Contrary to popular opinion, not everyone records only games, audio, video and data onto CD-R. Some people put photos on as well. Since Kodak's proprietary PhotoCD format for discs is no longer available (but see the section on formats for word on their files), the following is generic for contnuous-tone images, not specific to CD recording. Still, it seems worth a page of exposition. Be warned, though: there's a fair amount of elementary arithmetic to follow.
We are concerned here with printing from the computer, not by optics onto photosensitive paper. Unless you use a dye-sublimation printer (where dots and pixels are equivalent), we must begin with some treatment of pixels. A pixel here is an array of printer dots which represents a shade of a color. Since conventional printers only produce one 'shade' of dot in a given color, in order to have varying shades we must have a different number of dots considered as a group. That group is termed a 'pixel'. A pixel of 8x8 dots offers 65 shades of color (0 through 64 dots printed in the block). The extra dot is usually ignored so we speak of 64 shades for an 8x8 pixel. The original image probably has an eight-bit byte to represent that color, which means that there are 256 values available; to represent all of them, one would need a 16x16-dot pixel.
The resolution of a printer is specified in dots per inch. The manufacturers fudge a little sometimes, so a "1400-dpi" printer may only deliver that resolution on special paper with a particular sort of test pattern - but that's another story altogether. Suppose we have a true, 600-dpi printer. If its output is broken into 16x16-dot pixels, there are 37.5 pixels per inch - 600/16. That's about twice as coarse as a newspaper photo, four times worse than a good magazine's output. In short, it's too coarse to be acceptable. For a quality image, effective resolution of 150 pixels per inch (ppi) is a good target.
Photographic resolution is a somewhat different story. Photo resolution is measured in lpm: line pairs per millimeter. There are two dots in a line pair and 25.4 millimeters to the inch, so good film in a good camera can deliver 100 lpm; in computer terms, 5000 dpi! (Now you know why film scanners have such high resolution.) Incidentally, a photographic print is usually considered sharp at 7 lpm or 350 dpi. No doubt, that was a factor when laser printers came out for text (no gray scale needed) at 300 dpi. Note that a dot in photography, as in dye-sub printers, does convey shading. As a result, that 7-lpm, 350-dpi print corresponds to something like 3000 dpi in an ink-jet or laser printer.
Digital cameras make a useful reference point and it may be worth using their resolution - in megapixels (MP) with three colors each - as a reference. Here's a short table of values relating those continuous-tone pixels to performance of films, printers, computer screens and television sets.
| Application | MP |
| 35-mm frame @ 100 lpm | 35 |
| 8x10 photo print @ 7 lpm | 10 |
| 8x10 dye-sub print @ 300 dpi | 7.5 |
| 8x10 print @ 150 ppi | 1.8 |
| 4x5 print @ 150 ppi | 0.45 |
| 640x480 computer screen | 0.3 |
| 320x240 VHS playback | 0.075 |
There are two broad classes of bitmap image format: lossy and lossless. The common lossless formats include PCX, TIFF and GIF, any of which is quite usable for image storage and retrieval. GIF is usually used for images with only 256 colors, not enough for continuous-tone quality; if the 24-bit version is considered, the same rules apply as for PCX and TIFF.
In a continuous-tone image such as a typical photograph, lossless compression has little payoff. Since an uncompressed TIFF is probably the most universally recognized format, its requirement of three bytes (one per color) per pixel is a good reference point. As a result, it will take more than 100 MB to store all the information in a 35-mm film frame. Even a good 8x10 print at 150 dpi - magazine quality - is over 5 MB. Ouch!
There are now competing schemes for compression of continuous-tone images. By far the most common is JPEG. JPEG compression is quite similar to that of MP3: detail is sacrificed in ways least noticeable to the viewer in order to save storage space. The amount of compression is controllable in software. The more you save, the greater the chance of visible artifacts. The artifacts show up as pixellation of colors - regions of gradual shading turn into blocks of solid colors - and ragged diagonals. Most users seek about 10:1 compression as their compromise, but you should play with your own graphics program to see what its controls offer in price and payoff.
In general, a single compression can be chosen so its effects are acceptable. However, a second compression, even with the same settings, will result in serious image degradation. If you are delivering finished images which will need no further processing, JPEG is a good choice of format. If you are storing the images for editing later on, uncompressed files are far better. If that sounds familiar to those who have done (lossy) MP3 compression, it should.
The Kodak PhotoCD format is a modified form of JPEG with very similar properties. With appropriate software, the image can be retrieved in any of a number of sizes - a minor convenience when resizing in an inexpensive image editor is so convenient. (It made a big difference when 8 MB was a lot of RAM.) The drawbacks to that format are many: access often requires specialized software; the format is not easily displayed and is not suitable for the WWW; the file size is greater than that of a JPEG for the same maximum dimensions and compression. In short, unless you can write the nearly obsolete format for PhotoCD set-top boxes and use one, this file format has no advantage.
Here, the options are yours and there is no 'best' answer. On a PC, you may want to create an index with ThumbsPlus or a similar tool, though the ThumbsPlus CD writer is prohibitively expensive for personal use. Adaptec's Easy CD Creator Deluxe includes PhotoRelay with useful capabilities not only for slide shows but for video and audio as well. WinOnCD will let you write a VCD with up to 99 still photos. You can also write your own HTML for cross-platform capability.
Before you start your imaging project, clarify for yourself what the purposes will be. If you just want to have the pictures display well on the computer's screen, a 20KB JPEG will do for each and a standard CD-R will hold more than thirty thousand of them. In contrast, you'll have room for only half a dozen full-quality 35-mm frames on such a disc.
E-mail me at cdrecording@mrichter.com
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