Photo CD

Photo CD

Media type	Optical disc
Encoding	PhotoYCC
Capacity	Up to 2500 photos in 256 × 384 resolution
Read mechanism	780 nm wavelength semiconductor laser
Standard	Beige Book
Developed by	Eastman Kodak
Usage	Picture storage
Extended from	White Book
Extended to	Picture CD
Released	1991

Optical discs
General Optical disc Optical disc drive Optical disc authoring Authoring software Recording technologies Recording modes Packet writing Burst cutting area
Optical media types Compact disc ( MIL-CD, Mini CD Digital Versatile Disc ( DVD+RW, DVD-RAM, DVD-D, DVD-A, DVD-Video, HVD, EcoDisc, MiniDVD Blu-ray Disc ( Blu-ray 3D, Mini Blu-ray Disc, 4K Blu-ray (Ultra HD Blu-ray ) M-DISC Universal Media Disc (UMD) Enhanced Versatile Disc (EVD) Forward Versatile Disc (FVD) Holographic Versatile Disc (HVD) China Blue High-definition Disc (CBHD) HD DVD-RAM High-Definition Versatile Multilayer Disc (HD VMD) VCDHD GD-ROM Personal Video Disc (PVD) MiniDisc (MD): MD Data, MD Data2 Hi-MD LaserDisc ( LD-ROM, LV-ROM Video Single Disc (VSD) Magneto-optical discs Ultra Density Optical (UDO) 3D optical data storage Stacked Volumetric Optical Disk (SVOD) Fluorescent Multilayer Disc Hyper CD-ROM Nintendo optical disc (NOD) Archival Disc (AD) Professional Disc DataPlay
Standards ATAPI/MMC Mount Rainier (packet writing) Mount Fuji (layer jump recording) Rainbow Books File systems ISO 9660 Joliet Romeo Rock Ridge / SUSP El Torito Apple ISO 9660 Extensions Universal Disk Format (UDF) ISO 13490
See also History of optical storage media High-definition optical disc format war
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Photo CD is a system designed by

The Photo CD system was announced by

By 2000, over 140 Photo CD processing labs in the U.S. were active, with many more outside the U.S.[8] However, by the late 1990s, Photo CD was being eclipsed by alternate formats, mainly based on the industry standard JPEG format.^[9] In the consumer segment, the Photo CD format's relatively inefficient compression scheme meant that Photo CD files were significantly larger than JPEG files of similar quality, and thus less convenient for transmission across the internet, etc. For example, a 16Base Photo CD image of 5.5 Mb can be encoded as a JPEG image of 2.1 Mb at 80% quality, visually indistinguishable from the original.^[10] When the Photo CD format was designed in the early 1990s, a design goal was to allow low cost playback-to-TV devices. At that time the available technology precluded 2-dimensional compression schemes such as JPEG, but by the late 1990s, advances in microprocessor technology had moved JPEG/PNG compression to well within the range of even very low cost consumer electronics.

In the professional and advanced amateur segments, Photo CD had been eclipsed by low-cost

In the retail segment, while Photo CD was initially relatively popular with consumers, it was largely an economic failure for processing labs. At the time of its introduction, Kodak claimed that processing costs to labs would be close to $1 per image,[8] which would allow the lab profitably sell at the $3 per image mark. However this promise was never realized, often resulting in the scanning process being rushed, with a resulting fall in quality.^[12] As a result of Photo CD's loss of market share and substantial corporate losses, partially attributed by Kodak Management to its scanning business,^[15] Kodak abandoned the format over the period 2001-2004. By 2004, Kodak 4050 Photo CD scanners were being offered for free to anyone that would pay for their removal by more than one processing lab.^[16] This abandonment generated considerable controversy both at the time and subsequently as the Photo CD format's technical specifications have never been released by Kodak. Photo CD remains an often quoted example of an “orphan format” and of the dangers of proprietary image formats within photographic circles.^[17][18][19]

Despite Kodak not releasing the specifications for the Photo CD format, it has been reverse engineered, thus allowing images to be converted to more modern formats. The original reverse engineering work was performed by Hadmut Danisch of the

Photo CD images are stored as a hierarchy of components ranging from Base/16 to 64Base which relate to the varying image resolutions that can be reconstructed.[27] The Base image, which has a luma resolution of 512 lines by 768 pixels, is typically used for the TV systems. The higher-resolution images are usually used for photographic imaging. The Base and lower resolution images are usually used for file indexes and thumbnails. The components used to create the Base/16 through 16Base resolutions are stored in a single file called an Image Pac. The Base, Base/4, and Base/16 images are stored uncompressed in order to allow easy extraction for display. 4Base and 16Base images are stored compressed, and as deltas (incremental images) from the resolution below them. Note all resolutions up the maximum resolution in a file (actually an Image Pac) are all present simultaneously. So, for example, a 16Base file contains a Base/16, Base/4, Base and 4Base image as well as the 16Base components. The sixth component, 64Base, is stored in separate files on the Photo CD as part of the IMAGE PAC Extension (IPE). This only exists on Photo CD Pro Master discs.

PCD images use a gamma transformed PhotoYCC encoding.

In Kodak documentation, α = 1.099, β = 0.018. However, the full standard values (1.099296826809442, 0.018053968510807) provide mathematical continuity.[30]

As a result of this function, colors that are outside of the gamut defined by the CCIR 709^[a] primaries are encoded by the negative values.^[27] Linear RGB values are from -0.20 to 2.00, while nonlinear RGB' are from -0.43357 to 1.402278.^{[citation needed]} Reference white (perfect, non-fluorescent, white-reflecting diffuser in the original scene) is 1.0, just like in BT.709.

