Vector Brushes - Different Strokes

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The move from vector to object brushes

Tom Arah looks at vector brushes and gets to grips with the object-based future.

Object brushes in Satori PhotoXL 

Bitmap v vector

For a child the difference between drawing and painting is minimal and largely depends on whether they happen to pick up a pen or a brush. For the computer user they are two entirely disciplines built upon two completely different technologies: vectors and bitmaps. As we saw two months ago bitmap brushes have now reached a level of sophistication where it's often difficult to tell computer-produced output from hand-produced art. By comparison the drawing package's pen and brush tools have always seemed creatively crude. As we'll see this month, however, that's not necessarily the case.

The strengths and weaknesses of computer-based drawing both derive from the way the drawing is produced. Whereas all bitmap images are composed of a rectangular grid of coloured dots, vector drawings are created mathematically. Each object whether an open line or closed shape is defined by Bézier curves which are themselves defined by anchor points with two control handles that determine the arc of the line as it enters and exits the anchor point. Such mathematical definitions are extremely efficient, but more importantly they are easily editable and scaleable. The result is the total flexibility of vector drawing and the pin-sharp, resolution-independent quality of its output.

Stroke Formatting

Vectors are ideal for describing open paths and closed objects then but there is surprisingly little intrinsic control provided over the appearance of the line or the "stroke" as it is called in the all-important Postscript page description language. You can set the weight of the stroke and whether it is solid or dashed, but otherwise the majority of options are limited to controlling how corners and line endings are handled. Not exactly creatively exciting. Compared to the artistic freedom of bitmap images, where brush strokes can vary along their length and interact with the paper and other strokes, the basic images from drawing packages always seem cold and clinical.

The main problem is that by default the strokes are absolutely uniform across their whole length which immediately looks unnatural and computer-produced. Corel Draw was the first program to tackle this when it introduced its calligraphic effects. By allowing the user to control the angle, roundness and diameter of the pen tool's "nib" the width of the line could be made to vary depending on the angle of the line. The end results looked as if they had been drawn by an angled pen rather than a robot.

Power Lines

Corel also pioneered another major advance in the use of power lines. This took a completely new approach enabling the user to draw shapes along the length of the stroke rather than lines. Using a diamond-shaped power line, for example, it seems like the width of the pen widens to the centre of the stroke and then narrows again. With pressure-sensitive pens it even becomes possible to vary the width of the stroke interactively which really opens up creative possibilities with natural pen tools that finally act like real world pens.

Because each stroke is actually a closed path these pen variations also open up formatting options including the ability to apply gradient and textured fills. However the fact that each stroke is actually a shape is also a serious problem as it means that once the stroke is drawn it is much more difficult to edit. You can't retrospectively change the width of the stroke, for example, or edit its path. More to the point, even with power lines each stroke is still fundamentally regular so that the end results might have the fluidity and spontaneity of a marker pen but not the creativity of an artist's brush.

The arrival of Expression

The program that completely broke the vector strait-jacket was Metacreations Expression. This took the power lines idea to its logical conclusion with its "skeletal stroke" system based on much more complex, composite shapes draped along the length of the path to mimic traditional art brushes such as india ink, stipples, highlighter and finger paints. The multiple objects in the stroke had different grayscale settings which meant that the stroke could be colorized to produce a far more realistic range of colours rather than the single flat colour of power lines. More importantly "skeletal strokes" remained linked to an editable path so that you could quickly change the width of the stroke or its positioning retrospectively. Best of all, Expression allowed you to create your own skeletal strokes so that, for example, you could turn text into a brush that you could then squeeze out onto your drawing like toothpaste.

The Illustrator approach

At one time it looked like Metacreations was going to develop Expression into the vector art equivalent of Painter, but sadly it seems to have been allowed to die. The good news is that the underlying idea hasn't and recently made a big comeback in the art brushes of Illustrator 8. The Adobe system isn't as well implemented as in Expression, as you have to call up a dialog just to change the thickness of each stroke, but the majority of the power is there. As Illustrator 8 also offers calligraphic brushes, scatter brushes - which act like vector versions of bitmap image hoses - and pattern brushes - which can create strokes out of repeated blocks - it has largely taken over Expression's role as the drawing program of choice for the computer-based artist.

Illustrator's brushes have certainly come a long way from the uniform-width Bézier curve, but equally they are still a long way away from the creativity of the bitmap brushes in a program like Painter. While you can quickly add a watercolour-style swash to underline a logo, for example, there's no way that you could make a vector drawing look like a realistic watercolour. The reason is simple. The bitmap brushes can produce such realistic effects because every pixel can vary compared to its neighbour and because the pixel values produced by the brush can be made to depend on the underlying pixel values so that bitmap brush strokes can interact with themselves and even the underlying paper. In comparison even the most advanced vector brush remains a crude and self-contained approximation.

