Understanding Bitmap Filters

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Making the most of bitmap filters

Tom Arah helps you understand your applications bitmap filters.

The most common task in bitmap editing is enhancing your photo's existing qualities through colour correction and image adjustment. The most creative option lies in treating your starting image as just that: a jumping off-point to wherever your imagination and your photo editor's capabilities can take you. When you go down this route you're inevitably going to be drawn towards your application's bitmap filters and, whichever application you use, you'll find no shortage of creative power available.

In fact the sheer range of power on offer can be overwhelming. With dozens of effects from Accented Edges through to ZigZag, the big danger is that you'll suffer from filter fatigue before you've even begun. Even worse, the variety within each collection is so huge, ranging from simple one-off commands through to virtual mini-applications, that it's difficult to see what can possibly connect them. After a few minutes' exploration most users are left wondering just what a filter actually is.

The sheer number of filter effects can be bewildering.

To understand this, you have to remember that each image is a bitmap - a grid of pixel values. Bearing this in mind, it's clear that each filter works as a black box taking one set of pixel values and returning another. It's an idea that we're used to from colour-correcting image adjustments where colour values are consistently mapped from source to destination. However you only have to compare, say, a creative artistic watercolour filter effect to a levels-based adjustment to realize that there are also core differences. Each filter might be a black box, but the big question is what happens within the black box?

To get a handle on what makes a filter tick and what makes it different, it's useful to create your own. This is possible using Photoshop's Filter>Other>Custom,  Photo-Paint's>Effects>Custom>User-Defined and Paint Shop Pro's Effects>User-Defined commands. In each case what you are presented with is a customizable matrix centred on the current pixel being processed and into which you enter values. The filter then applies what's known as a "convolution" to the matrix. Effectively all the pixel colour values are multiplied by the values you've entered in the cells and summed and this sum is then used as the output value for the central pixel and this is repeated for each pixel in the image.

Unless you make sure that the sum of the values you enter into the matrix is close to one, the image will quickly turn toward white or black, but within this constraint you can produce some eye-catching results. The Custom filter's convolution effect isn't often useful in practice, but it is useful for what it shows about filters in general. In particular it shows how each pixel is reassigned a value based on the values of surrounding pixels. This is very different to a colour correction adjustment where every pixel is treated independently and every pixel with the same colour values is treated equally wherever it appears. With a filter, context is all.

Convolution effects show the importance of contextual processing.

This contextual processing can immediately be put to good use in two important areas - blurring and sharpening. By comparing a pixel's values to those of its surrounding pixels you can then change its values accordingly. By lowering the contrast between adjacent pixels the image loses sharpness; by increasing it, the image comes into focus. When Blur and Sharpen filters were first introduced they were crude fixed commands, but the filters and their results can be made more flexible and effective by introducing conditional processing so that, for example, a Sharpen Edges filter will only apply its effect if the existing tonal variation is already greater than a given threshold.

The Blur and Sharpen filters (and especially the Gaussian Blur and Unsharp Masking variations) are the most fundamental of filters and, by subtly enhancing the quality already there in your photos, they can prove just as useful as the main colour correction commands. Alternatively, the same underlying principle of identifying and manipulating tonal ranges can be used to produce very different results. By identifying small islands of isolated colour values - often introduced when scanning an image - a Remove Noise or Dust and Scratches filter can then replace them with an averaged value from surrounding pixels. A Find Edges filter, on the other hand, can remove all the colour information that doesn't differ strongly from its neighbours.

Rather than identifying edges and difference, filters can also be used to recognize areas of flat colour and similarity. With a Crystallize or Facet filter, for example, the image is broken up into cells and these are then averaged. Different cell shapes and sizes produce totally different results while introducing a coloured border between cells can produce Pointillist, Mosaic and Stained Glass effects. More advanced filters, such as a Silk Screen effect, are less rigid and more true to the original producing more artistic almost hand-produced results.

It might not seem that significant but breaking the image down into regions for processing like this reveals another important aspect of filters and another difference to colour correction adjustments - each pixel's processing can depend on its position in the image. In fact there are a whole host of filters where this is the major factor. From the simple tile-based displacement Puzzle effects through an almost infinite range of Twist, Pinch, Skew, Punch, Ripple and Twirl distortion effects to the most advanced, interactive Liquify manipulations each of these filters works by mapping original co-ordinates to target co-ordinates. The same is true of another important category of filter, the 3D effects such as Spherize, Cylinder and 3D Transform which work by mapping pseudo-3D manipulations back onto the 2D target bitmap.

