Using a sharpening workflow in Photoshop CS2
Table of Contents
Edge masking
Edge masks are an indispensable tool for both sharpening and noise reduction.
When sharpening, we use an edge mask to concentrate the effects of the sharpen on the edges, so that flat areas such as skies, and textured areas such as skin tones, don’t get oversharpened. For noise reduction, we use the same kind of mask, but inverted, so that the edges are protected from the noise reduction.
Figure 2 shows the steps for building an edge mask. The first step is to create a channel that has good contrast between the edges and the nonedges.
Sometimes one of the existing color channels will work—simply duplicate the channel to serve as the basis for the edge mask—but often you can achieve better results by using Channel Mixer or Calculations to create the channel.
Figure 2: Building an edge mask. Add a new channel, either by duplicating an existing color channel, or by using the channel mixer to create a grayscale version of the image. Then run the Find Edges filter to isolate the edges.
The raw image
The new channel
The new channel after Find Edges
A Gaussian Blur softens the transitions and blurs the noise.
Inverting the image creates white edges where we want sharpening.
A Curves move controls the contract between edges and nonedges.
Once you have a grayscale version of the image, run the Find Edges filter to locate the edges, then use a combination of blurring and contrast adjustments to control the relationship of the edges and non-edges. Once you’ve created the edge mask, you can load it as a selection through which you apply the sharpening, or you can add it to the sharpening layer as a layer mask. Each approach has advantages and disadvantages.
Edge mask as selection. To load the edge mask as a selection, Command-click on the channel’s tile in the Channels palette. We suggest hiding the selection’s marching ants (Command-H). Then, with the sharpening layer targeted, you can run Unsharp Mask. The white areas in the edge mask get fully sharpened, the black areas are fully protected from sharpening, and the gray areas receive sharpening proportional to the gray value. The disadvantage is that you have no control over the transition between sharpened and unsharpened areas once you’ve applied the sharpening. Edge mask as layer mask. Instead of sharpening through the mask as a selection, you can sharpen the layer globally, then add the edge mask as a layer mask: Load the edge mask as a selection, target the sharpening layer, and then click the Add Layer Mask icon in the layers palette.
Once you’ve added the layer mask, you can tweak the contrast of the layer mask with Levels or Curves to fine-tune the relationship between the sharpened and unsharpened areas. The downside to using the edge mask as a layer mask (rather than just sharpening the selection) is simply that it creates a larger file. Figure 3 shows the steps for applying the edge mask as a selection, or as a layer mask.
Edge mask for noise reduction. You can use approximately the same edge-masking technique to apply noise reduction instead of sharpening. Invert the mask (or omit the inverting step when creating the mask), leaving the edges black (so that they’re protected from the noise reduction), and the non-edges white (so that they receive the full benefit of noise reduction). It’s usually a good idea to use a slightly different blur, as well as different contrast, on the noise mask than on the edge mask—if you simply invert them you can exaggerate the transition between the edges and nonedges in both the noise reduction and sharpening layers. Making the masks slightly different helps a great deal.
Figure 3: Applying the edge mask
Command-click on the edge mask channel’s tile to apply the edge mask as a selection.
To apply the edge mask as a layer mask, first load it as a selection, then target the sharpening layer, and click the Add Layer Mask icon.
We find that the Despeckle filter does a great job of minimizing film grain and digital noise, but we generally apply it separately to each color channel, because typically one channel will need more applications than another. With film or print scans, the blue channel is almost invariably the noisiest, so we may run Despeckle once on the red channel, twice on the green, and three or more times on the blue. On digital images captured as JPEG, we look at each channel to determine where the noise lies, and Despeckle accordingly. With digital raw images, unless they’re extremely noisy, we rely on Camera Raw’s noise reduction features.
If the noise is primarily in luminosity, as it is with transparency film, we prefer to carry out noise reduction on a layer set to Luminosity blending. Using separate layers for sharpening and noise reduction offers more control, but at the cost of a larger file size.
We also use masks with the new Reduce Noise filter, especially on higher-resolution film scans, where the filter seems to want to preserve the grain as well as the detail. The ability to edit the layer mask adds a level of post-filtering control that we often find useful.
