10-RES-Handeling images

Resources > Handling Images

Concept of Pixels
The concept of a digital image is a children's 'peg board'. It is characterised by the number of pixels (the 'pegs') and the size of the monitor screen (the 'board'). A pixel for a computer is the information of colour and position. The available pixels are spread out on the monitor. The number of pixels per length and width of a monitor is called the resolution. A typical resolution of a computer monitor is 1024X768 pixels, although in newer computers it can be higher. If the resolution is higher, but the monitor screen is still the same, say 17'', what happens? The size of each pixel will become smaller.

Still camera and computer resolution:
What makes dealing with pixels difficult is that sometimes one knows the number of pixels without any mention of width/length (e.g. a still cameral may give you 5 Megapixels), in another case one has to deal with pixels per length and width and finally you may also have to consider the pixels per area. Confused?
A still camera gives you 5 Megapixel (=5 Million Pixels). Let's say your computer screen depicts the aforementioned 1024x768 pixels. The screen can depict 1024 pixels along its width and 768 along its length, i.e. one row of pixels along and one across.
How many pixels can the screen depict at all? That would be 1024x768=786,432.
Your camera gives you 5 Million Pixels, but your screen can only depict less than 1 Million. What happens? You can only see a corner of the image and have to scroll for the rest.
If a computer screen has a resolution of 1600x1200 can depict 1,920,000; nearly 2 Million. You would see more of a 5 Megapixel image, but still not all of it.

Printer and computer resolution:
A resolution is often given as ppi, which is pixels per inch, or dpi, which is dots per inch and it is a linear value (number of pixels along a length). You know that your computer screen has a resolution of 1024x768 pixels. But how many ppi does it have? Take a tape measure, measure the length and width of your screen (if it is a 17'' screen it that means that it measures 17'' diagonally). Say it measures about 14.8''x 11.8'' (length X width), than it would have about 69 ppi across and 65 ppi along. Most probably the measurement is not quite correct, but near enough. 72 ppi is a typical resolution in a scanner or graphic software, which is close to the calculated value.
So if you have an photo you want to depict on a monitor screen and you would like it to have more or less the same size as the physical photo, scan it with a resolution of around the value we just worked out (ca. 72 ppi).

Scanning of documents:
If you start scanning, you are asked if you want to scan an image or a document. Why this distinction? A document contains text, which means fine lines. If the resolution is too low these lines will not be very clear as lots of information is not being scanned. So for text, you need a higher resolution, usually 300 ppi. Can you see the problem? If you want to show it in a computer monitor, it will be too large and you need to scale it make it visible on a computer screen. Word is likely to scale down an image as long as it stays in the word document. You can downscale it with specific imaging software as well, if you want the smaller size permanently.