Enlarging images

Anyone who has a digital camera or regularly downloads pictures from the Web will eventually experience the problem of undersized images. Printing these low-quality images will lead to disappointing results. Even images taken at higher resolutions of 1 megapixel (approximately 1200x800 pixels) will struggle to produce a good-quality print larger than the standard 10x15cm.

The problem is not restricted to digital images — polaroids, old photographs from early last century or small snapshots taken in photo booths all present similar challenges. Scanning at higher resolutions can assist, but will not entirely be successful if you are trying to produce substantially larger images.

Finally, there may be times when you want to crop an image or zoom in on a feature. This can leave you with a small image needing some enlargement.

Graphics programs offer various ways to improve the size and print quality of small pictures. The results can range from mediocre to startling. Depending on the image, there may be some trade-offs: the larger image may not be as crisp as the original, or you may see the appearance of jagged edges. However, the impact of these unwanted intrusions can be reduced.

The basics of resampling

Resizing a digital image is not a straight­forward process for graphics programs. They have to redraw the entire picture for even the most basic adjustments. This is achieved through a process known as ‘resampling’. In simple terms, this means that the program makes a series of calcul­ations to ensure that the same proportions and colours are kept during resizing. This is easy for larger blocks of colour, but finer details can present problems. For example, when enlarging thin lines such as hair, will they be thickened and by how much? The answers to this question can be frustrating: yes, no and sometimes. In some cases the enlarged image will lose a little sharpness, but in other cases someone’s hair can start to look like it is made of Lego blocks. The outcome is dependent on a number of factors, and one of the most significant is selecting the best resizing technique.

When enlarging an image, most pro­grams offer a choice of Nearest Neighbour, Bilinear or Bicubic. The general agreement is that Bicubic is the best, but it takes longer for the program to complete. The time factor is no longer a problem on faster PCs, but if you are experiencing lengthy delays try Bilinear, then Nearest Neighbour.

How to enlarge

You will need a more advanced program than the free Paint program that ships with Windows. If you don’t have a program but wish to try your hand at enlarging, head to the cover CD and load up the trial version of Paint Shop Pro.

For any image that you are trying to upsize, you can get disproportionately better results if you start with a larger original file. An image that is only 100x100 pixels contains very little detail, and enlarging won’t be very successful. However, at around 300x300 you can start to squeeze some extra life out of your picture. An image sized originally at 1200x800 can be enlarged up to A3 poster size while retaining good print quality. Keep this in mind when setting up your digital camera or scanning an image.

Before enlarging, you should check the image’s current dimensions (in pixels). Then you will need to consider the desired final size of the image. There will be a drop in clarity as the image increases, so don’t go overboard. Picking the final size will depend on the image’s use — for example, are you trying to create a larger print or stretch an image to fit on your desktop?

After loading the image into your favourite graphics program, check that you are using 16 million colours (also called 24 bit). In Paint Shop Pro select Colours-Increase Colour Depth-16 Million Colours (24 bit) and for Photoshop select Image-Mode-RGB Color. Now head to Image-Resize (Paint Shop Pro) or Image-Image Size (Photoshop). The program should display the image’s current dimensions. Type in your new desired size and select your preferred resampling option — by default, it is usually set to Bicubic.

Caution: make sure that the units are listed as pixels. Do not use centimetres or other options. Also ensure that you check any boxes marked “Resize all layers” or “Maintain Aspect Ratio”. Click OK and view the results.

Once you have applied the new dimen­sions, you may notice that several unexpected changes have occurred. There may be some minor variation in colours, the image will be blurry and some lines and edges may now have steps instead of smooth curves. It is possible to remove some of these unwanted intrusions — and this will be the topic of the next column.

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Scott Mendham

PC World
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