Get a FREE fully-working copy of Steganos Security Suite 7 (worth £49.99) when you sign up to the Practical PC Newsletter. Scanning Pictures For Emailing
Don Bradbury has another look at graphics compression
This is the question for those who scan pictures and want to email their output. Which is preferable - to scan at high resolution at then resize and compress greatly, or to scan at low resolution and then resize and compress much less?
The answer may not, at first sight, be all that obvious, so a few tests were made for comparison. A high quality 10" x 8" colour photograph was first scanned (in Paint Shop Pro) at 300dpi on a Canon CanoScan N650U flatbed. That resulted in a highly detailed image of some 247MB. The image was resized to 25% of the original and then saved with JPEG compression set to 70%, producing a file for emailing of some 23K. Not much good, you might think, for printing, but a nice size if all that's required is for the recipient to view the image on screen. You would choose your own final image quality; these tests were for comparative purposes.
The same original was then scanned again, this time at 75dpi, producing output amounting to only 1.5MB (a mere 0.6% of the first). By trial and error, a setting was found for the resize operation that, combined with the same 70% JPEG compression, produced another 23K final image file. That resizing turned out to be 86% of the original.
Which final image would you expect to be the better? Or would they be much the same? Are you ready for the answer? Take a look at the hugely zoomed graphic. The top image was from the 300dpi scan; the lower from the 75dpi scan.
Comments
The first image shows less obtrusive compression artefacts, and so might be the better bet for emailing. Might? Well, you'll probably notice that the lower, 75dpi, scan produced the more "punchy", ie contrasty, final image, more closely resembling the original, in fact. This is more obvious if the whole image is viewed, or at least the full face.
So, as far as compression artefacts is concerned, the moral is, there's nothing like having as much information as possible present from the start. Subsequent downsizing and compression will do their worst, but a starting point containing more detail is preferable to a far less downsized and compressed but weaker starting image.
The preservation of image contrast, on the other hand, can be a different matter, though the actual scanner in use might play its part in that play-off. You pays your money and takes your pick, as they say, and it could be worth a few trials on your own equipment.