The preshaped RGB values are then converted to a luminance and two chrominance components via a CCIR 601-1-like matrix:^[27]

${\displaystyle Y^{\prime }=0.299R^{\prime }+0.587G^{\prime }+0.114B^{\prime }}$

${\displaystyle B^{\prime }-Y^{\prime }=-0.299\cdot R^{\prime }-0.587\cdot G^{\prime }+0.886\cdot B^{\prime }}$

${\displaystyle R^{\prime }-Y^{\prime }=0.701\cdot R^{\prime }-0.587\cdot G^{\prime }-0.114\cdot B^{\prime }}$

Finally the luminance and chrominance components are scaled to 8-bit values by the following equations:^[27]

${\displaystyle Y^{\prime \prime }={255/1.402}\cdot Y^{\prime }}$

${\displaystyle C_{1}=111.40\cdot (B^{\prime }-Y^{\prime })+156}$

${\displaystyle C_{2}=135.64\cdot (R^{\prime }-Y^{\prime })+137}$

Kodak claims that "the scale factors and offsets for the C1 and C2 channels result from the distribution of real world colors".^[27] Given the 8-bit integer range for each of the 3 components, 0 ≤ Y ≤ 1.402, -1.40036 ≤ B' - Y' ≤ 0.888689, -1.01003 ≤ G' - Y' ≤ 0.86995.

An oddity of this encoding scheme is that it allows Photo CD images to represent colors which are above 100% (up to 140.2%), "

In addition, although "toeroom" (Y < 0) is not provided, Kodak expressly allows the use of R, G, B < 0, allowing out-of-gamut (for Rec. 709) colors to be expressed.[27] The effect is similar to xvYCC, which came much later.

Photo CD images use three forms of compression in order to reduce image storage requirements.^[27] Firstly, chroma subsampling reduces the size of the images by approximately 50%. This subsampling is by a factor of 4 for 4Base images, and a factor of 2 (4:2:0) for all other resolutions. Secondly an additional reduction in size is achieved by decomposing the highest-resolution image data, and storing the 4Base, 16Base and 64Base components as residuals (differences from pixels at the previous level of resolution). Thirdly and finally, the Photo CD system employs a form of quantization and Huffman coding to further compress this residual data. This Huffman encoding is performed on an image-row-by-image-row basis. The Huffman tables are encoded into the Photo CD image itself, and have different lengths depending on the compression class. These Huffman classes are:^[34]

• class 1 - 35mm film; pictorial hard copy,
• class 2 - large format film,
• class 3 - text and graphics, high resolution,
• class 4 - text and graphics, high dynamic range.

For example, to recompose an image of 1024 lines by 1536 pixels, the 512-line by 768-pixel luma Base image (which is neither "residual" or Huffman compressed) is interpolated to a 1024-line by 1536-pixel image. A 1024-line by 1536-pixel 4Base residual is then decompressed from its Huffman-encoded form, and its elements are added to each corresponding pixel. The resulting image contains detail for the full 1024-line by 1536-pixel sampled image. To recompose an image to a resolution of 2048 lines by 3072 pixels, the process is essentially repeated, using both the 4Base and 16Base residual. A similar operation is subsequently used to recompose the 4096 line by 6144 pixel resolution. In each step, an identical process is applied to the chroma channels.

Kodak based the PhotoYCC scheme on some existing video standards, notably on aspects of CCIR Recommendation 601 used with PAL and NTSC digital television systems, and also on CCIR Recommendation 709 (now ITU-R Recommendation BT.709) used for HDTV. The PhotoYCC definition is defined in a manner that is not constrained by the limitations of the actual video display.^[27][31]

In practice the color space of Photo CD images varies significantly from Rec. 709. Firstly, the Photo CD encoding scheme allows greater than 100% values for color components, thus allowing Photo CD images to display colors outside of the nominal Rec. 709 gamut.^[32] In addition, in order to achieve accurate color reproduction, especially when scanning reversal film, Kodak found it necessary to provide ICC color profiles specific to film type and scanner.^[35] As a result, by the time that the Photo CD format fell into disuse, five different color spaces were in common use in Photo CD images (PCD 4050 is a Kodak scanner model number):

• Color Negative
• Universal E-6
• Universal K-14
• PCD 4050 E-6
• PCD 4050 K-14

Thus, while the use of the generic Photo CD color space will provide color reproduction that is acceptable for many purposes e.g., Web viewing of thumbnail images, in order to achieve fully accurate color reproduction, e.g., for photographic purposes, any Photo CD display or conversion software must use the correct color profile for the combination of the original media and scanner model.

For practical use, any Photo CD images will have to be converted to a modern format such as

• The ability to convert images at maximum resolution, ideally up to the 64Base (4096x6144) level.
• Correct highlight handling. Blown or clipped highlights are a common problem with many Photo CD conversion packages.[32] Once a highlight is clipped by a conversion program, the information cannot be recovered by later manipulation. Many Windows based software packages partially correct for the highlight problem by using a “hacked” DLL created by Ted Felix.^[36]
• The use of scanner and film type specific ICC color profiles. Unless such profiles are used, the color reproduction of the image will be incorrect,^[35] often, especially for reversal (slide) films, very noticeably so.^[37]
• Extracting metadata. Metadata is information such as the date that the scan was performed, the model number of the scanner, and film type. This information is not only useful to the user, but is also critical in selecting the appropriate color profile, as these vary by film type and scanner.^[35]

• Picture CD, a similar product also by Kodak