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Visible Quality of an Image


How we judge the quality of what we see depends on the medium on which it is displayed:

On Screens

Like a digital image, a computer screen is a grid of squares each one being a pixel of uniform colour.  You cannot change the size of the pixels of a screen:  they are fixed.  At most you can fake larger pixels by square lumps of real pixels to display a single faked big pixel (for example 2x2 or 3x3 and so on).  This only makes the image blurred.  You cannot go in the other direction and make it finer than the native definition of the screen.  That native definition is (unfortunately) called "resolution" by the salespeople.  You will find flat panel displays of 1024x768 pixels, 1280x1024 pixels, and so on.  When buying a flat panel screen, always ask how many pixels it really has.

Since the physical size of your screen is not flexible, its resolution is also fixed.  Say we have a 1280x1024 pixels screen that is 33cm wide, then the resolution is 1280pixels / 33cm = 37.87 pixels/cm or almost 4 pixels per mm.

Whatever you do, an image on a computer screen will have the quality of that screen.  Most screens available today are very similar:  they have a resolution of 3 to 4 pixels per mm.  There are worse screens, but not any better ones.  So screen presentation is going to be pretty much the same quality, whatever screen you use.

Our image tells us it wants to be displayed at 100 pixels per cm, but in fact our screen has only 38.8 per cm.  This is why the browser shows the image much larger than 8x6 cm:  the 800 pixels need 800pixels / 37.87pixels/cm = 21.1 cm on the screen.

Should we force the browser to display the image so that it is 8 cm wide, then we should use only 8cm x 37.87pixels/cm = 303 pixels on the screen.  But since we have 800 in our image, we have to throw away more than one in every two pixels.  This is exactly what the web browser will do:

It's now 8cm wide, but you can no longer read the panel above the door of the shop, and using a magnifying glass is no good:  the detail is not visible because had to be thrown away!


On printers the situation is very different:  most ink-jet printers can print at 300 pixels per cm(1) or more.  So we can print the image as small as 800pixels / 300pixels/cm = 2.6cm without any loss of detail:  we could use a magnifying glass to read the panel, because the printer did not throw anything away.

We may also choose to print the image very large, say 80cm x 60cm to hang it in a frame on the wall.  Should we want to print it at a resolution of 100 pixels per cm we will need 80x100 = 8000 pixels to fill the 80 cm width.  We have a problem:  there are only 800 pixels available, so we will have to invent 9 extra pixels for each one we have.  This can be done in two ways:  either we just enlarge every pixel ten times, or we "smudge" the colour cleverly between adjacent pixels.  You can see these two methods applied to a small part of the shop panel, the letters "THE" of "THE SMITHY":

220x140 pixels obtained from
22x14 pixels by growing each
original pixel ten times.

220x140 pixels obtained from
22x14 pixels by "smudging" the
original colours cleverly.

If you stand far enough from the screen, there is almost no difference between the left one and right one of these two images:  whatever we do, we can't get information that was not in the image we started from.  The enlarged result is blocky or blurred, but not more detailed.


Whatever we do, we can't get more detail than there was in the original image.  If you want to print something large and in great detail, you need a lot of pixels.  Here are some rules of thumb:

What you may want to do: criterion physical size resolution image size note
put a photo on the web nice size on screen irrelevant irrelevant 800x600 pixels or whatever it needs so that means you scan at about 30ppcm or 72dpi
scan a document to print as a "photocopy" legible printout A4 60ppcm or
irrelevant, but will be about 1200x1750 scan at 60ppcm or 150dpi which is good enough
print a large photo from a camera high detail in close-up viewing A5 or A4 irrelevant as large as you can get, 5 megapixel cameras give you 2560x1920 pixels go for the full set of pixels
scan a page of text to send via e-mail read it on screen should be close to original, e.g. A4 will be that of the screen, i.e. 3 or 4 pixels/mm 800x1200 may not be good enough scan at 60ppcm or 150dpi then sharpen, reduce etc.

This is an example of an A5 page scanned at 60ppcm (150dpi), then sharpened and then reduced to 500x725 pixels.  It's readable on the screen but does not print well.  However it is only 67kB big.

(1) in one direction, in the other direction it may be as high as 2000.  But here we are only interested in the worst case.

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next planned revision: 2009-01