Image Hose Brushes - Dab Hands

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Image-based Brushes

Tom Arah shows you how to make the most of your image-based brushes and looks at Painter's new brush engine.

Image hose

The underlying engine that powers the bitmap editor's most powerful tool, the brush, is built on an illusion. As we saw back in issue 56, the apparently perfectly smooth brush stroke is actually built up by a whole series of coloured dabs laid out along the stroke's path. Under normal circumstances the overlap is such that you can't see the individual dabs but, if you drag a large brush quickly and then zoom in on the edge of the stroke, you'll be able to see the slightly uneven corrugated effect produced by the overlaying circular dabs.

The dab-based stroke might be a workaround, but over the years it's proved to be a very effective platform. The first major advance came with the move to grayscale dabs which meant that the density of the colour laid down across each stroke could be varied. In particular this meant that the edge of the stroke could be made softer than the centre which produced a quantum leap in terms of realism. Almost as important was the ability to angle the dab which meant that the width of the stroke could be varied to produce a much more human calligraphic effect - an effect brought to life with the interactively resizable dabs produced by pressure-sensitive tablets. MetaCreations Painter is the program that has taken the dab-based engine furthest, culminating in realistic oil-paint brushes built on multiple bristles that each lay down slightly different colours while cycling slightly around the central stroke's path!

In the process of emulating traditional artistic media, Painter has managed to largely obscure the dab-based nature of its brushes, but an alternative approach is to highlight it. Photoshop's default brushes are all built on traditional grayscale circular dabs but, using the Brush palette's Load Brushes command to load the Assorted Brushes set, you can access a whole range of different effects. Using the hollow circle dab, for example, reveals the step-based nature of all strokes by producing a coiled spring effect. Alternatively you can paint with various crude bristle effects and even aeroplanes, ducks and deer. By default the dabs overlap by 75% so the actual shape is unrecognisably smeared but, by double-clicking on the dab icon and changing the spacing to over 100%, each individual dab is revealed.

Dab-based brushes

In traditional bitmap programs all brush strokes, whether smooth colour or recognisable shapes, are built up as a series of evenly spaced dabs.

The difference is huge. Now, rather than the brush laying down apparently smooth strokes of colour, you are suddenly made aware of the dab shape. This breaks completely new creative ground with the ability to paint dotted lines, for example, or to create a border built up of a repeating series of a simple shape. That's as far as Photoshop takes things, but other developers were quick to appreciate the potential. Rather than laying down a single, simple grayscale dab, why not lay down multiple, photo-realistic full-colour images? That's exactly what Photoshop's many rivals have come up with in their various "image hose", "picture tube" and "image spray" implementations. In each case the tool works by "loading" a series of themed images and then laying these sequentially or randomly along the path of the brush stroke.

In each case the series of images is stored in a single "nozzle" or "tube" which is actually just a standard bitmap file divided into a regular grid. The simple nature of the nozzle is a huge advantage as it means that you can edit the series of images and, even better, create your own. To do this in Paint Shop Pro, for example, you first specify the size of your cells by setting up a pixel-defined grid based on the largest image in your series. Next you create a new file with a transparent background where the horizontal and vertical size is an integer multiple of your cell size. For example, if you set your grid spacing to 100 pixels, you could create a nozzle image of 400 x 300 pixels which would contain twelve cells (four across and three down). You then place one image in each cell. Finally you use the Export to Picture Tube command and enter the number of cells you have created horizontally and vertically.

Tube-based image hose

Image hoses are built on the use of grid-based image nozzles. This means you can create your own effects but in most cases the results are neither controllable nor random.

In effect the image hose allows you to build up your image from any other existing image. The flexibility and creative potential are obvious and come as something of a revelation compared to the simple laying down of colour of the traditional dab-based brush. With a fish nozzle, for example, you can fill an aquarium in seconds while with different sizes of tree you can create a forest with a few strokes of your brush. And of course to create the trees in the first place you could always use another leaf-based nozzle. The efficiency, flexibility and creativity are clear but, perhaps most important of all, this power is made available to the complete non-artist just as much as to the budding Rembrandt.