The end of the road?

Ultimately then the pixel-based bitmap approach has a huge and unmatchable advantage when it comes to producing truly artistic effects. So is that the end of the road for the vector-based brush? Strictly speaking it is, but recently a new breed of program has appeared that attempts to move things onto the next level. These new programs attempt to combine the benefits of vector drawing and bitmap painting into a new breed of "object-based" imaging. The central idea is simple: rather than drape a shape along the vector-defined path, drape a pixel-based effect. The pixels offer the advantage of interaction and absolute variability while the underlying vectors provide flexibility and editability.

Into the mainstream: PhotoDraw 2000

The program that is most obviously bringing such object-based technology into the mainstream is Microsoft's all-in-one graphics application, PhotoDraw 2000. Select PhotoDraw's Painting option and its range of Artistic brushes become available. These act in a similar way to Illustrator's brushes draping a complex shape along the length of the stroke to mimic a real world brush, but there's a big difference. Thanks to their object nature PhotoDraw brushes can be given a pixel-based semi-transparency effect so that watercolour brush strokes can overlap realistically. This opens up huge potential for using PhotoDraw to stylize drawn images. You can take some basic clipart, for example, and give it a charcoal effect and if that doesn't appeal just move on to oil-paint or chalk.

Microsoft has also appreciated the other pixel-based possibilities of object-based strokes. The Photo Brushes wrap scanned bitmap elements along the length of the stroke so that you can draw a simple line onscreen and PhotoDraw will turn it into a photo-realistic snake. Even retouching is handled in the same object-based way. Put it all together and you have some serious power. Interactively draw a border of ivy around your Christmas card with a single stroke and you can even use the dodge and burn tools to add some realistic shading. The results are strikingly photo-realistic but also completely editable. You can move the line's anchor points or change its width and it will automatically redraw. You can even change the current photo brush to a border of paper clips, for example, and the retouching effect remains in place. If that looks a bit cheesy you can simply apply one of the Designer Effects to give the stroke a watercolour effect.

The Drawbacks

The creative potential of PhotoDraw's object-based handling is mouth-watering, but there are serious drawbacks. Essentially PhotoDraw is having to juggle the pixel-based processing of a layer-based photo-editor and the vector-based processing of a drawing package simultaneously. The end result is grindingly slow performance with the program in an almost constant state of redraw. Worst of all the program just doesn't operate in real time. To add an artistic or photo brush stroke to the image, it has to wait until the stroke has been finished before it can begin working out how to wrap the relevant shapes along its length and this then takes ages to process. The end results might look creatively artistic but the process is anything but spontaneous. For retouching, which depends on constant pressure-sensitive visual feedback, the system is simply unworkable.

There's more trouble when it comes to output. PhotoDraw advertises itself as an all-in-one drawing and painting/photo-editing solution so it's reasonable to assume that you can save output to a vector-based format, such as WMF or EPS, in the same way that you can with Illustrator's art brushes. In fact of course that's impossible as the pixel-based effects of the brushes can only be saved to bitmap formats. In other words you are immediately throwing out the scalability and resolution-independence of vector artwork. Ultimately PhotoDraw 2000 is only designed to produce bitmap output, but without the speed or naturalism of a dedicated editor like Photoshop or Painter.

Suddenly rather than the future of graphics, PhotoDraw is looking very much like a dead-end. In short it can't draw, can't paint and can't photo-edit - at least not as we currently think of the terms. In the longer run, however, the outlook is very different. Ever-improving processing power will tackle the speed issue - almost certainly with the help of a new instruction here and a tweak to the OS there. Microsoft is also in a unique position when it comes to tackling the output issue as by building support for PhotoDraw's MIX file format into its own apps it could create a new object-based file format standard with scalability and output quality restored.

Graphical Fireworks

In the long run the flexibility and creativity of object-based handling is too useful to fail, but in fact there are already practical solutions in place today. My favourite has to be Macromedia Fireworks. Its brushes are also pixel-based but are designed to mimic existing bitmap tools such as the common airbrush and chalk and charcoal tools. Crucially, because the effects are much simpler, they can be applied in real time. In fact with pressure-sensitivity and variable opacity and even blend mode control, the only difference to painting with a bitmap brush is that the Fireworks stroke can be selected for retrospective path and formatting editing.

