Photo CD - AbsoluteAstronomy.com

Photo CD is a system designed by Kodak for digitizing and saving photos in a CD. Launched in 1992, the discs were designed to hold nearly 100 high quality images, scanned prints and slides using special proprietary encoding. Photo CDs are defined in the Beige Book

Rainbow Books

The Rainbow Books are a collection of standards defining the formats of Compact Discs.Red BookYellow BookThe Rainbow Books are a collection of standards defining the formats of Compact Discs.Red Book...

and conform to the CD-ROM

CD-ROM

A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....

XA and CD-i Bridge specifications as well. They were intended to play on CD-i

CD-i

CD-i, or Compact Disc Interactive, is the name of an interactive multimedia CD player developed and marketed by Royal Philips Electronics N.V. CD-i also refers to the multimedia Compact Disc standard used by the CD-i console, also known as Green Book, which was developed by Philips and Sony...

players, Photo CD players (Apple's PowerCD

PowerCD

Apple PowerCD was a US$499 CD player sold by Apple Computer in 1993 and discontinued several years later. It was a re-badged Philips-designed product which was sold in addition to Apple's speakers and also included a remote control. The PowerCD was capable of reading Kodak photo CDs, data CDs and...

for example), and any computer with a suitable software (LaserSoft Imaging's

LaserSoft Imaging

LaserSoft Imaging AG is a software developer designing software such as SilverFast for scanners and digital cameras. The company's headquarters are located in Kiel, Germany, north of Hamburg, and another office in Sarasota, Florida, United States....

SilverFast

SilverFast is software for scanning and image processing, including photos, documents and slides, developed by LaserSoft Imaging.- History :...

DC or HDR for example).
The system failed to gain mass usage among consumers partly due to its proprietary nature, the rapid decline in the costs of scanner

Image scanner

In computing, an image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass...

s, and the lack of CD-ROM

CD-ROM

drives in most home personal computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s of the day. The Photo CD system gained a fair level of acceptance among professional photographers due to the low cost of the high quality film scans. Prior to Photo CD, professionals who wished to digitize their film images were forced to pay much higher fees to obtain drum scans of their film negatives and transparencies

Transparency (photography)

In photography, a reversal film is a type of photographic film that produces a positive image on a transparent base. Also known as dias or slide. The film is processed to produce transparencies or diapositives instead of negatives and prints...

. Photo CD was replaced by Picture CD

Picture CD

Picture CD is a product by Kodak, following on from the earlier Photo CD product. It holds photos from a single roll of color film, stored at 1024×1536 resolution using JPEG compression. The product is aimed at consumers...

, also from Kodak.

Designation	Resolution		Size	Intended typical use
Designation	(px × px Pixel In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled.... )	(Mpx)	(MB Megabyte The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000... )
Base/16	128 × 192	0.025	0.07	Preview (index print, thumbnail)
Base/4	256 × 384	0.098	0.28	Web
Base	512 × 768	0.393	1.13	Computer screen, TV, Web
4 Base	1024 × 1536	1.573	4.50	HDTV screen
16 Base	2048 × 3072	6.291	18.00	Print-out up to ca. 20 x 30 cm
64 Base	4096 × 6144	25.166	72.00	Professional print, pre-press, archiving (optional)

Variants

The Kodak Pro Photo CD Master Disc contains 25 images with maximum resolution of 6144 x 4096 pixels (six resolutions per file, Base/16 to 64 Base). This type is appropriate for 120 film, 4x5, but also for small picture film, if highest resolution is required.

History

The Photo CD system was announced by Kodak in 1990. Photo CD targeted a full range of photographic needs, ranging from consumer level point-and-shoot cameras to high-end professionals using large format 4x5 sheet film. The first Photo CD products, including scanners for processing labs and Photo CD players for consumers, became available in 1992. The project was expected to be a $600 million business by 1997 with $100 million in operational earnings. Kodak entered into a number of partnerships grow the usage of Photo CD. This included, for example, an arrangement with L.L. Bean in 1992 by which the catalog would be distributed in Photo CD format, and an arrangement with Silicon Graphics in 1993 to make all Silicon Graphics image-processing workstations capable of accepting Kodak Photo CD optical disks. These measures, together with the then relatively low cost of $3 per image and convenience, made Photo CD the digital imaging solution of choice for many photographers in the mid to late 1990s.

