Filter Tips: Handling Photoshop Filters

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Making the most of your Photoshop filters

Tom Arah helps you get the most out of Photoshop's filters.

Two months ago we looked at the huge power and creativity that the use of bitmap filters can open up, from fundamental blurring and sharpening through to advanced artistic effects. With dozens of filters to choose from, the potential is undeniable but making the most of it is another matter - and one that we're going to explore this month.

To begin with, of course you have to make the most of the power offered within each filter. This is easier said than done as the sheer range of creative options on offer means that no-one can intimately know the effect of every parameter in every filter. More to the point, while most parameters are provided as gauges, because of the underlying and often unpredictable interaction of different settings, there's often no obvious connection between increasing or decreasing a parameter and the effect it produces.

Filter effects often depend on the unpredictable interaction of multiple parameters.

In other words, the best way to get to grips with each filter is to experiment. A very useful tip in this regard for Photoshop and Elements users is to hold down the Alt key when dragging on a parameter's gauge as this updates the small preview dialog in real time. Pressing Alt also has turns the Cancel command into a Reset command which restores all parameters to the last applied settings. If the filter allows it, it's desirable to preview the effect on your image as a whole. For screen work it's important to set your view to 100% to see how your actual pixels are affected, while for print work it's also useful to see how the effect will work to scale by switching to Print View (Photoshop lets you swap views even while the filter dialog is open).

Most filters are capable of producing vastly different effects each of which might be desirable in different circumstances. More advanced filters recognize this and provide presets which are simply stored parameter settings. Even more useful would be the ability to save and load your own commonly used settings. In fact with Photoshop you can do just this thanks to the ability to record filter settings as part of an action using the Action palette. You could, for instance, set up separate Unsharp Mask actions with different diameter and threshold settings for images destined for screen and print or set up Chalk & Charcoal artistic effects with different colours and weightings.

The reason that it's crucial that you get the effect exactly as you want it is because all Photoshop's filters are "destructive" which means that, once applied, the original input pixel values are lost. In other words, unlike changes made with adjustment layers or layer styles, you can't recall the dialog at some later point and retrospectively fine-tune the effect. As such, if the effect on the image as a whole isn't exactly what you expected or wanted, hit Ctrl+Z to undo the filter and then call up the dialog again to have another go with the Alt+Ctrl+F shortcut.

In fact both Photoshop and Elements offer slightly more flexibility than this via the undo History palette which lets you return to an earlier image state. With Photoshop you also have the option of storing an image state with the palette's Snapshot command. This is particularly useful with filters as it means that you can store variations of an effect as snapshots and then view them quickly in turn to select your favourite. You can even create an action to automatically apply different filter settings and store the results as selectable snapshots.

Photoshop's history states and actions are useful for finding the best filter and settings.

Ideally, using these tips, you will be able to get exactly the results that you want, but usually you will still want to adjust the end effect. Sometimes the effect will appear too weak and your best option is simply to re-apply the filter (shortcut Ctrl+F), say to boost sharpening. More commonly, the effect will be too striking and you'll want to tone it down. You can do this in Photoshop using the Edit>Fade command (shortcut Shift+Ctrl+F) which effectively lets you merge the before and after image states.

Essentially the Fade command is a filter in its own right using each pixel's before and after values as its raw materials. Set the Fade value to 50%, say, and the pixel value is calculated by splitting the difference between the original and filtered values. While the most common use of the Fade command is simply to tone down the filtered effect, it can do much more thanks to the use of blend modes. By setting the blend mode to Darken, the filtered pixel value will only be used if it is darker than the original value. Set the blend mode to Difference and the final colour value is set as the difference between the original and filtered values producing very unpredictable and eye-catching results.

This mixing of original and filtered image states is great for producing both subtle and striking effects and in many ways the Fade command should be seen as the most useful of all filters. However its major drawback, as with every other filter, is that its effects are again destructive so that you can't retrospectively fine-tune your filter's strength. However you can get around this - and even provide similar capabilities within editors, such as Photoshop Elements, which don't offer a Fade command - by the intelligent use of layers.