Fireworks also solves the issue of pixel-based retouching very neatly as any stroke or shape can simply be converted to bitmap format. This means that the stroke loses its object-based editability but it enables local retouching. Double-clicking on the converted object takes you into image edit mode where dedicated pixel-based options, such as the magic wand and dodge and burn tools, become available. The problem of bitmap-only output also melts away as all Fireworks output is destined to appear as simple GIFs and JPEGs in any case.

With simpler brushes and web-centred output the problems seen in PhotoDraw's over-ambitious approach disappear. The underlying imaging engine is so impressive with its combination of drawing-style editability and bitmap-based creativity that I had hoped that Macromedia would develop Fireworks into an all-round graphics program to replace its uninspiring Photoshop-clone, xRes. With Fireworks 2, however, nothing new has been done with the core object-based technology and the program has clearly settled for its web-based niche.

High End control: Satori PhotoXL

There is one object-based program, however, which takes the challenge directly to the high-end creative bitmap editors Photoshop and even to Metacreations Painter itself. The program is Satori PhotoXL. When you use a brush like the Satori airbrush it's clear that you are producing a completely naturalistic bitmap-based effect. With a few strokes you can produce an attractive artistic-looking apple. Turning to the retouching tools you can select the highlight and darken options and quickly give the apple realistic depth. Everything is fast and smooth and the end results couldn't be bettered with Painter.

Turn to the Object List, however, and you can see that the various brush and retouching strokes have been stored. You can select each tool's strokes, or "brush objects" as they are called, and retrospectively change their colour or width or simply delete them - though sadly not edit their path - and the image will update accordingly. This really is impressive object-based power with Painter-style bitmap creativity allied to vector flexibility. Call up the Brush Setup command and you'll see how the magic is achieved. Each Satori brush truly is object-based with the fall-off of the density of the airbrush nozzle, for example, determined mathematically.

The sky's the limit

Ultimately then the distinction I made at the beginning of the article between dot-based bitmap brushes and mathematically-based vector brushes is an oversimplification as of course everything on the computer is mathematically generated (and indeed final perceived colour is always produced by rasterized dots). The difference is that in a traditional bitmap environment the movement of the mouse is used to determine the path of the algorithmically-defined bitmap effect but then the resulting colour values are committed permanently to the pixels and the stroke information thrown away. Object-based brushes on the other hand retain the link between the bitmap effect and the vector path so that the stroke information in the image is never lost and can be used for future editing. Effectively the object brushes work by joining the dots, retaining the link between the pixel effect and the original vector path.

Theoretically then the skies are the limit with the possibility of recreating any current bitmap-based brush - after all, as we saw two months ago, even the most artistic of brush effects is simply determined by adding more parameters to the equations. But hang on. With the strokes of the image effectively on their own layer, able to interact with all other layers and able to be updated what does this mean in terms of resource demands and speed? Surely if PhotoDraw is slow, Satori must be glacial?

Pulling out the stops

But this is where Satori pulls off its greatest trick. It recognises that the only pixels it actually has to update accurately as you work are the pixels of the screen image. The end result is that working in Satori is actually far quicker than working in a normal bitmap environment. Sometimes astonishingly so. If you are working on an A2 poster, for example, you can happily use a 1000-pixel wide airbrush that would bring Photoshop to its knees. Zoom in on an image and click the Hi-Rez command and Satori will pull out the relevant co-ordinates and just process that section of the image. By converting imported bitmaps to its own RIR format it can even do the same with huge scans so that handling images of over 100Mb is as fast as handling those under 10Mb.

Of course there has to be a payback time and this comes when you finally output your image and render the pixel values across the whole image to TIFF or PSD format. This process is inevitably time-consuming, but there are two pieces of good news. The first is that you can leave images to be batch-processed while you go off for lunch. The second is that Satori PhotoXL restores resolution-independence as you can set any output dimensions even up to multi-gigabyte advertising hoarding size and the program will automatically scale the effect of all its originating and retouching brushes (though clearly it can't add detail to any scanned images).

Satori's approach is so different to current solutions that it takes a lot of getting used to and certainly it isn't the right program for the majority of users. However it does show the potential of object-based handling. In particular it takes the speed and resolution issues that crippled PhotoDraw 2000 and shows that they aren't intrinsic to object-based processing. Indeed it turns them into strengths.

Future dreams

Eventually no doubt one set of developers will get all the issues right and finally provide users with Painter's creativity and Illustrator's flexibility in a single package. Hopefully they'll also throw in Satori's speed, PhotoDraw's effects and Fireworks' simplicity. When they do the computer user will finally be able to forget the distinction between vector drawing and bitmap painting and, like the child choosing a pen or brush, just get on with being creative.

Tom Arah

05/99


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