Another crucial difference between colour correction enhancements and filters is that the former work purely with the information that is already there in the image. With filters this isn't a restriction so that external pixel information can be brought into the equation. This is most obvious with texture filters that can be used to apply fills based on seamlessly tiling textures to create wood, brick, grass effects and so on. By blending the texture information into the existing pixel values more subtly, surface effect filters can make it seem as if the image is printed or painted onto canvas.

Rather than relying on existing bitmaps, the external pixel values can be generated from scratch. There are a whole host of filters for adding mathematically generated fills, fractals, clouds and advanced gradients. Lighting effects can also be seen as another form of external pixel information that is realistically and seamlessly blended into the existing image by interacting with the existing colour values. And why just add patterns? There are also filters designed to add recognizable objects such as realistic clouds, fireworks, lightning, snow and so on.

Advanced creative filters like these can suddenly bring a dull photo to dynamic life, but the filters that are most impressive in this regard are undoubtedly the artistic filters. There's something profoundly satisfying about turning an uninspiring photo into a beautiful work of art with just a few clicks. But how can a filter possibly achieve results that look as if they've been painted with watercolour or oils or sketched with charcoal or produced with a woodcut?

Art filters are still based on a combination of blind mathematical operations.

In fact of course we've already seen the answer - or rather answers. Each of the art filters works on the same principles that we've already seen - identifying and applying colour blocks and outlines, blurring and sharpening, distorting and displacing, applying textures as paint strokes and other textures as canvas materials and so on. There isn't a ghost - or in this case artist - in the machine. The results of the art filters might be beautiful but ultimately they come down to brute maths in just the same way as the crude convolution effects. Compared to the relatively simple mapping of a colour-correcting image adjustment, an artistic filter involves complex, contextual and conditional processing - but essentially it all boils down to the same thing: number crunching with a purpose

So far we've explored how filters work and along the way seen just what sort of work they can achieve. Now let's look at just what filter-based power each of the main bitmap editors provides.

Inevitably most attention focuses on the market leader Photoshop especially as over its long life it has built up a collection of just under a hundred filters. There's no doubting the range of power but it's difficult to feel on top of the filters as they, and the 13 categories into which they've been organized - Artistic, Blur, Brush Strokes, Distort, Noise, Pixelate, Render, Sharpen, Sketch, Stylize, Texture, Video, Other - have evolved over time and seem fairly random. The artistic effects, for example, have been split into three categories though many would fit just as well under any heading. And I don't suppose that there are many users who can tell what effect the Reticulation or Sumi-e filters will produce before applying them.

When you do apply a filter, you are immediately reminded of just how old and idiosyncratic many of these filters are. The dialogs tend to be small with just two or three mysterious parameters that produce unpredictable results when changed and sometimes the end results are actually dependent on external factors such as your current foreground and background colours. That wouldn't matter if the dialog told you as much, but there's no information on what effects can be achieved or how and there's no direct access to the online help to find out. There's also no Reset button to restore the default settings (though one does appear if you hold down the Alt key!)

The real sign of the filters' age though is the fact that with the odd honourable exception, such as High Pass, there's no way of previewing the effect of the filter on the current image as a whole. Instead each filter has a tiny in-built preview. This has the advantage that its zoom factor is 1:1 so that you immediately see an accurate sample of the final effect, and you can also zoom in and out to get a rough idea of the overall effect. Generally though, trying to judge anything via such a tiny window is ridiculous, especially as today's vastly increased processing power means that many filters could be previewed on the image itself in near real-time.

By comparison the Corel Photo-Paint filters feel like a breath of fresh air. And Photo-Paint can even claim to have the edge on Photoshop with its range of 103 filters - though many of these are separate texture effects that could easily have been consolidated. The effects also tend to be more self-explanatory than the Photoshop collection and better organized into Photo-Paint's twelve categories - 3D Effects, Art Strokes, Blur, Camera, Color Transform, Contour, Creative, Custom, Distort, Noise, Sharpen and Texture.

Corel Photo-Paint provides a more modern solution.

Each filter's dialog also seems much more modern partly because they are based on the normal Windows look-and-feel, but also because more effort has been put into making them transparent and consistent. Other improvements include a direct link to the online help for each dialog, a Reset button and the ability to swap filters (and adjustments) from a fly-out menu without having to close and reopen the dialog. What really makes the difference though is that each dialog offers three ways of working: with a large inbuilt 1:1 preview; with smaller before and after previews; or live on the current image.

By comparison, Paint Shop Pro (PSP) just falls under the hundred mark with its range of filters, but I particularly like their arrangement. Rather than settling for a purely alphabetical approach the range of categories are sensibly divided into core effects - Blur, Sharpen, Edge, Noise, Enhance Photo - followed by creative effects - 3D, Artistic, Geometric, Illumination, Reflection and Texture.

