderfer ('DHIATENSOR') and on the right,
you can see the Linotype machine that is in the foyer of
the Derby Evening Telegraph in Derby.
The Linotype's main purpose was to cast lines of text
into little bars of metal. These were then lined up and
used to produce the pages of print that the paper
published (it's all done with DTP programs now).
If you look at the photograph on the right, at the
keyboard and then move to the left, you can see a grey
coloured wheel and if you follow its spindle back, you
can see a red circle. If you look up from that, you can
see a red bar that is leaning just to the left in the
photograph. To the right of that is a silver-coloured
structure (to the right of which is a black panel). That
is a pot of molten metal (or it would have been if the
machine was running) around 10" in diameter.
|This is it from the other side. You can see
at the top, at an angle of around 45 degrees, the trays
with the different typefaces on and some of the wiring.
machines were electrical so there was no need to arrange
the keyboard so that levers would not get tangled with
each other. With this in mind, the keys were arranged in
columns according (roughly) to the popularity of the
letters. The keyboard arrangement went so...
|Computer keyboards are entirely electric
as well so there is also no reason to hang onto the
QWERTY layout as this is slow. (The only reason that I
can see for the QWERTY layout to prevail is that there
are sufficient of them out there and there are so many
established writers that use it that the manufacturers
and users cannot justify setting out the required capital
for the changeover.) So, how can it be improved?
Dvorak layout is a substantial improvement on the normal
QWERTY keyboard layout. With a right-handed person, the
left hand sits over the vowels and the right hand covers
the most common letters with the last five letters in the
Linotype list (KQJXZ), well out of where either hand
would normally go.
|As a result, (if you are right handed)
your least dexterous hand just does the vowels whilst
your most dexterous hand dances around with the most
common letters either under the home row or above it -
this does lead to the common questions; 'where is the
K?', 'where is the W?' and so on.
So, it's clearly a
good thing to have but how do we get one?
However, if you've ever tried to find new ones on the
Internet, you might find that you cannot find new ones or
you have to pay through the nose for them (you can get
old IBM keyboards in US Dvorak layout). There is one that
I am aware of that has a switch on it so that you can
switch between QWERTY and Dvorak but you are probably
better off making your own - if you think you can do it.
This is not as bad as it might at first sound and I
did it in around 30 minutes.
|First, catch your keyboard - this will have
to be a QWERTY because Dvorak keyboards are a bit thin on
However, many keyboard designs on the
market will not do: you need to get one that has all of
the keys on the three letter rows at the same height and
Reject those that have the top-letter-row keys higher
and/or at a steeper angle than the middle row or the
reverse for the bottom row such as the keyboard on the
|Also, the keys on all three letter rows need
to be the same shape and size - this is because you are
going to swap them over. The most likely candidates are
cheap, flat keyboards which you can pick up for less than
£10.00 such as the one on the right which I got for
One other thing to consider is that the
home keys (on a QWERTY, they are the 'F' and the 'J'
keys) will not end up in their normal places. The home
keys are designed to feel different so they either have a
smaller radius of curvature, a raised spot in the middle
or they have a little raised bar across the bottom of the
key face. This last type is the best because if it is
particularly annoying, you can take that off with a knife
and still leave the legend on the key. Your new home keys
will be 'U' and 'H'.
|You can prise the keys off from the
front but you run a serious risk of breaking or snapping
something internally if you do that. This will also
increase the tendency for this to happen in normal use.
is better to take the keyboard apart carefully
(remembering where each screw goes - get a piece of paper
and draw a map of the screw locations on it then, as you
take them out, push the screws into the paper so that
when it comes to putting them back, you know where each
With the back off, you will probably be met with
either a single sheet of rubber with conductive pads or,
with a lot of separate silicone cones.
the conductive sheet, you will probably see a
printed circuit board. On the underside of the
rubber cones, there is a conductive patch that
comes into contact with the printed circuit
board, thus making electrical contact when the
key is pressed. Take off the sheet and put it
with the printer circuit board; or,
the silicone cones, make a note of the position
of any different ones (such as colour-coded cones
for the Enter or Shift keys) and put them to one
side. With this type, you have two printed
circuit boards but instead of board, they are
printed on thin, plastic sheet. Between these two
sheets is another, but this time with holes that
correspond to the key positions. In this case,
the two printed circuits come into contact when
the key is pressed, forcing them together with
the silicone cone, through the hole in the middle
sheet. Put these to one side if you can (or, if
you cannot safely remove the printed circuits,
just make sure you look after them whilst you do
the next operation).
