Convert ops

Despite the best efforts of graphics-software developers, the gulf between bitmap and vector formats is set to remain. Perhaps this isn't surprising - after all, they're entirely different. Bitmap images are simply a collection of individual pixels, while vectors comprise a collection of mathematical instructions.

Some bitmap editors, namely Photoshop and PaintShop Pro, are bundled with professional line-drawing tools such as CorelDraw and Illustrator. They also feature bitmap tools, but converting between formats has never been easy.

In most cases, the difference between bitmaps and vectors is of purely theoretical interest. Many of us never go near vectors. Your scanner uses bitmaps exclusively, so does your digital camera. If you're editing photos, working with bitmaps is all you'll ever need to do.

But there are occasions when I wish bitmaps and vectors were freely interchangeable. This is true when rescaling small images or preparing a logo from a low-res original. While there are resizing cheats you can employ - I sometimes double a picture's resolution and apply a Gaussian blur - you're fighting a losing battle against the limitations of pixels.

The only other option in those circumstances is to convert the image to a vector. Once that's done, you can scale it to any size because it's resolution independent. As vectors can be adjusted using a pen tool, you can edit without losing detail. Bitmap-to-vector converters are hardly spring chickens - I used Adobe's Streamline conversion app back in the 1990s. But they were never very good. Only in the past few months have vector-conversion tools become worth looking at.

Illustrator CS 2.0 introduced LiveTrace last year, setting a new bar for file conversion. But this converter isn't as good as CorelDraw X3's similar PowerTrace feature (see Figure 1), which I've been using heavily this month. LiveTrace is easier to use and more feature-laden.

Unlike PowerTrace, LiveTrace does its conversion in the main document window. The LiveTrace button and preset scanning modes are easily accessible from Illustrator's toolbar. Illustrator's 12 presets allow you to match the accuracy of the bitmap you're converting to its vector equivalent.

If the default settings aren't good enough, you should rescan using a higher preset - you'll find most detail retained in the photographic quality settings. The drawback of higher settings is that the vectors in the resulting line drawing may have too many points, resulting in a jagged appearance and huge file sizes. It's always a matter of balance. Thankfully, you can develop and save your own preset to work from.

Illustrator's Tracing Options palette lets you specify just about everything, from the resolution at which you sample the original bitmap, to how much you should blur the original before conversion. This process helps remove artefacts from the original.

Compared to that, PowerTrace looks Spartan. To convert a bitmap in CorelDraw you have to navigate a dedicated but clunky dialogue box. Its preset choices are limited and there's no access to blur settings.

For all that, PowerTrace provides me with better results. I like the way it compares before and after views and the way you can merge and edit bitmap colours before you convert.

This feature alone is a boon. Tiny bitmaps, particularly JPEGs, often suffer colour degradation through compression, which means you get small shade shifts between pixels in what should be flat areas of colour. Too often, conversion software recognises these shifts as discrete areas, so imperfections are transferred to the final illustration.

PowerTrace avoids this by building a palette of colours, allowing you to select similar colours and merge them into one - see Figure 2. Even better, you can select the merged colour and adjust it to match a white or black point, or specific Pantones used in a logo.

PowerTrace is a delight to use. Often developers feel the need to add every bell and whistle, but Corel has done the sensible thing: make it easy to use and people will use it.

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Tom Gorham

PC Advisor (UK)
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