Live Trace and Live Paint

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Back to the drawing board

Tom Arah looks at the best way to produce creative drawings on the computer and with Adobe Illustrator in particular.

live trace

Producing a creative drawing on paper tends to be a two stage process involving “inking” and “colouring”: sketching the defining lines in black and then filling this framework with colour. The first mainstream program to attempt to translate creative drawing to a computer environment was Adobe Illustrator back in 1987. Based on Adobe’s PostScript technology, Illustrator and subsequent competitors such as CorelDRAW and Macromedia FreeHand build their drawings as mathematically-defined vectors or “paths”. “Stroking” an open-ended path produces a line and filling a closed path produces a coloured shape. And with a stack of stroked and/or filled paths you have all you need to reproduce any illustration – or technical drawing, graphic design or even text-based layout. It’s a beautifully efficient system and one that has become second nature to generations of computer artists.

Second nature maybe, but no-one could say that drawing with vectors is truly natural. Compared to the simple creative freedom of sketching on paper, the whole process of drawing onscreen is intrinsically awkward and indirect despite important advances such as freehand tools and digitizing tablets. And there are further limitations imposed by the underlying vector architecture. For creative drawing, you often want lines that are fluid and expressive, but that’s just not possible with stroked paths which are intrinsically uniform along their length. Again advances such as vector brushes that treat each stroke as a filled path radically improve the end results – Creature House’s Expression deserves especial credit here - but no-one could claim they were as natural as pens and pencils.

It’s not just the computer equivalent of inking that’s limited; the situation is even worse when it comes to colouring. Here traditional artists expect to be able to simply fill in the holes of the sketched framework but that proves impossible where these areas have been produced by overlapping and intersecting lines – as far as the program is concerned there just isn’t a shape to fill. The problem is that to apply a fill, vectors want a single neatly closed path to work with, not multiple open paths. In other words to apply colour to a hand-drawn vector sketch you first have to recreate the areas that you want to fill in as objects!

Ultimately then it’s possible to recreate any drawing as vectors, including hand-drawn illustrations, but to do so you have to construct the apparently spontaneous end effect by laboriously defining the shape of each coloured region as separate closed paths and even do the same for each inked stroke if these aren’t to be a uniform width. In other words the fundamental unit of the vector drawing program isn’t the sketched line but the defined shape which explains why almost all vector-originated artwork immediately looks artificial and computer-generated. Overall it would be hard to come up with a system less like the creative freedom of pen on paper both in terms of process and results. As such, programs like Illustrator and FreeHand have always failed the one group of users they were designed to help: freehand illustrators.

illustrator cs2 vector

Vector drawing’s fundamentally shape-based approach is artificial in practice and results.

But there is an alternative: bitmaps rather than vectors. Each of today’s major bitmap editors, such as Adobe Photoshop and Corel PAINTER, provide far more responsive and realistic hands-on drawing tools. And of course bitmap programs provide another unbeatable advantage compared to vector drawing applications: you can create your sketch on paper and then scan the results. Even better by creating a new layer above your scan, you can use a quick pencil-based sketch as the basis for final inking using the bitmap editor’s brushes.

The benefits when it comes to colouring your original or scan are just as clear. Because, by default, each bitmap canvas is a single layer, it’s possible to fill any enclosed area simply by clicking in it with the Paint Bucket tool. Of course the bitmap program can’t “see” the shape any more than the vector program, but rather works like the Magic Wand tool based on colour similarity to the pixel that you click on – which means that you’ll have to apply anti-aliasing or feathering to the fill to avoid undesirable fringes around anti-aliased lines. In other words you can consistently apply flat colours, gradients or patterns to your artwork in seconds; slightly longer if you remove unwanted gaps, stray pixels and anti-aliasing.

photoshop brushes

With its range of hands-on brushes Photoshop provides a more natural drawing environment.

With superior inking and colouring it looks like it’s game, set and match to the bitmap editor. Maybe Illustrator and the other vector apps should forget about creative drawing entirely and concentrate their efforts on logo design and technical work where precision is more important than creative expression. It’s certainly true that for painterly art built on subtle brush work and continuous tones, bitmaps are the way to go. However for the typical creative illustration built on clearly-defined lines and areas of colour, vector handling still proves unbeatable. It all comes down to the underlying nature of vectors.

To begin with, the object-based nature of vector design provides extraordinary creative control. Once you’ve applied a stroke or a fill in a bitmap editor the changes are pretty much permanent; in a vector drawing everything remains infinitely and instantly fine- tunable. In other words you can always redraw and reformat your lines and shapes until you find exactly the effect you are looking for. And with advanced features such as Illustrator’s gradient meshes, multi-fill styling and non-destructive effects, the extra control offered by vector handling leads directly to new creative possibilities.

