These are some of the tricks I've picked up in the last few years of working in digital imaging. Most are tips for Photoshop, but some are probably applicable to other applications as well. Some tips are Mac-specific, sorry about that PC people! Email me if you have any better solutions for the problems I address here or if you have any good advice you'd like to add.

Last updated on the 15th of February, 1996:

Fixes for Moiré

Moiré is the name commonly given to the patterning that often occurs when you scan an image that has been printed using a halftone screen. The pattern results from the differences between the dot frequency in the halftone and the pixel frequency in the scan.There are many different ways in which you can tackle moiré, but all have the net result of averaging the pixel data and thus introducing softness into the image. There is no way around this. A halftoned image has already lost image data which can never be retrieved.

If you are prepared to (or if you have not choice but to) accept the limitations in acquiring images in this way, the following methods may be used to improve the overall visual quality of these images: If you are getting a lot of strong patterning, try scanning at a different resolution. Try matching your scanning resolution to the halftone screen frequency, so if the image has been printed with a 133lpi screen, try scanning at 266 dpi. (note that a lot of scanners interpolate such resolutions very badly (my Powerlook is one) and so using odd resolutions like this can result in worse scans)

• Try scanning at a resolution higher than you need and then sample the image down. Often the resampling process fudges the image data enough to remove the patterning.
• Apply the Despeckle filter (under Noise in the filters menu). This will often yield acceptable results. It has the advantage that it does retain some edge information.
• Apply the Despeckle filter to one or more colour channels. (Often one channel will have more patterning than another)
• Apply the Median filter (under Noise) to one or more channels, specifying just enough radius to remove patterning. Some people use Gaussian Blur instead of Median. I prefer the edginess you get from Median and believe it yields a sharper-looking result.

This is the high-tech approach that I use. If you are using Photoshop 3 you can try it too. First, use the first two tips above to get the best scan you can. Then, make a duplicate of the image layer, apply Median with sufficient radius to remove the moiré, set the medianed layer to either lighten or darken mode (whichever yields the smallest pattern of fine visible dots over the image), duplicate the merged layers into a new document then use either Despeckle or Dust and Scratches to remove the fine dots. Adjust your levels if necessary then follow this with Unsharp Mask with a large (proportional to dpi) radius setting (to avoid sharpening any halftone dot artifacts) and small amount percentage (say 40-50%) and a threshold of 0.

Photoshop performance

For peak Photoshop performance on a Mac (some of this might apply to PCs):

• Buy as much RAM as you can afford. Ideally, you should have RAM allocated to Photoshop that is 3 to 5 times greater (in Mb) than the biggest Photoshop file you'll be working on. So if you'll be working on 40Mb files you'll need (ideally) 200Mb of RAM allocated to Photoshop.
• Forget about RAM Doubler or any of the other RAM management software, they won't help except to wrestle small amounts of RAM off other applications that you might be running at the same time. They won't make Photoshop think that your 8Mb of RAM are really 16Mb.
• Using your disk formatting software, create a partition on your fastest Hard Disk that can be defined as your scratch disk and keep it empty. Again, it'll need to be 3 to 5 times as biggest as the biggest Photoshop file you'll be working on. Switch off Apple's Virtual Memory scheme.
• Make sure that you have at least as much scratch disk space as you have RAM allocated to Photoshop.
• If you can't afford enough real RAM, consider getting a SCSI-2 or 3 card to speed the transfer of data to and from your scratch disk.
• If you have enough RAM, hard disk space etc and you STILL WANT MORE, get a PhotoEngine card or similar. These suckers can really make those filters fly, even on a Power Mac. (BTW, has anyone tried a Daystar Genesis MP yet? I'd be interested in hearing from anyone who has.) If, while you are working you get a "Can't perfom the operation because the secondary scratch disk is full" message you can try:
• Clearing your clipboard and undo buffers (you can do this by making a small selection and choosing copy twice, this clears the Undo buffer too).
• Unfragment Photoshop's memory by creating a new small document and saving it.
• Try running your operation on individual channels separately.