Within each dialog, Paint Shop Pro offers a Reset button and quick access to detailed help and also offers enhanced preview capabilities with in-built Before and After windows, and options to either preview the current settings on the full image or to automatically proof all changes as you make them. It's just a pity that the power is rather let down by PSP's ugly shareware look-and-feel and by a tendency to over-complicate matters.

PSP also offers another welcome innovation in the form of the Effects Preview that lets you see the default settings of each effect applied to a reasonably large preview thumbnail. It's not an accurate reflection of the final result but it does give enough of an idea of the effect to help you decide whether to open the filter. The downside is that each preview is generated one at a time as you work your way through the list so that exploring all 97 filters isn't really an option.

Paint Shop Pro's Effect Browser lets you explore your filter options.

Finally there's PhotoImpact. This offers a smaller range of 67 filters under its Effects menu arranged into ten categories - Blur, Sharpen, Noise, Photographic, Distort, Illumination, Artistic, Video, Creative and Material Effect. Many of the filters are comparatively crude and their dialogs are less than helpful. Open the Watercolour effect, say, and you are presented with nine tiny preview options that are much too small and inaccurate to base any sensible decision on, while clicking on the Advanced option leads to a very rudimentary set of controls. Open another filter such as Painting or Particle, however and the opposite is true. Here you are presented with a bewildering host of presets and parameters - the power is unquestionable but it's difficult to know where to begin to try and produce a particular effect.

If this mixed bag was all that PhotoImpact offered it wouldn't be a contender, but the program has an important trick up its sleeve. Rather than calling up the relevant filter dialog through the Effects menu commands, the most common way that effects are applied is through the many galleries in the EasyPalette. This makes a huge difference. Because all effects are represented by thumbnails you don't have to know what effect, say, the Craquelure filter will have - you can see it. And, unlike with Paint Shop Pro's Effects Preview, you can see all effects within a given category.

To then apply the effect with the default settings all you have to do is double-click on the thumbnail. If you don't like the result you can simply undo it and choose another. If the filter needs fine-tuning, you can right-click on the preview thumbnail and select the Modify Properties command to call up the associated filter dialog. And if you do call up the dialog for fine-tuning you can save your settings as a new thumbnail to be applied in future. In fact PhotoImpact has taken this approach itself to provide literally hundreds of preset effects based on its main filter engines.

PhotoImpact's gallery-based visual approach makes working with filters simple - and fun.

PhotoImpact's visual gallery-based approach is a huge advance. It helps tap the full potential of the filters on offer and, at the same time, brings the huge creativity that they open up under control. It means that PhotoImpact is an excellent choice for the casual user wanting to add creativity to their images while enjoying themselves at the same time. But it's not the right choice for the professional. There's just not the necessary attention to detail and, more importantly, there's just not the speed. Working with screen-sized images or digital photographs this isn't so apparent but, if you're working with multi-megabyte images, things soon slow to a crawl.

Sadly this is just as true of Photo-Paint and Paint Shop Pro. When working with large images you might assume that you can rapidly fine-tune an effect using the dialog's small in-built preview and then apply it to the image itself when you're happy with it. Bizarrely however, with the majority of effects, it seems that the filter settings are still applied to the whole image even if all that is shown is the small in-dialog preview! And as the underlying processing is less than speedy, with larger images this soon becomes untenable. Ultimately it seems that Photoshop is the only viable option. Because it only updates the small filter preview sample, setting up your effect is near instant and then, thanks to the program's unbeatable machine-code optimised pixel processing, you can be confident that the application of your final effect is as fast as possible.

It looks as if we have to choose between power and ease of use - but that's not quite the case. There's one application we haven't looked at yet - Photoshop Elements. Despite its budget price, this offers almost exactly the same range of filters as its bigger brother. In addition it offers its own dedicated floating Filters palette which presents all effects as easily selectable visual thumbnails. You can also set the palette to work in two modes so that double-clicking on a thumbnail either calls up the associated dialog or applies the default settings. It's certainly not perfect - why not double-click to apply and right-click to modify? - but for the moment Photoshop Elements gets my vote for providing the best combination of functionality, usability and value.

Ultimately, all the photo-editing applications still have a long way to go to help their users unlock the full potential of their bitmap filters. Until they do, it's down to us to put in the effort to master them. Thankfully the extraordinary creative power that the use of filters opens up, means that the effort is richly rewarded.

Tom Arah

December 2002

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