As well as absolute control, vectors provide absolute quality. Because each vector path is mathematically defined, it is intrinsically resolution-independent. This means that vectors can be scaled to any size without loss of quality. Crucially it also means that, unlike bitmap images where the output resolution is determined by the halftone screen (see RW79) and so typically 150 lines per inch or less, the black lines in a vector drawing can be output at the maximum device resolution. With a PostScript typesetter outputting at over 1500 dots per inch the result is lines as crisp as the text on the page. And you don’t need bitmap-based anti-aliasing to try and fool the eye into seeing smooth lines – at that resolution they just are.

There’s a clear gradient at work here, moving from the maximum freedom and expression offered by pen on paper to the maximum quality and control of vector drawing with bitmap editing falling pretty neatly in the middle. Obviously the ideal would be to combine the different strengths of all three. Scanning and reworking hand-drawn artwork takes care of integrating paper and bitmap workflows, so what’s needed is a way to then turn the edited bitmap into vectors. The obvious solution is tracing.

I still remember the excitement when GEM Artline first introduced the ability to place a partially grayed-out bitmap into your layout which you were then expected to manually trace with the path-based drawing tools. Don’t knock it. It’s hardly state-of-the-art but this manual line-by-line approach means that you can swap between stroked paths and filled paths as appropriate and that the artwork is kept as clean and editable as possible. You’ll even find dedicated Illustrator add-ons such as LogoSpruce designed to make the bitmap redrawing process and its end results as smooth as possible.

Manual tracing still has an important role to play but it’s obviously a major chore to recreate an entire drawing like this. So why not offload the task to the computer? That’s exactly what Illustrator’s Auto Trace tool (for some reason hidden away as an option of the Blend Tool) enables. Using it you can simply click on a shape in the placed bitmap and, based on colour similarity, its outline is automatically converted to a filled path. This means you can click on a white hole in an inked drawing created by multiple intersecting lines to automatically create a closed path ready to accept colour. Accuracy and editability suffer compared to doing it yourself, and you’ll still have to manually draw any stroked paths, but convenience and productivity are boosted enormously.

illustrator auto trace

Illustrator’s manual Auto Trace tool is handy though underpowered.

The obvious next step is move on from working one line at a time to automatically tracing the entire image. The program that defined this category of bitmap-to-vector conversion software was Illustrator’s natural partner, Adobe Streamline, which was launched back in 1990. It’s not just productivity that is improved; by automating the process you can trace far more complex images even continuous-tone photographs. In fact if you remove the restriction on the number of colours and set the minimum area to 1 pixel the results can be indistinguishable, and as vectors they are fully scalable too. But don’t be fooled – yes they are scalable, but no extra detail is added when you do and so the massive extra processing involved compared to handling a placed bitmap is wasted.

Tracing colour images is a useful sideline but only makes sense when dealing with posterized work in which the palette of colours is reduced. Ultimately the core purpose of bitmap-to-vector conversion remains tracing black-and-white drawings as it’s only the solid black vector objects that can be output without halftoning (cyan, magenta, yellow and spot colours apart). And here Streamline, offers a major advance. As well as producing its tracings as filled closed paths with the default Outline method, it can also identify uniform width lines up to a maximum of 6 points in weight and recreate these as stroked paths with the Centerline option.

adobe streamline trace

Streamline offers both outline and centreline-based tracing.

The resulting stroked lines are guaranteed to be absolutely uniform compared to outlined paths and are also more editable as it’s much easier to redraw and reformat a stroked path than rework a filled shape. And by selecting both Outline and Centerline in the main Conversion Setup dialog, Streamline automatically selects between the two approaches as it thinks appropriate to offer the best of both. No-one could say that tracing like this is natural or integrated and the results certainly aren’t perfect, for example intersecting lines are treated either as multiple separate stroked paths or a single filled path neither of which are fun to edit. However there’s also no doubt that such tracing plays a crucial role enabling a degree of freehand creative expression to be put back into vector illustration.