Fabulous Filters

Here is a list of my favourite freeware/shareware filters:

• Rotate Color by Hugh Kawahara. I don't really have any idea what it's doing but I love what it does!
• Create B&W by Hugh Kawahara. Allows you total control over how RGB images are converted to greyscale, specifically what ratio of RG and B is used.
• Kwick Mask by Hugh Kawahara. A turbo charged "Color Range" and Quick Mask in one.
• Retroscan (368k, sit.hqx) by Deep Devices. Creates video effects, like a bad TV signal, with lots of control over the parameters. Fun to incorporate with other effects.
• Sucking Fish filters. (305k, sit.hqx). Range includes a filter (DekoBoko) for creating button effects, handy for multimedia people.
• Synthetic Texture (7k, sit.hqx) by Jeff Mock. Adobe gave this one a lobotomy and called it the Clouds filter. Synthetic Texture gives you control over x and y parameters, plus control over random number generation. It's a lot more fun than the Clouds filter, and much more useful too.

Scans of damaged/textured photos

Photos that are damaged or that are on textured paper are sometimes hard to scan well because the rough surface of the image creates shadows (esp. on my Umax Powerlook, which seems to cast very long shadows). Fortunately there is an answer. The following tip creates a scan that has effectively been lit from two sides. This eliminates a lot of the shadows, and makes retouching a damaged photo much easier.

Carefully scan the image twice, rotating it on the scanner glass by 180° for the second scan. (A good way to get the second scan exactly aligned to the first is to tape the image to a phone book. These are nice and heavy and have exactly parallel sides.) Then, in Photoshop, unrotate the second scan and place it into the first scan as a new layer. Set the transparency of this second layer to 50%. Move the scans until they are exactly aligned. (Use the Offset filter, it's much faster than nudging with the arrow keys) You may need to do some arbitrary rotation as well (provided you've been reasonably accurate, less than ±0.5° should be sufficient). When you're satisfied, flatten the image and you're done!

Correcting for desktop scanner imperfections

If you've used the full scan area of your scanner before, you'll probably realise that the scanner does a better job of scanning some parts of the scanner glass than others. For a better quality full page scan, try this:

First, find a perfectly white and smooth sheet of card as big as the scanable area on your scanner. Scan it 4 or 5 times in grayscale and at the resolution you need for your final image, using the scanner's "auto" settings. In Photoshop, apply Auto Levels to each scan. Composite them all into one document by using one of the scans as the host and adding all the other scans into it as separate layers. Set the transparency of each of the new layers to 50%. Flatten the image. Apply a gaussian blur of sufficient radius to create a soft looking image with even gradations. Invert the image to create a negative of the resulting composite. Do Auto Levels again.

Now scan your picture, using the same selection rectangle in the scanner driver that you used for the card scans. It is important that you are able to align the images precisely, or else the corrections will be out of sync. Once your picture scan is in Photoshop, select Quickmask mode (Q) and copy and paste the composite into it. Exit Quickmask mode (Q), Hide Edges (cmd-H) and adjust the curves (cmd-M) and/or Levels (cmd-L) on the selected areas. Usually, the necessary correction is
to darken the shadows and midtones. When you're satisfied that the image is more evenly balanced, Deselect All (cmd-D) and, if necessary, do a final curve adjustment to correct for any unwanted shifts that have occurred.

Retouching with confidence

Here's a good general-purpose tip for anyone working in Photoshop 3.0. When retouching an image put all your retouching on a new layer so that if you make a mistake you can use the eraser tool to reinstate the original image.

Selective Blurring/Sharpening

Most images have areas that need sharpening as well as areas that could benefit from softening. Make two duplicate layers of your image. Sharpen one and blur the other. Using the eraser tool and a well feathered lasso switch from one to the other erasing the bits of each layer that don't need the effect that layer applies. Adjust your layer opacities and flatten.

Blurring Backgrounds

Rather than painstakingly masking the subject, try loosely selecting the background. Float this as a selection and then make it into a new layer. Apply sufficient Gaussian Blur to emphasise the subject. Add some noise if the background now appears too flat. Any fuzziness from this layer that overlaps the subject may now be erased away with the eraser tool before the image is flattened.

The secret to younger looking skin?

On old portraits, the tones on the face are often disproportionately grainy. To correct this, select the skin areas, excluding the mouth, eyes and any defining lines like those around the nose. Float this selection and apply enough Gaussian Blur to remove the graininess. Add some noise (4 usually works for me) and then adjust the opacity until you attain a good balance between detail and smoothness.

Guy Morton 1995. All Rights Reserved