After the initial excitement though the basic image hose soon begins to reveal its limitations. To begin with there is the fact that, after you have set up the series of images, you really lose all control over the effect apart from being able to scale it before you begin painting. Paint Shop Pro allows you to choose whether the next image is chosen sequentially or randomly and whether it is regularly or irregularly spaced, for example, but otherwise the end results come as a complete surprise. That's not much use if you are creating an insect border, for example, where you need to be able to control which way the insects are pointing as they walk around the image. Unfortunately the crude image hoses aren't really unpredictable enough either. If you are trying to create a stony pebble background, for example, you want each pebble to be subtly different to the next in terms of its size, placement, angle and colour. Unless you go to the trouble of ensuring your image tube is infinitely long and varied, however, the results will always look too regular to be convincing.

Suddenly, rather than supremely flexible, the image hose looks caught between being too random and too regimented; useful only for the occasional cheap effect. For many programs' implementations that's a fair assessment, but it's not actually an inherent problem. What's needed is a much more advanced image hose engine such as the one provided by MetaCreations Painter. The central advance this offers is the ability to interactively control the size, placement, angle and even colour of the next image to be laid down. This is all achieved with the Brush Controls palette's Expressions settings where you can set the size, angle, jitter (degree of randomness) and grain (percentage of current background colour that is mixed into the image) to be tied in to user-controllable factors such as direction, angle, velocity and pressure.

Image hose

Painter's interactively controllable image hose enables advanced images to be created with a couple of strokes.

The difference this makes in practice is enormous. By setting the nozzle size to be determined by pressure, for instance, the harder you press the bigger the image becomes - assuming of course that you have a stylus rather than a mouse. Just as usefully, by setting the angle at which the image is placed to be determined by the direction you move the cursor, you can immediately create borders and strokes where the images you add follow the direction of your brush. Alternatively, by adding in jitter and setting other parameters to randomize, you can ensure truly unpredictable results when you want them. To make life a little easier, MetaCreations even offers the twenty most useful combinations such as "Linear Size/P(ressure) Angle/D(irection)" or "Spray Size/R(andom) Angle/R(andom)" as immediately accessible brush variations.

Painter's interactive engine immediately tames the image hose and turns it from an occasional trick into a seriously useful tool. However there is still one factor that is not yet under control. While the next image's size, angle, placement and even colour can all be user-determined, the image itself can't as this is chosen either sequentially or randomly. Not surprisingly Painter has a solution for this too: indexing. By ensuring that there is some sort of progression in the images stored in the nozzle file you can then tie the image sequence to a user-controllable input such as pressure. If your image hose is of an increasingly smiley face, for example, the harder you press the happier the results.

In fact Painter offers even more control than this by allowing you to create up to three series or ranks in the same image file. By setting each series to a different input you can then paint with a sequence where the individual image depends on a combination of pressure and angle, for example, but where the colouring is randomly chosen. Painter even offers an alternative option of tying the image chosen to the current underlying source. By first setting up a simple grayscale base image, the image hose can then be used to lay down images based on the underlying luminance.

Indexed nozzles

By indexing nozzle files you can gain control over the individual images that the image hose lays down.

It's clear that the Painter image hose has come a long way from the cheap and cheerful offerings of the budget bitmap editing brigade. A nice demonstration of the tool's power is the ability to create a photo-realistic panorama of a rocky mountain with picturesque Greek village, wispy clouds, inviting forest and foreground flowers, all produced with a few strokes of the supplied nozzles. That's only the beginning, however, as with the ability to both create your own nozzles and precisely control their use, you're really only limited by your imagination. As such, for many jobs, Painter's image hose proves just as useful as its natural media brushes. In fact in many ways the two tools can be seen as natural complements with the various dab-based brushes used for laying down continuous strokes of colour and the image hose coming into its own for laying down individual objects.

With two such advanced brush engines you might think that MetaCreations would be happy to rest on its laurels. Instead, with Painter 6, the company has chosen to go back to the drawing board and effectively reinvent the brush from scratch. In particular it has come up with a new category of "rendered" brush whose strokes aren't built on the illusion of overlapping dabs at all, but rather are directly computed on the fly. The result is strokes that don't just look smooth, they are smooth. You can't move the brush fast enough to reveal the dabs as they just aren't there.