Next, you need to take out the keys.
usually travel in a cylinder with the key pad expended on
top. They can be hold in by a catch and if you examine
the underside of the key - where it comes through the
panel, into the back, you might be able to see a latch of
some sort. You can probably unlatch this with a
screwdriver or similar instrument but be careful not to
Rather than take them all out (there are some that
don't change position), Take out one, say the 'S/s' and
put that to one side. Where the 'S/s' used to go, you
need to put the 'O/o' key so take that out and put that
where the 'S/s' used to go. Next, take out the 'R/r' and
Once you have, swapping them so that they end up in
the order in the photograph - only the 'A', 'M', '\',
back-tick and the numbers don't move - screw back the top
again and you have your keyboard.
Just in case you were interested, there are other
versions of Dvorak keyboard: a design for use only with
the left hand, one for use only with the right hand and,
the US design. The UK designs keep the double quotes
above the '2' and the '@' goes where the 'Q' on a QWERTY
keyboard goes so that in our world of electronic mail,
the most used key is within easy access (if we wanted to
stretch to get it, we would have used a QWERTY keyboard
layout, now, wouldn't we?).
Dvorak keyboard - using
'QWERTY' is not the only keyboard arrangement
supported by PCs and whilst you might not expect to find
or a Linotype ('ETAION SHRDLU') layout, you should expect
to find an English Dvorak.
On SuSE Linux, open YaST 'Control Centre' (this is the
personal settings and not the administrator settings) and
under 'Regional and Accessibility', choose 'Keyboard
|Under the 'Layout' tab, select the country
layout you want and then add to the active layouts,
choosing 'dvorak' as one and 'basic' as the other.
|In the panel, you should now get a little
Union Jack with 'gbd' or 'gb' in it. Click on it to
change the global layout.
If you want to change the
layout on a more global scale, open YaST 'Control
Centre'/ 'Hardware'/ 'Keyboard' and under 'Layout' you
have two configurations. Under the primary set-up, change
the variant to 'Dvorak'. You can check that the letters
match by using the test box at the bottom. This is just
what it says it is - United Kingdom Dvorak. Click 'OK',
then 'Save' then 'Exit Sax2'.
On Windows Vista, click on 'Start'/ 'Control Panel'/
'Classic View' then double-click on 'Regional and
Language Options'. On the 'Keyboards and Languages' tab,
click on the 'Change Keyboards...' button.
In the 'Text Services and Input Languages' dialogue,
under the 'General' tab, click on the 'Add...' button, go
down the list and under 'English (United Kingdom)'/
'Keyboard', check the 'United States-Dvorak' box - there
is no other choice at the moment. If you click the
'Preview button, you can see that the double quotes, '@',
'£' and '#' keys are in different places or non-existent
and that the tilde ('~') is a shifted back-tick.
To get a '£', hold down [Alt] and use the number pad
to key in . When you release the [Alt] key,
the character will be displayed.
Bogomips on scalable processors
The bogomips value can be an invaluable measure of
just how fast your computer is (although this is a
measure of floating point maths in the integer part of
the kernel so it is just about as meaningless as any
other performance value on any system - doesn't stop it
being an indicator of speed though, you just have to
realise that it is no more meaningful than any other
So, if your rig has an AMD 3700+ processor but when
you click on on 'Menu', 'System', 'Monitor',
'KInfoCenter' and then on 'Processor', although it says
that it is an AMD Athlon 64 ... 3700+, it also says the
bogomips value is only 2,010.94.
'Processor' page in KInfo Center is predominantly the
output of 'cat /proc/cpuinfo'. The bogomips figure is
calculated during the boot process and stored along with
the rest of the information in '/proc/cpuinfo'.
The Athlon64 3700+ we used here runs normally at
1,000MHz but scales up to a maximum of 2,210MHz when it
is needed. The figure you see on your computer could well
be the one that was calculated at 1,000MHz which is what
the Linux Kernel now does. If you run a recent version of
KNOPPIX (such as 5.1.1) and look at '/proc/cpuinfo', you
will see that the CPU speed is reported as 1,000MHz which
gives 2,010.94 bogomips.
Using an earlier version of KNOPPIX such as 4.0.2
(where the CPU speed is full during the bogomips
calculation), looking at '/proc/cpuinfo' shows a CPU
speed of 2,209.868MHz and 4,358.14 bogomips. Mystery
Liquid nitrogen cooling
nitrogen cooling produces performance gains but you also
need to consider the hazards. So, if you want to replace
the dust collector on the right with something out of a
Frankenstein film, do so by all means but put safety
In articles about liquid nitrogen cooled computers,
the only protective equipment a lot of the people in the
photographs seem to be wearing are goggles and gloves. As
somebody who has worked in a laboratory, I am utterly
appalled by this as they appear to be putting across the
message that goggles and gloves are safe enough and, the
only protection and precautions required. It is more
dangerous than that.
