How to preserve an archive of PCX files

Modern image file formats can display 16 million colours, not just 256 chosen out of 262,144, and they do a far better job of compressing these images than does the feeble RLE (run-length encoding) algorithm employed in the PCX format. As many modern software packages don’t support the PCX format, a legacy collection of PCX files is rapidly becoming a liability rather than an asset, and I’d be inclined to convert any PCX files I have to a format such as PNG or TIFF.

Which format to convert to?

I wouldn’t convert them to JPEG because that’s a lossy format that throws away some information in order to make the file smaller, and once that information is lost you can’t ever retrieve it. Also, because PCX files often had to contain areas of dithering – adjacent dots of different colours used to simulate a colour that’s unobtainable in the current palette – converting them to JPEG could even make the files grow larger as JPEG doesn’t cope well with dithered colours.

I wouldn’t convert to BMP either, because that usually employs no compression at all and so makes for much larger files (256-colour BMP files may have RLE or Huffman compression, but it’s optional and so depends on the application being used).

GIF is another possible format to consider but it, too, is both old-fashioned and restrictive. It was invented back in 1987 and so is stuck with a palette of just 256 colours. It was also hobbled because for ten years (1994-2004) Unisys insisted that anyone who supported it within their application had to pay a licence fee to use the LZW (Lempel–Ziv–Welch) compression algorithm on which it relied. The PNG format was designed to replace GIF, and it has much greater colour depth (16 million colours), better transparency support (an 8-bit alpha channel, where GIF has only a single-bit alpha channel) and more efficient compression than GIF. The only feature PNG lacks compared to GIF is the ability to animate images. PNG is now the best lossless image format available for most applications, and is the default format used internally by Microsoft Office applications.

Most image-editing packages that can read PCX files can write PNG and so are able to convert files from one format to the other, but if you have many files in the old format you’ll probably want to use a batch conversion routine to convert all the files in one operation. There are many packages that can do this for you: IrfanView is one possibility that’s free for non-commercial use. Commercial users should buy a licence for €10 per user, but there are discounts available for bulk purchases.

Once you’ve converted all your PCX files, which takes only a few seconds per file, you can archive all the original PCX files to CD or DVD and then use the PNG files in their place. Most batch converters will happily search down through subfolders to find images to convert, and put the converted files alongside the original image or in another folder if you’d prefer. If you have tens of thousands of old PCX files you may need to leave the batch conversion running overnight, but so long as you don’t run out of disk space you’re unlikely to have any problems.

Inspect a few sample images to see how much bigger or smaller the converted files are than the originals, and then use that ratio to estimate how much extra storage space you’ll need. Depending on which conversion utility you use, it may stop if it encounters a corrupt file or may just log the problem and carry on converting. Unless you can find a good copy of any corrupt file from a backup, there probably isn’t much you can do about these (trying to fix the image header with a hex editor to retrieve a corrupted image usually isn’t worth the effort).