BOOT CAMP 187 (09/08//01)

PICTURES AND PIXELS part 1

What's the difference between 256-Color High-Color and True Color and what's a megapixel what it's at home? Welcome to the weird and wonderful world of PC displays and imaging. It's a vast subject that covers a lot of ground, from monitors and printers through to scanners and digital still cameras, so if you're in the market for any sort of visual input or output device or wonder why, when you open some picture files or email attachments they appear to be big enough to cover the side of a house, then it pays to know a bit about what's going on behind the scenes. This is the first in a short series of Boot Camps devoted to PC imaging and we'll begin with some basic principles.

It all comes down to dots and pixels they're the fundamental building blocks of all computer-generated images. For the most part pixels (short for picture element) and dots are one and the same but you are more likely to see pixels mentioned in relation to monitor displays and digital cameras and dots when talking about printers and scanner – however, the point is a pixel or dot is the smallest individually controllable element in an image. That simply means that the computer controls the intensity or brightness and the colour of each pixel or dot. 

The pixels and dots on a monitor screen are made up of three separate red, green and blue elements, which can be mixed in varying proportions to create any one of the millions of colours and shades that our eyes can distinguish – we'll be looking at colour in more detail next week. The pixels in scanners and digital cameras are also made up of three elements but this time they are sensitive to red green and blue light. The dots in printers are 'painted' on the paper using microscopic droplets of black and coloured inks or toner particles, in the case of laser printers. It's clear then that we're not talking about fixed entities, a pixel can be the size of a dinner plate, like the tri-colour display module used in one of those giant video screens you see in sports arenas and concerts whilst a pixel on the surface of an CCD image sensor chip in a digital camera is only a few microns across.  It follows therefore that the amount of fine detail contained in a computer-generated image varies according to the number and size of the pixels, how close together they are, the overall size of the image or display and how far away you are from it.

To complicate matters further the number of pixels in an image is only part of the story when talking about image 'quality'. Of equal importance is the ability of an imaging and display system to capture and faithfully reproduce or render a range of colours and shades. This is usually referred to as 'colour depth', and 'greyscale' and they all come under the general heading of resolution, a blanket term that describes how much information an imaging device can process.

Believe it or not that was the easy bit… Resolution, in the context of a PC monitor can mean two things. First there's the ability of the display to reproduce fine detail, texture and colour, and that is controlled by the number, size and spacing of the pixels on the screen, the efficiency and performance of the materials used and how well the electronic circuits in the monitor process the video signal or data coming from the computer. Second, we can also talk about the resolution of the video image displayed on a monitor screen, and that is determined by the monitor and the PC, the way the computer is configured and the performance of its video processing components.

The latter causes the most confusion, mainly because Windows lets you change resolution from the Display utility in Control Panel. Many people will have fiddled, or been tempted to fiddle with the 'Screen Area' setting and most recent PCs with 'standard' 15 or 17-inch monitors have at least three options, (selected by moving the slider), namely 640 x 480, 800 x 600 and 1024 x 768 pixels. On systems with larger screen sizes the options may also include 1280 x 1024 and 1600 x 1200 pixels. In most cases it will be set to 800 x 600 or 1024 x 768; 640 x 480 is the 'VGA' default and the minimum resolution Windows can be displayed.

Windows 98 & ME will let you change the resolution temporarily so you can see what effect it has – have a go but before you do make a note of your current setting. On older Windows 95 systems there is a chance you could end up changing the setting to something your PC or video adaptor cannot handle and you'll be faced with the scary sight of a blank screen, and no obvious way of getting it back to normal. (In case that happens to you the solution is to restart the PC in Safe Mode by pressing F8 immediately after switch on and the PC will start in the default VGA display mode and you will be able to access Control Panel).

If you change the resolution to 640 x 480 you will see that everything on the screen becomes much larger and graphics and text looks coarse and blocky. Change it to a higher setting and the opposite happens, everything is smaller and sharper looking but text may be so small that it becomes difficult to read on a 15-inch screen, but it would be just the right size when shown on a 17 or 19-inch monitor. The conclusion you can draw from this is that the video display generated by your PC should be scaled to fit the size of the monitor screen and that higher resolutions work best on larger screen sizes. Most modern PCs, monitors and Windows 9x onwards carries should carry out this adjustment automatically using a system called Plug and Play (PnP) whereby the monitor tells the PC about its capabilities and which resolutions it supports.

Next week – Pictures and pixels, part 2, Colour

JARGON FILTER

CCD

Charge Coupled Device, type of microchip used in digital cameras, web cams, camcorders and video cameras, containing thousands, sometimes millions of light sensitive picture elements or 'pixels' 

GREYSCALE

The number of shades of grey, between white and black – typically 256 -- that a PC imaging system can handle

VGA

Video Graphics Array - standard display format used on PCs, typically made up of 640 x 400 pixels and 256 colours.

TOP TIP

If you are going to be out of the office for a few hours or maybe the whole day you can easily let anyone sending you emails know that they may not get a reply straight away. Outlook Express has a sort of e-mail answering machine facility built-in that will automatically reply to any incoming email messages. (Note that the PC and Outlook Express both have to be running and online or connected to a network).

Start by creating the message that you want anyone sending you an email to receive, something along the lines 'Sorry I'll be away from my desk until …'. To do that click on New Mail, type in the text of your message then go to the File menu and use Save As to name and save the message in a location of your choosing. Next go to Tools > Message Rules > Mail and click the New button. In the first box select 'For All Messages', in the second box choose 'Reply With Message' and in the third box click on the underlined Message hyperlink and direct it to your reply email. Click okay and it's done.