Liquid nitrogen ('LN', boiling point -196C) is
actually very dangerous and goggles and gloves are not
enough. Nitrogen is roughly 78 per cent of the air we
breathe - the remainder being mainly 21 per cent oxygen
and one per cent argon.
The level of oxygen is quite important.
- below around 17 per cent and people start to lose
- above 25 per cent and fires become difficult to
If you want to try LN cooling, you need to consider
the following brief, non-exhaustive list first:
- If LN comes into contact with any skin, it will
destroy it. In addition to a protective overall
and shoes with spats, you need to use a full-face
visor and gloves that go all the way past your
- You need to have somewhere safe to store LN when
you are not using it - a well-ventilated, locked
compound, outside, under shelter and in the
shade, away from any buildings;
- Only transport LN in a dewar flask - one with a
special top to relieve the pressure - or, a
special container designed specifically for the
purpose and supplied by the LN suppliers
themselves or a specialist equipment outlet;
- Don't carry it up stairs if there is a
possibility that others might be below;
- Don't use LN in a confined space. Vaporised LN
lowers the oxygen level, leading to
unconsciousness and eventually death. Examples of
confined spaces include:
- any part of your house;
- Don't use LN if you are in flats or near other
business units in the same building. Nitrogen gas
might leak into other people's premises and cause
- Don't put your fingers in the gas above the
liquid surface - this can cause frostbite that
you won't notice until it's too late; and,
- When the LN has boiled down a long way, you might
see that the liquid has turned blue. This is
liquid oxygen (LOX) which has condensed out. LOX
is also extremely dangerous, pushing the air
oxygen level above 25 per cent, making fires
difficult to put out. Oxygen can saturate fabrics
and build up in confined spaces.
So, if you read up further on the issues and take
appropriate precautions, you might be able to do this
One problem that a lot of LN cooled systems have is
that they attract frost. This is caused by the air being
cooled below its 'dew-point'. The dew-point is a
temperature below which water will condense out from the
air - in other words, when the relative humidity goes
above 100 per cent. To stop this from happening, you need
to have the circuit board and anything else that gets
cold, surrounded in air that has a low dewpoint - one
that is lower than the temperature of the equipment. So,
rather than trying to get dry air from somewhere, you can
use the evaporated LN as a source of dry air. If you have
your computer in a case of some sort, just let the
nitrogen from the CPU LN cooler purge the case and
without water vapour in there, you shouldn't get any
Linux Elevator types
played around with your Linux computer, you might have
noticed that in Grub, there is a kernel option;
'elevator=cfq' and wondered what it is.
Well, the elevator is a system-wide setting for the
In/Out scheduler, affecting all of the disc drives. When
your system boots, it uses the default scheduler (usually
cfq) unless you have specified otherwise. This is done in
the grub menu and seen as a boot option.
CFQ ('cfq') stands for 'Completely Fair Queuing' and
it is not the only option. There is also: 'Anticipatory'
('as'); 'NOOP' ('noop'); and, 'Deadline' ('deadline').
They differ in the way that they work.
- Basically, CFQ maintains a
scalable I/O queue for each process and tries to
distribute the available bandwidth equally
amongst all of the I/O requests. This produces
good results where you have multi-processor
systems and where you have many LUNs (Logical
Unit Numbers - channels/drives/partitions/slices)
as part of a system such as a storage area. Both
queries and transactions perform well with CFQ.
- The deadline scheduler minimizes
the latency for any given request using a
round-robin algorithm, visiting 5 queues and
producing close to real-time performance with
processes being seen to quickly. It re-orders
requests and whilst it's transaction rate is
slightly lower, it is faster at queries than CFQ
making it a better option if your system is
preoccupied with database queries.
- NOOP uses a simple FIFO (First
In: First Out) queue and as a result is
substantially lighter on CPU time - assuming that
the disc controller or the host adapter will sort
out anything else that needs doing. NOOP runs at
about 75 per cent of CFQ's speed so if you need
to run something that is processor intensive, you
can use NOOP and let the controller do the
- Anticipatory adds a short delay
so that it can collect and sort I/O requests and
as a result, reduce the seek operations required
on the drive itself. The problem with this is
that it increases latency but it excels in
environments with a limited number of LUNs and
slow drives. Most home systems use only a few
LUNs so this one is worth a go.
The best way to find out which one works best for you
is to try them all and benchmark them under normal use.
This will not affect what is written to your drive so it
is safe to play around with.
To change your configuration, either:
- open 'Yast'/ 'System'/ 'System Settings' and then
click on the Kernel settings, selecting the I/O
scheduler you need;
- open /boot/grub/menu.lst (as 'root') in a text
editor and add 'elevator=cfq' to the end of the
'kernel' line; or,
- add it to the kernel options when you boot up in
'elevator=as' should work all right on home systems so
give it a try.
