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emco: english >Know-how >Computer and Internet >Scanning and Printing >
Scanning and printing basics
Media students need to work with images: handling them in page layout for print or web display and manipulating them as part of graphics. Digital technology offers a vast array of software processes designed to cover every conceivable requirement. This short article deals with two of the most important - scanning and printing - and provides some basic guidelines.
Scanners
The price of scanners has fallen dramatically in the last few years and for less than £150 it is possible to acquire a scanner that will allow students to capture reasonable quality images for web and print production. Every scanner should come with appropriate software ('capture' and image editing: better deals might include a version of Photoshop LE, providing an inexpensive introduction to the delights of working with industry standard software).
Simple rules to remember: | decide where the final image will appear - on a web page, in a print product
| | scan the image at the size you want it to appear in the finished product |
In order to follow these rules, you need to know something about image resolution.
Bitmaps and dpi
Photographic images are 'bitmaps' - arrangements of rows and columns of pixels (picture elements), each of which can be represented as a different shade. The more data in the file, the bigger the bitmap and the more detail in the final image. Bitmaps are generated by scanners and digital cameras (a drawing or graphic that is scanned will also produce a bitmap). Computer software represents image resolution both by the matrix of pixels (e.g. 537 x 756) and by 'pixels per inch' (ppi). A computer monitor shows images at 72 ppi (Mac) or 96 ppi (PC). The resolution of a monitor is also referred to as 'dots per inch' ('dpi') and if you want the final image to appear only on the screen, you can select '72 dpi' in the scanner software.
But what if you want to print the image? This is where some confusion arises. You might have a printer that is described as a '600 dpi' or even '1440 dpi' machine. But the description 'dpi' is used differently in scanners and printers. A printer might use several dots to represent a single pixel. For most inkjet and laser printers, the maximum image 'resolution' they can achieve is the equivalent of 150 ppi/dpi and this is what you should select in the scanner software. If your print product is going out to a professional printer using an Imagesetter, increase the dpi figure to 220. There are only two reasons why you should scan at more than 220 dpi. The first is if you want to enlarge the original. If you have a photograph that is 6" by 4" and you want to enlarge it to 12" by 8", then just double the dpi to 300. You should also select 300 if you are scanning text or 'line art' (select 'line art' rather than photograph for text and diagrams).
Image size
Scanning at too high a resolution will produce very large data files of a megabyte or more. These will often have to be scaled down to fit in publications and the extra resolution is wasted if it can't be printed or seen on a monitor. (You will also be very unpopular if a large image means a web page takes a long time to download.)
Once you have the image scanned and displayed in Photoshop or equivalent, you should be able to check 'image size' given in inches or centimetres. You can always scale down the image here, but check the 'resample' box. If this is selected, your scaling down will result in the whole image being re-organised with image detail being commensurately reduced. Leave the box unselected unless you need to resample (see below). Never 'scale up' an image - the result will be 'blocky' as all you are doing is increasing the size of each pixel, not adding detail. If you need to enlarge, scan at a higher dpi.
Processing the image
If you have Photoshop available and your scanner software has a Photoshop 'plug-in', carry out scans direct from within Photoshop using the 'Twain Acquire' menu. This means the scanned image will appear in an open Photoshop window. If you are scanning an original photograph, the default settings on the scanner software will probably be OK. If the photograph is over or underexposed you can change these settings, but it is probably easier to use Photoshop to work on the scanned image.
Photoshop is a very sophisticated program, but a couple of commonly used tricks might help. First try the Image Menu and select 'Adjust' and then 'Auto Levels'. This will 'standardise' the spread of dark and light levels in the image. If the image seems no better, cancel the action and try 'Variations'. This will show you a range of possibilities. An alternative is to go first to the 'Filters' menu and select 'Unsharp Mask'. This should generally sharpen the image and increase the contrast. When you gain confidence you can manually adjust Levels using the slider controls.
If the original image is already printed as a 'halftone' in a book or magazine, there are two problems. First, the image is copyright and shouldn't be republished without permission and second, scanning is likely to create an unpleasant clash between the pixel matrix you are using and the matrix used when the image was printed as a halftone 'screen' (look closely and you will see the arrangement of dots). The resulting moiré pattern can leave an ugly grid of blocks across the image. Your scanner software might provide a setting to deal with this (look for a 'descreen' option). If not, here is a possible solution. Scan the image at high resolution. In Photoshop go to the Filters menu and select 'Blur' or 'Despeckle'. This should soften the edges of the blocky pattern. Now go to Image (Image Size) Menu and reduce the resolution to normal (150 dpi), selecting 'resample'. The resulting image can be 'sharpened' in the Filters Menu and should be much closer to an acceptable image for screen or page.
'Printing'
If your image is going straight to the web, you will need to save it as either a JPEG (for photographs) or a GIF (for line drawings, or graphics). These are 'lossy' file formats - they compress the image data and some detail is lost. Your image editor should offer you a range of compression options and since web images are already quite small, you shouldn't need to compress too much: 20-50k is OK for a photographic image.
Images that are to be printed should be saved as 'TIFF's (make sure you save for Mac or PC as appropriate when offered the choice). The screen image is not necessarily a good indication of how the image will print to paper. Some printers will produce lighter or darker prints than shown on screen and trial and error will produce the best match between screen and printer. Colour printing needs an article by itself, but as a basic measure, you can experiment with colour profiles on the monitor and adjustments via the printer software to get close to a match. More important is the question of the selected print resolution.
Modern inkjet printers offer resolutions of 1440 dpi and over. Beware that high resolution printing takes a very long time and uses plenty of ink: a media department is unlikely to have either the time or the budget to allow this on a regular basis. 360 dpi is fine for text and for most images used in DTP exercises etc. (some inkjets have a 'fine/photo' setting which will improve the quality of 360 dpi). A good quality image (i.e. an original photograph with plenty of detail) could be printed at 720 dpi, but above that the only justification is for an exhibition print on 'photo quality' paper.
More info
If you want to find out more, check out the manuals for Photoshop and for individual printers. Companies like Epson produce good advice for use with their scanners and printers (much of this will now be online). Focal Press publish a guide to Photoshop and there are numerous other guides of varying quality available from computer stores. Good luck!
Author: Roy Stafford. © Roy Stafford 2001. With kind permission of MediaEd.
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