All you need to do is to select your original image, copy it and then paste it as a new layer (Ctrl+A, Ctrl+C, Ctrl+V) and then apply your filter effect to this new layer. To then fade the effect you can change the layer's opacity, while to create blend-based effects you can select from the full range of layer blend modes. Crucially, this approach means that you can change opacity and blend mode at any time not just immediately after applying the effect. And, because you've kept a copy of the original pixel values, you can always delete the filtered layer and start the whole process again. It's certainly not the non-destructive flexibility of a layer style or adjustment layer, but it's a definite step forward.

By controlling layer opacity and blend mode you can take non-destructive control of the Fade command.

So far when talking about filters I've assumed that we've been applying them globally to the entire image but often that's not the effect that you want, say if you want to sharpen particular features, motion blur a stationary object or apply a canvas effect to just the white areas in an image. In fact controlling where filters are applied is very simple - all you need to do is make a selection first using any of Photoshop/Element's selection tools or commands.

If you're using Photoshop rather than Elements, as well as applying filters to selections, you can also apply them to individual channels by first selecting one in the Channels palette. This can be used for subtle effects, say applying a sharpen filter just to the colour channel with the highest contrast, or for striking effects, applying a coloured pencil effect just to an image's blue channel.

You can also apply filters based on Photoshop's user-defined alpha channels or masks. This is particularly powerful as you can control the effect of the filter just as we did with the Fade command but this time intelligently vary its strength across the image. By creating and selecting a radial gradient alpha channel before applying a Gaussian Blur, for example, you can produce depth of field effects that draw attention to the focus of your image. Do the same and apply any Brush Strokes filter and you can produce the effect of a photograph seamlessly turning into a work of art.

Using selections and masks with your filters offers more creative control, but the need to know exactly what effect you want to produce and set it up in advance is a serious block to creativity. An alternative approach within Photoshop is to turn to the History palette to take before and after snapshots of the original and filtered image and then to select the original snapshot. Using the History brush (shortcut Y) you can then target the filtered snapshot in the History palette by selecting the source check-box next to it and then paint with the filtered colour values onto your original image. You can even change the History Brush's opacity and blend mode to take further control, or target the original snapshot to undo changes. Effectively you can interactively paint onto your image with any variation of over a hundred totally different filters!

In many ways the History Brush is Photoshop's most powerful and adaptable tool but it still has a drawback: when you close the image, all snapshots are lost and any changes are made permanent. Alternatively, you can maintain maximum non-destructive flexibility by again turning to layers. First you need to store your before and after image states as separate layers and then add a layer mask to the overlying filtered layer with Photoshop's Layer > Add Layer Mask > Reveal All command. When the layer mask is selected as the current target, painting with a solid black brush reveals the underlying original image layer, while painting with solid white re-applies the filter effect to any original areas that you've revealed. Choosing a gray colour, on the other hand, creates a semi-transparent mask that merges before and after image states. By adding a gradient to the layer mask you can even manage global fade effects and, because you can always change the gradient, this time the mixing effect remains live and editable.

With selections and layer masks you can control the strength of a filter's effect locally.

Through the intelligent use of layers, we've taken total and fully editable control of the mixing of an original image with any given filter - but let's not get carried away. Effectively we have managed to make the Fade command non-destructive but there's a major limitation that immediately becomes apparent if we want to apply a second filter. Filters can't be applied like an adjustment layer to all underlying layers, and so our only option is to flatten our layers which of course makes all changes permanent and immediately loses the flexibility and editability we've laboriously built up.

This isn't simply a theoretical point. One of biggest strengths of filters comes through their combination. Before applying a Find Edges filter, for example, it's often useful to apply a Colour Reduction filter while, if you want to produce a brushed metal fill, the best way to do it is to apply some Noise and then apply a Motion Blur and so on. If you thought that filters on their own provided enormous creativity, when used in combination the results that they can produce becomes virtually infinite.

It's worth thinking about this in a bit more detail. Effectively each filter works by taking original input pixels and then applying a mathematical operation to produce a new set of values. In more advanced filters, such as some of the artistic effects, these new values are then in turn used as the input values for further operations so that the final filter effect is effectively the result of a stack of serial operations. And, crucially, there's no reason why this stack shouldn't actually straddle multiple separate filters in which the separate filters themselves act like the parameters of a larger overall effect.