Bayer mask interpolation
You might be quite happy with your digital camera on
the whole but you could find some interesting and
unwanted effects if you have areas of saturated red or
blue, next to some other colour, resulting in a 'blocky'
The image on the right is of a Cinnabar moth. As the
name suggests, it has brilliant red areas on its wings.
The image is 33 per cent of its normal size so one pixel
here, represents 3 pixels on the original.
|This is part of the above picture, blown up
to three times its size so three pixels here represent
one pixel on the original.
You can see that the detail
appears to be fairly consistent as the light grey lines
travel across the darker colours of the wing. However,
where the border is between red and the darker grey, the
level of detail is half of that - giving it the blocky
|This is an image of a computer that appeared
at the Computer Trade Show in May 2006.
The 'R's of
'CORSAIR' definitely have a clipped appearance about
|And here, you can see just how clipped - you
can also see that the pure red only has half the detail
again. So, what's going on?
|The image sensors in your digital camera are
sensitive from blue right down to red (and beyond, into
the infra-red range) and therefore are not capable of
differentiating between colours. To get over this, the
sensor manufacturers use filters so that a given cell
will be sensitive to red, green or blue - corresponding
to the primary colours that our eyes use.
A cell can
only detect one colour so they have to be placed next to
each other - a demosaicing algorithm being used to guess
the levels of the two missing colours so that the cell
can form a three-colour pixel. A hexagon- or
triangle-based pattern would be best for fitting the
colours on the sensor but unfortunately, colour monitors
and every image processing/handling software and hardware
component down the line uses a rectangular raster. To get
over this, each 2x2 sub-square of the layout of the cells
has the three primary colours and the fourth cell is used
for an additional green - the Bayer filter mosaic. Our
eyes are more sensitive to green so having twice as many
of these than the other colours allows for more detail in
the final image.
Whilst green is all right for detail, the red and blue
are impoverished and you can see in the images on the
right that every other row/column doesn't have either a
red or a blue.
|This has the unfortunate effect that where
you have a narrow, saturated red or blue detail that
crosses one of these blind rows/columns, there is nothing
there to form a guess with.
In the photograph, you can
see how the Bayer mask overlays the memory card displays
and that the bottom of the 'C' in 'CORSAIR' on the
display falls in one of these blind spots.
Unfortunately, you are stuck with this and even using
a RAW image will only give you the same information. The
solution is to use a camera with a completely different
type of sensor - one that uses a trichroic beam splitter
and three different sensors so that three cells (red,
green and blue) map onto each other leaving no blind
spots for thin lines to disappear into. But first, you've
got to find one.
Raw image format
Your camera's image is captured by the cells on the
CCD sensor. This works because of something called the
photoelectric effect - where electrons are displaced by
individual photons. So, in effect, the output of each
cell is a crude count of electrons and has a very large
dynamic range, although this is reduced quite
substantially by thermal noise which also messes around
with the electrons.
Normally, this is reduced down to eight bits so that
the image can be saved as a JPEG file but if your camera
can save RAW files, you can keep a lot of this dynamic
The image on the right was taken in a poorly lit room
under tungsten light. You can see from the histogram at
the top left that little of the light range of the sensor
|The result of this is that if you were
interested in a range of detail that only existed in the
darkest one per cent of the image density, a JPEG image
(being 8-bit) would give you two or three values to
choose from whereas you might well find that you could
extract a whole eight-bits-worth of data from that range
Another advantage of cutting down the
dynamic range to 8-bit is that you are able to perform
corrections to: colour temperature; linearity; gamma;
saturation; and, so on - at the end of this, you still
have an 8-bit per pixel per colour image and haven't
introduce any JPEG artefacts.
|Here is the White balance tab. You can
select the level of green you need to have, along with
the colour temperature.
|If you click on the dropdown combo, you can
see a number of different white balance choices.
|The colour tab allows you to choose an input
profile and an output profile, changing the gamma and
linearity in the process.
Notice the file icons on the
right hand side. You can load or save curves and other
settings as you go so if you encounter these conditions
again, you can just call them up when you need them.
|Here, we have the interpolation algorithms
to use. AHD should do for most operations but you can
choose any you like.
|You can also take spot measurements either
by clicking on the image and reading the values or...
|...dragging an area.
|In the column on the left, at the bottom,
there are a couple of buttons and check boxes: for under-
and over-exposed pixels. Clicking and holding down the
buttons will show you which channels are over exposed and
where on the image; whereas checking the checkboxes will
mark the under exposed as black pixels and the over
exposed as white pixels.
So, whilst RAW files are
normally at least twice the size of a corresponding JPEG,
the amount of extra data you have to play around with
makes it all worthwhile because you can look at sections
of the image that are usually destroyed by the process
that makes the JPEG image for you.
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