HP Photosmart scanner scans a 30-bit image, but it outputs it as a 24-bit image file. I assume that this is because HP decided that there was not a significant enough difference in quality to justify the larger file size. Is this true?

However, Hamrick's Vuescan software will allow you to generate the actual 30-bit image from the scan. What I want to know is if there is a significant improvement in quality, shadow detail, etc., with the 30-bit over the 24-bit images? If so, does it justify the larger file size?

-- Michael Klemmer (nitrogen@worldnet.att.net), July 18, 1999

Answers

24 bits is generally good enough for nearly every output device you're likely to encounter (Monitors, Printers, Etc). Scanning at 30 or 36 bits really helps the internal algorythms of software packages to interpolate colors and detail from pixel to pixel when resampling for various purposes. Simply said - having more information available helps the softwares decision making more accurate.
Moving 30 or 36 bit images around is like pushing a rope. They're big, bulky and require more resources than are typically available in a national defense computer - and then you need another 20 Gig drive. improperly sampled 24 bit images can ruin your day - so a good program is necessary. HP typically does well, Photoshop, etc. But stay away from the cheap stuff when it comes to photo editing.

Des

-- Dan Desjardins (dan.desjardins@avstarnews.com), July 18, 1999.

If the scanned image is the final image, with no further processing required, then 24 bits is probably enough for colour. If you are going to manipulate the image (which generally involves losing information), then it is better to start with more bits, so you can finish with 24. If you had started with 24, you might finish with 16, with a visible loss in quality.

For B&W, the situation is even more critical. A 24-bit scan might contain only 8 bits of 'grey', so starting with more than 24 bits is even more useful.

Of course, the software that performs the post-scan processing has to be able to handle the greater number of bits.

-- Alan Gibson (Alan.Gibson@technologist.com), July 19, 1999.