With the combination of Illustrator’s simple, hands-on Auto Trace tool and the powerful automated Streamline, it looked like all bases were covered and that there was little more that could be done. Certainly that seemed to be Adobe’s opinion as it stopped the development of Streamline after the launch of version 4 back in 1997 (which explains why it doesn’t support JPEG!). It was a serious mistake as Illustrator’s rivals quickly proved that this was by no means the last word in bitmap-to-vector conversion. Compared to the increasingly antiquated Streamline, for example, CorelDRAW’s bundled TRACE application provides greater control and makes it easier to fine-tune your settings. Even more impressive is FreeHand’s Magic Wand tool which offers its power directly on the drawing canvas, can trace either single paths or entire objects and works with both bitmaps and vectors.

Fortunately Adobe has recognized its mistake and, with the recent release of Illustrator CS2, has completely rethought its approach to tracing and, in the process, its whole approach to creative drawing. In particular the Auto Trace tool has been dropped - disappointingly as it still has an occasional role to play - and replaced by the new Live Trace capability which incorporates all of Streamline’s power, and more, directly within Illustrator. And it couldn’t be easier to use. Select a placed bitmap and the new context-sensitive Control Palette offers dropdown access to a whole range of Live Trace presets most of which are aimed directly at creative illustrators: Hand Drawn Sketch, Comic Art, Inked Drawing and Detailed Illustration. Select one, and a few seconds later your bitmapped lines appears as pin-sharp vectors ready for the highest quality output at any size.

live trace

Illustrator CS2’s Live Trace offers powerful tracing and round-tripping.

What makes Adobe’s new implementation unique is that the tracing is “live” which means that you can swap between presets to see which works best or call up the Tracing Options dialog to fine tune parameters. And thanks to the speed of the system and its preview capability you can see the effect of changes as you make them. Powerful options include control over the image’s black/white threshold, path fitting and minimum area. Best of all, for black and white images, you can now combine outline and centreline tracing of lines up to 100 pixels wide and, as you can distinguish between the resulting stroked and filled paths in the preview, you can hone in on the best possible mix for the current image.

Adobe has another trick up its sleeve. Because the tracing is live, you can edit the original linked bitmap and the vectorized version will automatically update to reflect the changes. You might want to do this to quickly remove noise or fill in holes, but there’s nothing to stop you making larger changes, redrawing lines and so on. This round-tripping is ideal for inking as it means that you can use an original scanned pencil sketch to work up a more solid but still fluid drawing framework with the natural brush-based bitmap tools, fine-tuning as necessary, and ending up with the pin-sharp output of true vector lines. The best of all worlds.

That’s great for inking, but what about colouring? If the image is traced as fills only, it’s a relatively simple case of expanding and ungrouping the tracing and then filling the shapes that define the white holes of the drawing. However for tracings recreated wholly or partially as stroked paths we crash back into the problem we looked at earlier: as far as Illustrator is concerned an apparent shape created by multiple intersecting open paths isn’t a shape at all. It looks like the only option is to laboriously recreate the apparent holes in the drawing as closed paths so that we can apply colour to them!

But help is at hand. As well as entirely reworking its inking capabilities through Live Trace, Illustrator CS2 sees the introduction of a ground-breaking new approach to colouring: Live Paint. By turning a selection of paths into a Live Paint group, you can treat it much more like a bitmap canvas. In particular you can fill any apparent hole or region in the drawing simply by clicking with Illustrator CS2’s new Live Paint Bucket tool just as you can in a bitmap editor. In fact Illustrator goes a stage further and even lets you automatically detect and close gaps up to 72 points wide. And, as its name suggests, the effect is live so that if you move a border element, the fill adjusts accordingly.

live paint

Illustrator CS2’s Live Paint breaks down the boundary between vector lines and shapes.

This is exactly what we need for efficient colouring and is a massive step forward. To apply a fill to a shape in Illustrator CS2 you no longer need to have a single closed path – any shape can now be defined by multiple overlapping paths whether stroked or filled. What matters is what appears onscreen not how the drawing was constructed. Behind the scenes this magic is enabled by Illustrator treating the Live Paint group as a flattened layer in which the existing paths create new closed paths as required, as you can see if you expand and ungroup a filled Live Paint group.

Ultimately this means that Live Paint is a workaround and an imperfect one too as the system can’t handle brush strokes, live effects, gradient meshes or transparency - though these can always be applied to the expanded results. However, for simple colouring of a Live Trace drawing, the Live Paint approach works brilliantly enabling the end user to finally break out of the closed-path straight-jacket of traditional vector handling. With Live Trace and Live Paint, Adobe has come up with two ground-breaking innovations that work hand-in-hand to revolutionize the two core stages of creative illustration: inking and colouring.

With Illustrator CS2 Adobe hasn’t just gone back to the drawing board – it has reinvented it.

Tom Arah

June 2005

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