ImgEdit

Image Editing

THE DIGITAL CORNER

Last month I wrote about acquiring digital images. So once you've got an image into a computer, what do you do with it?

There are quite a few photo editing software packages available, ranging widely in cost and usefulness. In fact, there are several relatively inexpensive programs that work quite well. But there is only one that is widely accepted as the leader against which all are measured: Adobe's Photoshop. Therefore, I will limit this discussion to Photoshop, although much of it applies to other programs.

Photoshop is available for both Windows and Mac platforms and works almost identically on both. Beyond the usual system requirements Photoshop needs two things: A good display system and lots of memory. You can use Photoshop with a typical 640 x 480, 256 color display, but image quality will be a major hindrance to producing good images. I would recommend a minimum of 1024 x 768 and "true color" (16 million colors, or 24 bits per pixel). For modern computers such display system are readily available, but a good monitor (17 inch or better) may cost a few hundred dollars more than you would otherwise spend. As for memory, the rule of thumb is to have enough available memory (excluding that used by the operating system and Photoshop itself) to store three copies of the typical image size with which you will be working. So how much is that? Well, it depends on your choice of input and output devices. If you are going for the highest possible resolution, a single image can be hundreds of millions of bytes, perhaps 75 MB for a 35mm original. More typically, resolution is limited by the output device. For example, most full color (continuous tone) printers have a resolution of about 300 dots per inch (DPI). Multiplying this number by desired print dimensions gives you the number of pixels. Multiply this number by three (each pixel requires 3 bytes of storage) to get the number of bytes for one image. Its a lot. It will work with less memory by using "virtual memory", which means trading secondary storage (hard disk space) for main memory. Using virtual memory slows things down considerably, but is often the only practical way to go, even though memory prices have dropped tremendously in recent years [all still true!].

Once you get the system set up, here's a very small sampling of the things you can do to an image in Photoshop:

* Make global adjustments to brightness, contrast, saturation, or hue. Adjustment can be guided

by immediate preview or by graphical or numeric analysis.

* Make such adjustments to limited areas selected by various techniques including manual

outlining, automatic boundary selection, or color range selection.

* "Spot" the image by copying adjacent pixels over the bad pixels (roughly 1000 times easier

and faster than non-digital spotting).

* Remove scratches (more or less) automatically using the "dust and scratch" filter.

* Crop the image and adjust the size up or down.

* Eliminate unwanted parts of the image by covering them with image data from another

part of the image, or from a different image.

* Intensify the main subject by blurring the background or other distracting elements.

* Combine graphics with the photo, such as a title or border.

* Apply various filters to soften, sharpen, or apply special effects.

* Apply lighting effects (works well on flat backgrounds)

This much power does not come without cost. Photoshop is quite difficult to learn. (It's also not cheap, with a typical retail price over $500. If you're buying a good scanner there's a good chance you can get Photoshop bundled with it for a lot less than the retail price.) There are many good books available, but I think they can have a negative affect on learning Photoshop because they try to teach you everything while you will probably use only a small subset of features. Also, the typical Photoshop user is not a photographer but a graphic artist or designer. The User Guide and tutorials included with Photoshop should get you going. Then you can use reference material to answer specific questions as they come up.

I view using Photoshop as analogous to the printing process. I adjust the image dynamics and color (either to be "correct" or for a desired affect), crop, and spot. There are some weaknesses, to be sure. Most importantly, unless you have outrageous amounts of memory and/or the patience of a saint, resolution is limited much more than it is with film and paper. Dynamic range is a trickier subject. Although some scanners will provide as much as 36 bits per pixel (approximately a 3.6 density range), Photoshop is limited (for the most part) to 24 bits per pixel (2.4 density range). (You might think from the preceding that density = bits per pixel divided by ten. It's not, that's just a coincidence.) 2.4 is a quite adequate range for prints, but it doesn't leave much "headroom", or margin for error. This means that anything more than the most subtle adjustments to brightness, contrast, etc. must be done during scanning. One exception to this rule is that, theoretically, PhotoCDs provide a somewhat larger dynamic range which you can "extract" to 24 bits as you read the CD.

Update: Photoshop 5.0 supports up to 48 bits per pixel for most operations. Unfortunately, most scanners come with older software that still provides only 24 bits, but this will change [I was right - 48-bit color is now available from many scanners].

On the plus side, many of the common tasks in printing are much easier to do digitally, and can be repeated with absolute perfection. Personally, I like the fact that I don't have to work in a dark, smelly, cramped room to produce a print. Also, I can work just about any time and leave my work right in the middle of things.

But the biggest problem in the whole process is that output devices (i.e. color printers) are technologically far behind the rest of the tools. This means that an output device that matches the quality level I want is far too expensive to own. So while I can get instant "preview" of changes as I make them, I have to take my final image file to someone else to make the actual print, and I lose control at that point. That brings us right up to the topic for next month: Image Output.

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