PhotoCD vs. JPEG

One of the most recent topics that became a subject for much discussion regards the image data file format JPEG and the newer Kodak PhotoCD format. So for any interested (especially those who requested it), below is a summary of the information exchanged...

JPEG is a compressed file format. That is all of the original information (each pixel that made up the photograph or whatever when it was created or scanned) is not necessarily present in the saved JPEG file.

JPEG is known as a LOSSY compressed file format. That means that some of the original data is discarded (lost) in the interest of making the file smaller. Maintaining greater quality means a larger file, smaller file means less quality. While a compressed TIFF files best compression ratios can only reach 2 or 3 to 1, JPEG compression ratios with no appreciable loss of quality can approach 20 or 40 to 1. Even higher ratios can be achieved with JPEG.

Essentially JPEG is a very simple file format that requires a very complex method for reading and writing it. JPEG requires a compressor to create/write the file and a decompressor to read/view the file.

PostScript Level 2 printers will be able to perform a decompression on an embedded JPEG file on-the-fly. Thus making it possible to use very high resolution JPEG compressed color images in desktop publishing documents without the performance penalty associated with using large un-compressed bitmaps.

PhotoCD PhotoCD is a file format that is created by Kodak expressedly for arch iving photographs. The most common media for PhotoCD images is a CD-ROM disk, however it is possible to have PhotoCD images on any media accessible by your computer. It stores files in YCC color space and most retrieval software can sample this into one of three bit depths 8, 15, 24 (and technically provides developers with the ability to add more in the future).

The PhotoCD file is actually three or four images in one file. Whereas JPEG, TIFF, etc., can have virtually any resolution (horizontal and vertical amount of pixels), PhotoCD has specific fixed resolutions. There is the original high resolution file, a 2x version which is half the resolution of the high res, a 4x version which is one fourth the resolution of the high res, and a 16x version which is one sixteenth the high resolution version (also known as a thumbnail). It is not necessary for all versions to be present in the file.

PhotoCD files are not compressed in any way. Each original pixel is represented in the file. This can obviously make the file sizes rather large when dealing with the full res files.

Currently there is only one way to create a PhotoCD file, through Kodak or an authorized Kodak PhotoCD production company. Kodak requires the purchase of a custom series of hardware and software to produce the PhotoCD file. There are, however, many ways to read a PhotoCD file, including Photo Shop plug-ins, PageMaker import filters, etc.

SO WHICH IS BETTER

JPEG for some things, PhotoCD for others.

JPEG has wider support and is a more well developed specification than is the PhotoCD format.

PhotoCD is a proprietary format and Kodak wants to keep it that way at least for a while.

JPEG files (even ultra high resolution ones) can take up far less space than even the medium resolution PhotoCD files because of the lossy compression.

PhotoCD files maintain all of the fine detail and tonal quality since none of the original scan is sacrificed for space.

JPEG files could require longer to open and save on slower machines because they must be decompressed or compressed first before viewing.

SO WHAT DO YOU USE?

We use compressed TIFF files for temporary and short-term storage, but then we migrate all our long-term storage image files into a JPEG file with varying rates of quality (but never below 70%) depending on the image.

We see PhotoCD as a resonable format for Stock catalogs on CD-ROM but we dont see it as a viable format for in-house work until either high capacity erasable optical (or better storage) gets cheaper and the license to write PhotoCD becomes affordable.

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