By 2000, over 140 Photo CD processing labs in the USA were active, with many more outside the USA. However, by the late 1990s, Photo CD was being eclipsed by alternate formats, mainly based on the industry standard JPEG

JPEG

In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....

format. In the consumer segment, the Photo CD format’s relatively inefficient compression scheme meant that Photo CD files were significantly larger than a JPEG

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

JPEG

image of 2.1 Mb at 80% quality, visually indistinguishable from the original. 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 1990, 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 desktop scanners such as those from Nikon

Nikon

, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...

and Minolta

Minolta

Minolta Co., Ltd. was a Japanese worldwide manufacturer of cameras, camera accessories, photocopiers, fax machines, and laser printers. Minolta was founded in Osaka, Japan, in 1928 as . It is perhaps best known for making the first integrated autofocus 35mm SLR camera system...

in the mid range, and by drum scanners at the very high end. While the pixel resolution of Photo CD was still comparable or better than the alternatives, Photo CD suffered from a number of other disadvantages. Firstly, the Photo CD color space, designed for TV display, is smaller than what can be achieved by even a low cost desktop scanner. Secondly, the color rendition of Photo CD images changed over time and with different scanner versions; 4050 scanners had different color rendition to earlier versions. Thirdly, the dynamic range of scans was lower than for desktop scanners. Test at the time indicated that the dmax rating (a measure of maximum density obtainable) of Photo CD was 2.8-3.0, while commonly available desktop scanners were reaching 4.2, a substantial difference. As a result of this, and Photo CD’s problems with color rendering, by 2004 the professional segment of the user community had generally turned against Photo CD.

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, 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.
As a result of Photo CD’s loss of market share and substantial corporate losses, partially attributed by Kodak Management to its scanning business,Gavetti, G; Henderson, R; Giorgi, S: Kodak and the Digital Revolution (A), pages 5-6. Harvard Business School, 2005. 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. 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.

Despite Kodak not releasing the specifications for the Photo CD format, it has been reverse engineered, so allowing images to be converted to more modern formats. The original reverse engineering work was performed by Hadmut Danisch of the University of Karlsruhe, who deciphered the format by studying hex dumps of Photo CD files, and subsequently wrote hpcdtoppm, which converts Photo CD Images to PPM format, in the early 1990s. During the early 1990s, hpcdtoppm was extensively distributed as part of various Linux distributions, but has since been almost entirely abandoned due to concerns about hpcdtoppm’s restrictive license conditions and lack of color management. However, Hadmut Danisch’s reverse engineering work has been used to create a number of other open source

Open source

The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...

implementations of Photo CD decoders such as ImageMagick. In 2009, pcdtojpeg was created under the GPL open source

Open source

licence. Also acknowledged by the author to be based on Hadmut Danisch’s reverse engineering work, pcdtojpeg allows Photo CD metadata to be decoded, is color managed, and can decode all known variants of Photo CD files. Currently hpcdtoppm and pcdtojpeg together form the core of information available in the public domain on the technical details of the Photo CD format.

Image Components

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. 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 simultaeously. 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.

Encoding

PCD images use a gamma transformed YCC encoding. Under this scheme, in order to encode scanned images into Photo CD file, the first step is to preshape the RGB data via a non-linear transform:

The preshaped RGB values are then converted to a luminance and two chrominance components via a CCIR 601-1 conformant transform matrix:

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

An oddity of this encoding scheme is that it allows Photo CD images to represent colors which are above 100%, "whiter than white". At the time of Photo CD's introduction, this allowed the display of images on the analog televisions of the day to show brighter, more vivid colors than would otherwise be the case. This was acceptable as the analog televisions of the day were designed to be driven beyond their 100% level without abruptly clipping. However, this aspect of Photo CD encoding is not compatible with modern image formats, and is the cause of the well documented "Blown Highlights" problem when converting Photo CD images to more modern formats. All modern formats enforce hard clipping at 100%, resulting in highlight clipping in converted images unless some form of Photo CD specific luminance and chrominance mapping is performed.

Compression

Photo CD images use three forms of compression in order to reduce image storage requirements. 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 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

Huffman coding

In computer science and information theory, Huffman coding is an entropy encoding algorithm used for lossless data compression. The term refers to the use of a variable-length code table for encoding a source symbol where the variable-length code table has been derived in a particular way based on...

to further compress this residual data. This Huffman encoding is performed on a 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 classes are:

class 1 - 35mm film; pictoral 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.

Color Space

Photo CD images nominally use a CCIR Recommendation 709 color space

Color space

A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...

and a D65 white point

White point

A white point is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results...

. This color space, originally designed for analog PAL and NTSC television use, was chosen to allow easy playback of Photo CD images to televisions. However in practice the color space of Photo CD images varies significantly from CCIR 709. Firstly, the Photo CD encoding scheme allows greater than 100% values for color components, thus allowing Photo CD images to display colors ouside of the nominal CCIR 709 gamut. In addition, in order to achieve accurate color reproduction, especially when scanning reversal film, Kodak found it necessary to provide color profiles specific to film type and scanner. 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 refers to a 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.

Backing up and Archiving Photo CD Images

Kodak’s Photo CD specification indicates that Photo CDs should have a useful life of over 200 years if stored under the recommended dark storage conditions (25°C, 40% RH).Douglas Stinson, Fred Ameli, and Nick Zaino: Eastman Kodak Company: Lifetime of KODAK Writable CD and Photo CD Media, 1993. However, this has been called into question by more recent studies that have cast doubt on the BLER methodology used to arrive at this estimate. In addition, reports have surfaced of disk failures. In any event, good practice dictates that any important digital asset be backed up to alternate media. Thus backing up or otherwise archiving Photo CD images is recommended.

