Photoshop 4

Although Web graphics are a huge growth area, Photoshop gives the impression that they're unworthy of attention. Instead, the program concentrates firmly on the high-end professional output of printed images. It's here that Photoshop made its reputation, and it's here too that the latest release makes the most impact. Version 3's biggest advance was the introduction of layers - the equivalents of acetate sheets allowing sections of an image to overlay and interact with the image below - which opened up the whole world of photomontage and composition. Version 4 takes the idea further to put layers at the very heart of the photo-editing process.

To begin with, layers are far more integrated into the whole operation of Photoshop. When cutting and pasting from one file to another, for example, rather than creating a floating selection, Photoshop automatically creates a new layer. To begin with, this can be very disconcerting. For example, if you try and save the new file you'll only be offered the choice of Photoshop's own PSD format, since it contains a layer. To save to a TIF file, you'll first have to use the new Merge Down command to combine the layers.

While this takes slightly more effort, the general move away from temporary floating selections to semi-permanent layers is a sensible step. This is even more apparent with the handling of text. Any text added is automatically assigned to a new layer. Again this means that there's no chance of inadvertently losing the floating selection and so making undesired changes to the overall image. It also means that to reposition the text at any time in future you can simply select its layer and use the move tool.

Photoshop 4 also extends the use of layers through an entirely new feature called adjustment layers. These are effectively masks through which an image adjustment is applied. Each adjustment can be seen as a lens that's slipped into the pile of layers, adding its effects to everything below it and so bringing colour correction and effects into the compositing process. Moreover, as each adjustment is a mask, it's possible to limit the effect by painting out portions of the layer. This means that colour and tonal adjustments can be interactively painted onto the image by using any of the tools.

Each adjustment layer is added either with the command under the new Layer menu or by holding down the key when selecting the New icon on the Layers palette. A dialog appears in which the user can specify a name for the layer and choose the type of effect from all of the options such as levels, curves, colour balance, brightness/contrast, hue/saturation, selective colour, invert, threshold and posterise. Clicking on OK then calls up the appropriate dialog where these parameters can be set. When the desired changes have been made, the new adjustment layer will appear in the Layers palette, indicated by a half-filled circle after its name.

One of the beauties of having the adjustment as a layer is that it's possible to control how the effect interacts with the image below. With a posterising adjustment layer, for example, you could change the opacity of the effect to 75 per cent so that some underlying detail still comes through. Alternatively, by setting the effect to only blend with the lighter sections of the underlying image, you could posterise only the highlights of the image. Or by changing the blending mode to dissolve or hard light, for example, you could create completely new effects. Putting everything together, it would be possible to have a text-based mask with a 50 per cent posterising effect that only affected the hue of the mid-tones of the image under the text.