But there is a problem when it comes to controlling the final result of these combined filters. As we've seen, it's difficult enough to predict and control the interaction of parameters within a single dialog so how can you experiment to get the best result from the combination of different settings when the parameters are split between different filters? With adjustment layers and layer styles this isn't a problem. If you're combining Levels and Hue/Saturation adjustments for example, you can simply fine-tune each in turn while Photoshop's Layer Style commands let you independently manage multiple interacting styling effects from a single central dialog.

What we need is some way to keep the stack of filter operations live so that we can experiment with different settings to produce the best results possible. And this is exactly what the UK company Fo2Pix promises to provide with its artistic buZZ.Pro 2 plug-in filter (www.fo2pix.co.uk). In fact buZZ.Pro is actually a collection of 19 filters for applying colour reduction, edge detection, blurs and so on. What makes buZZ.Pro different is that you don't apply the effects individually but rather combine them in an ongoing stack. To produce its Oil effect, for example, the stack is built up of colour edge detection, emboss and two simplifier effects. At any time you can reorder your stack, add or delete steps or fine-tune each filter, say to cut down on the number of colour areas in the image, each of which will produce completely different end results.

buzz.Pro shows the benefits of non-destructively combining effects.

You can save stacks that work well for future use and buZZ.Pro comes with a selection of 13 presets ranging from Bright Colour Wash through to Watercolour. buZZ.Pro certainly works as a proof of principle and shows the huge creative advantages that flow from being able to combine multiple live effects but it has a number of intrinsic and unavoidable limitations. To begin with, it can be difficult to judge an effect by the tiny preview. More to the point, while the range of filters is reasonable, there's no way that Fo2Pix can ever hope to match the full breadth and depth of Photoshop's existing filters. And, most telling of all, while each individual filter remains live within buZZ.Pro, the overall effect is still destructive - once applied, there's no going back for future fine-tuning.

Clearly what we need is a more general solution that answers all of these criticisms. And the answer is staring us in the face - why can't we simply apply the existing Photoshop filters in a non-destructive way either as adjustment layers which would affect all underlying layers or as layer styles which would affect just the pixel information on the layer that they were applied to? After all, exactly this sort of bitmap-processing power is available in another Adobe graphics application - the vector-based Illustrator!

One of Illustrator's big strengths is that it provides the vast majority of Photoshop's filter collection from its Filters menu, ready to be applied to any imported bitmap. Much more powerful is the fact that the same filters can also be applied from the Effects menu. At first this duplication might seem strange but applying a filter as an effect has a number of advantages. The most obvious is that you aren't restricted to applying the filter to bitmaps, you can apply them just as well to vector objects which are automatically rasterised and masked.

What makes Illustrator so special is that this rasterization isn't destructive as it is within Photoshop where you have to permanently render a shape layer to pixels before applying a filter. Instead the effect is live and works like a layer style so that you can still edit your objects by resizing, repositioning, reshaping, reformatting and so on and, when you do, the filter is automatically reapplied. In short you have the best of both worlds: the precision and editability of vectors and the creativity and naturalism of pixels.

And because Illustrator's effects are non-destructive they are also retrospectively editable. All you have to do is select an object which has had a filter effect applied to it and open the Appearance palette (Shift+F6) and you will see the effect listed along with any other formatting options. Double-click on the filter name and its dialog reopens and you can fine-tune your settings. Even better, there's nothing to stop you applying multiple filter effects each of which is listed in the Appearance palette. Reorder the filters or change settings, and the overall effect is recreated accordingly. Effectively you can create buZZ.Pro-style stacks with any combination of any filter - and edit the stack at any time in future!

Illustrator offers truly non-destructive filter effects managed as stacks.

It's exciting creative power but the system isn't perfect. To begin with, Illustrator only allows filters to be applied directly to objects so there's no option to apply effects to all underlying objects or sections of objects as there is with Deneba Canvas's lenses. In other words you can apply filters like layer styles but not like adjustment layers. It's also important to realize that there is inevitably a very serious performance hit involved in keeping filters live especially if you target your effects to produce 300 dpi print work rather than 72 dpi screen work. Hopefully Adobe is listening and will add options to temporarily switch off or lock effects in the next release of Illustrator.

More to the point, I hope that Adobe is working on adding a similar non-destructive approach to managing filters to where it makes most sense - in Photoshop itself. In the meantime the best way to make the most of Photoshop's bitmap filters is to get to grips with the Actions, History and Layers palettes - and Illustrator!

Tom Arah

February 2003



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