Two broad strategies can be used for archiving Photo CD images - either backing up the entire CD, or just backing up the actual images:

Backing up entire Photo CD disks

While in principle simple, backing up an entire Photo CD has never been supported by Kodak and cannot be achieved by simply copying the Photo CD’s directory structure. In addition, conventional CD copying equipment and software will not properly copy a Photo CD as some aspects of the Photo CD format are proprietary to Kodak. Photo CDs copied in this way will have all image files present, but will not play in a Kodak Photo CD player. However, various unofficial methods have been created that will allow Photo CDs to be created and/or copied although these methods require specialist tools and technical knowledge.

Backing up only the Photo CD images

If all that is required is to backup the images on a Photo CD rather than to create a CD that will play a Photo CD player, it is possible to just backup the individual image files:

For normal (non-Pro) Photo CD’s, all that is required is to back up the *.PCD files from the CD’s PHOTO_CD\IMAGES directory.
For Photo CD Pro CDs, those with 6144x4096 64Base images on them, it is critical that the entire CD file directory be backed up. Image extension files are stored in the PHOTO_CD\IPE directory, and unless whatever program you later use to convert the images can find those files, you will not be able to recover the full resolution 64Base images - the images will in effect be downgraded to 16Base.

Converting Photo CD Images

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

JPEG

, TIFF or DNG

Digital Negative (file format)

Digital Negative is an open raw image format owned by Adobe used for digital photography. It was launched on September 27, 2004. The launch was accompanied by the first version of the DNG specification, plus various products including a free of charge DNG Converter utility...

. While there are a large number of image conversion programs available, both commercial and freeware or open source

Open source

, most are capable of only basic, low resolution (often only to Base resolution, 512x768) conversions. The main distinguishing features between programs as regards Photo CD capabilities are:

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. 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.
The use of scanner and film type specific color profiles. Unless such profiles are used, the color reproduction of the image will be incorrect, often, especially for reversal (slide) films, very noticeably so.
Extracting metadata. Metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...

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.

Achieving Correct Color Reproduction

Correct color reproduction can only be achieved by programs that use film and scanner specific color profiles. Unfortunately, as at March 2010, only two solutions exist that allow this:

Photoshop CS2 and CS3. While neither Photoshop CS2 or CS3 are still available from Adobe, many photographers still have copies available to them, making this a viable option for many. Photoshop CS2 was shipped with a Kodak supplied image conversion plug-in. This was not supplied with Photoshop CS3 or later versions of Photoshop, but can be installed on Photoshop CS3 by following Adobe’s instructions. This plug-in is however not compatible with Photoshop CS4 or later. Also, the appropriate color profile must be manually assigned to each image. If this is not done correctly, the image will suffer from incorrect color, and blown highlights. This requires that the user have separate records on the scanner and film type, or extract that data using a different program e.g., pcdtojpeg.
pcdMagic. pcdMagic allows the use of either the original Kodak color profiles, or its own internal profiles. In addition, it is capable of decoding the Photo CD image’s metadata, and assigning a profile automatically based on that. pcdMagic is available for both the Mac and Windows.

Comparison of Photo CD Conversion Software

The table below compares some of the better known options for converting Photo CD images.

Name	Platform	Supports 64Base	Correct Highlights	Color Profiles	Extracts Metadata	Cost	Notes
hpcdtoppm	Win, OS X, Linux, Unix	Yes	No	No	No	Free	Only converts to PPM (Netpbm format)
ImageMagick	Win, OS X, Linux, Unix	Yes	Partial	No	No	Free
Picture Window	Win	No	Partial	No	No	$90
ThumbsPlus	Win	No	Partial	No	No	$79
iPhoto	OS X	No	No	No	No	$79/Free	Part of the iLife suite of programs, often provided free with new Macs
IrfanView	Win	No	Partial	No	No	Free	Can use Ted Felix’s hacked Photo CD DLL for highlight correction
pcdtojpeg	Win, OS X, Linux, Unix	Yes	Yes	No	Yes	Free	Color managed, but supports only a generic Photo CD color profile
Photoshop	Win, Mac OS 9	Yes	Yes	Yes	No	n/a	Only Photoshop CS2 and CS3 with restrictions - see text
pcdMagic	OS X	Yes	Yes	Yes	Yes	$79	Can use either the original Kodak color profiles, or internal color profiles. Can convert to DNG.
pcdMagic for Windows	Win	Yes	Yes	Yes	Yes	$79	Comes with its own Photo CD color profiles; converts to JPEG JPEG In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality.... or TIFF.
SView5	Win, Amiga, MorphOS	Yes	No	Yes (other)	Yes (other)	Free	Resolutions beyond Base require external tool (hpcdtoppm/pcdtojpeg)

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.