IrfanView tutorials.

IrfanView is a very fast, small, compact and innovative FREEWARE (for non-commercial use) graphic viewer for Windows 9x/ME/NT/2000/XP. It is available for download at http://www.irfanview.com/.

IrfanView Class 3

Pixels, RESOLUTION and dots oh my!

Resolution

Resolution is the number of dots in a square inch.

The dots can refer to pixels on the screen, or dots that a printer puts down on a page.

A standard screen resolution is referred to as 72 pixels per inch (although they can go higher). So a square inch contains 72x72 pixels.

A standard black and white desktop printer resolution is 600 dots per inch (although they can go higher), ie a square inch contains 600x600 dots. (Colour printers are more complex as they use combinations of overlapping dots to create different colours.)

Monitors

So what happens to the dots when you zoom in and out?

If you have a picture that is 72 dots or pixels per inch, and you display it at 100% (ie no zooming), the monitor automatically maps each dot in the picture to one pixel on the screen.

If you zoom right in on a picture, you can see the dots that it is made up of, eg this flower picture zoomed in at 1600%:

Obviously the monitor (and printer) will use more than one pixel or dot to represent the dots shown here, but it shows the dots that make up this 72 dpi mage.

Your turn

Open up the image flower72dpi.tif.

Zoom in or out so that you are viewing it at 100% size, so that one pixel is used for each dot. You can see what percentage your view is in the area under the picture:

Now zoom in as far as you can, so that you can see the actual dots the picture is made up of.

Remember that to quickly zoom in a long way, you can click and drag to make a selection around a small area, and then click inside the selection to zoom in.

Move around the image (hold down the right mouse button and drag the picture to move around it), so that you can see where the dots are most obvious. This is usually at the edges between regions of different colours, as they tend to blend into one another in areas of fairly even colour.

Remember that to zoom back out again quickly (to save clicking the zoom out button many times), use the shortcut key F (for ‘fit image to screen’).

Other resolutions

So what happens if you have an image that is 144 dots per inch? (ie 72dpi x 2).

If you open up an image in IrvanView and display it at 100%, it will show you it at 72dpi, even if the image was saved at 144dpi, as it matches the dots to the pixels. So a 5x5 image at 144dpi will show on the screen as 10x10.

Basically IrfanView ignores the size information encoded within the picture, and just shows you one dot per pixel.

Resizing images

If you want to use an image from your digital camera (or scanned image) for either uploading to a website or emailing to someone, you’ll need to reduce the size of it, as most cameras these days capture more information than you need for these purposes (although they are needed when printing).

With flower72dpi.tif still open, go to the Image menu and select Resize/Resample. This box will come up:

It gives you the current size in terms of pixels.

Alter the Width setting to 400. As the “Preserve aspect ratio” box is ticked, it will automatically alter the height proportionally.

Notice that it also has options for cm and inches in the Units section, which shows you the dimensions the picture would be if you printed it out (at the current 72dpi).

Select Save As from the File menu (shortcut – S), and save it as a jpeg file, with a save quality of around 80. This will give you a file that is only 22kb, quite small and ideal for web sites and emailing.