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perhaps interesting - about washed out highlights:
Concepts and encoding of Photo CD are described in
Giorgianni + Madden, Digital Color Management.
For instance, negative values R,G,B and values
larger than 255 are allowed.
Reading this book diagonally leads to the impression,
that not only standard sRGB images can be derived
but also images with wider gamut and more bits
per pixel (besides the variable spatial resolution).
Best regards --Gernot Hoffmann
Hello Gernot, thanks for your reply.
Yes, I've seen the Ted Felix site, downloaded his DLL, and used it with Paint Shop Pro. Unfortunately, although it does preserve more of the highlights, in other respects the Photoshop conversion looks better.
As you say, it seems that it would be possible for an intelligent conversion tool to convert a standard PCD image to a TIFF with genuinely useful 16-bit colour. Unfortunately, no-one seems to have produced such a tool. I tried asking here just in case anyone has found a better solution than the ones I've tried.
I get the impression that Adobe merely bundles the conversion tool provided by Kodak without attempting to correct its defects or even to document it properly (the Photoshop help says very little about it).
Although PCD is more or less a dead format now, there must be lots of old Photo CDs at large in the world, and quite a few people interested in converting those images. I suppose most people innocently convert the images with whatever tool they have, and wrongly blame the PCD format itself for the imperfect results they get.
There were also two level of PhotoCD scans - the standard and the Pro - the Pro not only being higher resolution, but also having a slightly greater density range. Then you also had the option of having automatic corrections built in at the scan stage, which usually clipped both ends of the tonal range. PhotoCD scans were done on a proprietary CCD scanner which was never really great quality anyway. It's entirely possible that whatever method you try to extract your images with will meet with disappointment.
Hello Peter, thanks for your reply.
I'm aware of the Pro version of Photo CD, but all of my own Photo CDs are the standard version (6 megapixels).
I also realize that the quality of the scan is inherently limited. When I still have the original negative or slide, I can get better results by rescanning with my Nikon CoolScan -- although scanning with the CoolScan takes up a considerable amount of my time.
However, for instance, I have some 50-year-old slides that have deteriorated since I put them on Photo CD in the mid-1990s. I therefore have an interest in getting as much quality as I can out of the Photo CD images.
Today I found an article in the Adobe Knowledgebase recommending that Photo CD images should be imported in the Lab colour model rather than RGB. I'm doubtful about this as it doesn't seem to make much difference at first sight; but perhaps there is a slight improvement.
I've now experimented a bit more with importing Photo CD images in Lab. The result looks much the same as importing in RGB; if you import as Lab and then immediately convert to RGB, it seems to be the same as importing in RGB (naturally enough).
However, when I use the Levels command and look at the histogram, I see that more information about highlights is preserved in the Lab version than in the RGB version. So, if I import as Lab, go to Levels, and move the middle slider before converting to RGB, I can get some improvement. This is a manual operation requiring some judgment, but it is possible and seems somewhat useful. The improvement isn't startling, but it's perceptible.
I'd like to correct point 2 in my original post here. I've now discovered that Photo CD images imported as 16-bit do, in fact, contain 16 bits of information (all 16 bits seem to be used).
A PCD file basically contains 8-bit colour information. However, it's YCC-encoded, and this doesn't seem to translate precisely into 8-bit RGB (nor, perhaps, 8-bit Lab). So using 16-bit RGB (or Lab) is a way of getting a more accurate conversion of the 8-bit YCC information. Unfortunately, Photoshop's converter doesn't make the best possible use of this opportunity (highlights are washed out), but nevertheless it evidently makes some use of the 16 bits available to it.
Because the information is basically 8-bit information, there's no way of getting real 16-bit quality out of the conversion. However, the 16-bit conversion ought to be at least slightly better than the 8-bit conversion. Whether it's worth doubling the file size for this slight and perhaps imperceptible improvement is up to the user, but it does after all make sense for Photoshop to give us the option.