Smoother strokes are undoubtedly an advance but, as no-one was complaining before, there clearly have to be other benefits. There are. If your brush Dab Type is set to either "line airbrush", "projected" or "rendered" in the General settings of the Brush Controls palette, you'll find that a new "Source" drop-down list appears. By default this is set to "colour" but, depending on your brush type, you'll find up to five new alternatives. The first two allow you to paint with a "gradient" or "repeated gradient" enabling you to vary the colour across the width or length of your strokes to produce anything from subtle colour variation through to amazing rainbow effects. The experience is eye-opening and liberating. Suddenly it seems absurd that you should have been limited to painting with a single colour for all these years.

The other source choices are even more impressive. Using the "pattern" source option allows you to paint on your image with any bitmap pattern. In many ways the effect is more like wall-papering than painting and Painter even offers a Victorian wallpaper pattern option along with paisley, celtic knot, checkers and so on. Even better than the supplied options is the ability to create your own patterns. All you need to do is select any rectangular area of your image that you want to be your "paint" and then use the Art Materials palette's Pattern section's Capture Pattern command to create a repeating tile. If you're one of those people still trying to buy a pot of tartan paint, here's your solution.

Pattern brushes

The ability to paint with gradients and bitmap patterns rather than just uniform colours is a huge creative advance.

Using Painter's new pattern pens might work like wallpapering to an extent, but it's important to realise that you aren't limited to laying down uniform strips of the pattern, you really are painting. In particular, using the Expression settings, you can set the brush's opacity, resaturation, bleed and size to respond to pressure, direction and so on. There isn't an option to control the pattern's angle but that's because this happens automatically so that as you turn your brush the pattern turns too. The creative freedom this offers is quite magical in practice and it's worth thinking of how the effect is being achieved. Clearly Painter must be stripping each bitmap pattern down to successive pixel-wide raster stripes that it then lays down according to a width, scale and angle determined by the velocity, pressure and directional information from the pointing device. The system isn't perfect and doesn't cope brilliantly if you move the cursor too slowly or at sharp angles but, if you keep the brush moving reasonably quickly and smoothly, the results are stunning.

The one disappointment is that, unlike the image hose, the pattern brushes don't offer a grain option so that there's no way to vary the colours of the pattern on the fly. There are workarounds however. To begin with, you can always edit the bitmap using the Check Out Pattern command and then save the changes back to your library. Alternatively you can set the source of your computed brush to "pattern with opacity". Now when you paint with the brush the stroke's pattern is rendered in tints of the current colour ranging from the pattern's areas of white which appear transparent through to areas of black which come through as solid. You can also choose further methods and subcategories for your brush so that, for example, if you pick the "grainy hard cover" variant you end up painting with a brush which combines the current pattern, colour and paper texture!

The ability to interactively paint with gradients and patterns is impressive enough, but the real show-stopper is Painter 6's fifth source option "pattern with mask". If you select this option and then choose a recognisable bitmap object complete with transparency mask as your pattern, such as the supplied picket fence or chain options, you will be amazed as soon as you start painting. In some ways, because the results are photo-realistic and instantly recognisable, the effect is like painting with the image hose, but the huge difference is that you are creating a single connected image along your entire stroke. If you take the bramble option, for example, you can create a completely photo-realistic border from a single bramble stem snaking around your image. Even more impressive is the fact that the effect seamlessly changes size in response to your pressure on the stylus. It's a bit like squeezing out toothpaste but much more creative and a lot more fun - and the end results are considerably more striking.

Pattern brushes

Painter 6's new pattern with mask brushes offer the amazing ability to interactively paint with photo-realistic objects such as brambles, thorns and picket fences.

All in all it's clear that Painter's new computed brushes mark a major breakthrough in creativity. In many ways the new computed stroke engine can be seen as the natural third complement to the two existing engines. Whereas the dab-based brushes lay down smooth colour and the image hose lays down discrete bitmaps, the computed brushes lay down smooth bitmaps. In fact it could be argued that it actually makes more sense to see the new computed brush stroke engine as the integrating culmination of - and perhaps natural successor to - the previous dab-based brushes and image hose. After all, as we've seen, all the brushes in fact work by spreading a bitmap effect along a vector-defined path it's just that the computed brushes take this to its logical conclusion.

Having said this, it's important not to get carried away. I don't suppose that Photoshop for one is going to throw away its dab-based brushes quite yet - after all it still hasn't embraced the image hose. In the longer term, however, no bitmap program can afford to ignore the huge leap in creativity and flexibility offered by rendered brushes and where Painter leads others will follow. If so, bitmap editing in general is about to be taken onto a new level.

Tom Arah

December 1999

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