DAKX - PhotoCD recompression

Lossless Transcoding of PhotoCD images

Kodak PhotoCD "Image Pac" files use a "PhotoYCC" color space designed for the storage of images acquired from film. The file format (extension .pcd, Mac file type 'PCDI') usually contains the image at 5 resolutions:

requiring 4-6 megabytes of storage.
[Professional files can contain a 6144x4096 version and be correspondingly larger]

According to the Kodak web site the first three sizes are stored uncompressed with 2x2 chroma subsampling. The 1024 line version is interpolated from the 512 line version, using quantized huffman-encoded correction factors for each pixel to recover equal luma and chroma resolution. The last two sizes are successively interpolated and quantized-huffman-corrected, again with 2x2 chroma subsampling.

Conversion to RGB display format is nonlinear and usually not reversible. Hilights in particular are easily lost. Most software performs this conversion automatically. For example, Apple provides a PhotoCD extension which displays the CD's as if containing RGB PICT files in separate folders for each resolution. The actual PCD files are also available in a separate PHOTOCD directory.

Kodak supplies a free import plugin for Adobe Photoshop which gives much more control over the color conversion. This plugin also allows the direct importation of PhotoYCC data as a pesudo RGB image, which can subsequently be manipulated or recompressed without loss. Conversion of the "transcoded" images back to Image Pac format requires a license from Kodak. One can also send PhotoYCC TIFF files to an authorized regional processor for PhotoCD transcoding.

Assuming the lower-resolution images could be adequately regenerated from the 3072x2048 size, DAKX transcoding averages about 20% smaller files. DAKX decompression for this size image requires about 300 second-mips (e.g. 3 seconds at 100 mips), much faster than software reconstruction using the Image Pac file (e.g. 12 seconds for Apple's implementation)

Of course the smaller images are useful for rapid access to thumbnails and previews. Here is a breakdown of the compression in YCC space for several images:

Bridge at night

Image Pac file size = 5,283,840 bytes
DAKX transcoding:
                  Y          C1         C2
   96 x  64       -         2,502      2,640
  192 x 128     16,551      7,388      7,650
  384 x 256     59,179     25,602     26,064
  768 x 512    216,044     91,079     89,960
 1536 x1024    789,824    584,614    556,501
 3072 x2048  2,820,738       -          -

Parrots (Kodak PhotoCD Photo Sampler #15-1010-1, image 23)

Image Pac file size = 3,944,448 bytes
DAKX transcoding:
                  Y          C1         C2
   96 x  64       -         3,177      3,022
  192 x 128     15,273      9,137      8,600
  384 x 256     52,821     29,169     27,461
  768 x 512    194,510     98,265     91,324
 1536 x1024    704,959    473,459    382,649
 3072 x2048  2,542,414       -          -

Kodak, Photo CD, and Image Pac are trademarks of Eastman Kodak Company.
PhotoShop is a trademark of Adobe Systems, Inc.
QuickTime is a trademark of Apple Computer, Inc.
All other brand or product names are trademarks or registered trademarks their respective holders.
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