Improve your enlargements

If you have attempted to enlarge a photograph, you may have noticed the appearance of many unwelcome changes to the image. There may be some minor variation in colours, the image may be blurry, and some lines or edges of the image may now have steps instead of smooth curves. With some clever tinkering, it is possible to remove these unwanted intrusions. Before applying these techniques, ensure that your image is set to 16 million colours (24-bit colour) or RGB mode in Photoshop. If you are unfamiliar with resizing an image, see last month’s column covering the basics of enlargement.

Get sharp

Most enlargements will make the image blur or ‘fuzz’. This problem is not unique to digital photographs — traditional film enlargements can also suffer from this aberration (but for different reasons). Luckily, there is a simple way to restore some clarity to the image — the Unsharp Mask. This filter locates pixels where the contrast differs from surrounding pixels by a certain amount, and then increases these pixels’ contrast. Most programs allow you to set the contrast levels and the size of the surrounding area. If you have made a substantial enlargement, then it is important to experiment with these settings to improve the outcome.

Taking the stairs

When enlarging images by a factor of three or more, you will notice the appearance of the dreaded jagged lines. These are evident in areas where there is curved, fine detail — particularly with dark lines on a light background. They are tricky to remove and sometimes the cure can be worse than the affliction. Applying a light blur filter may have some impact (use it before the Unsharp mask mentioned above). The jagged edges are a result of the mathematical techniques used to enlarge an area. The details are a little too technical to cover here, but that doesn’t prevent an easy solution.

The quickest way to treat the problem is to change the way enlargements are made. One common approach is called ‘stair interpolation’ (SI). Don’t be scared off by this piece of jargon, it can turn out to be the best way to avoid the jags. The trick with SI is to perform a number of smaller enlargements, rather than a single jump. If you were enlarging an image from 1000x1000 to 5000x5000pixels, your first enlargement will be to 2000x2000, then this is enlarged to 3000x3000 and so on. In this example there are five steps up (hence the word ‘stair’ in SI) to the desired size.

The number of steps to take will depend on your enlargement, but five steps are suitable for smaller enlargements, and 10 should be enough for larger images.

This approach is only necessary if you are making substantial enlargements. A small increase of, say, 20 per cent will not benefit from this approach. As a quick shortcut, see if you can get away with the enlargement in a single step. On some images the effect is barely noticeable, but it may stand out on other types of photographs.

Performing the same function 10 times over for hundreds of photographs will probably drive most people crazy. Luckily, there are alternatives. If you are the adventurous type, you could write a script in Photoshop or use its Automate tool. Everyone else can try any one of the automated tools on the Web. Most will only work for Photoshop, but you may find one for your program if you persevere. One of the best tools is the Fred Miranda script ($US15) available at www.fredmiranda.com/SI. It takes the monotony out of SI by performing the steps automatically, and it has an option to run the sharpen tool at the end of the enlargement process.

If money is no object, then try Lizardtech’s Genuine Fractals Photoshop plug-in (www.lizardtech.com). It is a little pricey but is widely regarded as highly effective, although some Web sites conclude that GF’s enlargements are similar quality to stair interpolation. There are three versions, priced at $US50 (GF 2 LE for the home user and mainly for digital cameras), $US159 (GF 2.5 for semi-professional users) and $US299 (GF Print Pro 2.5 for professionals).

To try the plug-in, run the installer and open your image in Photoshop. Select File-Save As, and from the format list choose Genuine Fractals. Specify a file name, click Save and the program will spring into life. If it is your first time, click the Help button for additional instructions.

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Scott Mendham

PC World
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