Sustainability of Digital Formats: Planning for Library of Congress Collections

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Adobe Photoshop Family

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name Adobe Photoshop File Format Family
Description

The Adobe Photoshop Family includes the two native formats for Adobe Photoshop, PSD (Adobe Photoshop Document) and PSB (Adobe Photoshop Large Document), which support all Photoshop features as stated by Adobe Photoshop's online user guide. PSD (Adobe Photoshop Document) files are raster image files, which include support for image manipulation such as layers, masks, transparency, text, alpha channels, clipping path, additional color settings, and a variety of other Photoshop-specific elements. Along with Adobe's Large Document Format (PSB), it is Adobe Photoshop's native file format, "that supports all Photoshop features," as stated by Adobe Photoshop's online user guide. The PSD file format has a maximum height and width of 30,000 pixels and a length limit of two gigabytes. The PSB (Adobe Photoshop Large Document Format) file format is almost identical to the PSD file, however, it allows for a considerably bigger file size. The Adobe Large Document Format supports documents up to 300,000 pixels in any dimension and file sizes can range up to 4 exabytes (over 4.2 billion GB). Photoshop file formats differ slightly depending on the operating system. According to Adobe's specifications, "For cross-platform compatibility, all information needed by Photoshop is stored in the data fork. For interoperability with other Macintosh applications, however, some information is duplicated in resources stored in the resource fork of the file."

The traditional file structure for PSD and PSB files consists of five major parts:

  • File Header: The file header contains the basic properties of the image including; height and width of the image, depth, color mode, number of channels, and signatures. There are a variety of possibilities for color mode in Photoshop, See Factors below. Adobe provides ample documentation about Photoshop's color modes. The file header has a fixed length of 26 bytes while the other sections are variable in length. In PSD files, the header value of version data is always equal to 1, while for PSB files, the version value is 2. As mentioned above, the height and pixel width is detailed in the file header. For PSD files, these values can range from 1 to 30,000 while PSB files support a max of 300,000.
  • Color Mode Data: This section holds data required to render images in Indexed or Duotone color modes.
  • Image Resources: This section of the file stores the non-pixel data for the images. These image resource blocks building unites are similar to other image formats such as TIFF and JPEG. As stated in Adobe’s format specifications, "They are referred to as resource blocks because they hold data that was stored in the Macintosh's resource fork in early versions of Photoshop." The resource block consists of Image resource IDs where each ID consists of a hex number, decimal number, and description. Adobe maintains a list of image resource IDs.
  • Layer and Mask Information: This section of the file contains information about layers and masks including the flattened image data and the global mask. If a PSD file has no layers or masks, then this section only has 4 bytes while a PSB file with no layers or masks will have a length of 8 bytes.
  • Image Data: The last section of the file contains the image pixel data. Additionally, to maximize compatibility, PSD files can be saved to create a composite (flattened) image that allows for easier compatibility with other software applications. Data about the composite image is stored in this section as well.

See Format Specifications.

Production phase Most often an initial-state format.

Local use Explanation of format description terms

LC experience or existing holdings The Library of Congress has received and inventoried PSD files in the past but does not have a substantial collection of this particular file format and prefers other non-proprietary image formats.
LC preference The Recommended Formats Statement for Still Image Works Section ii. Photographs – Digital designates PSD files as an acceptable format.

Sustainability factors Explanation of format description terms

Disclosure PSD and PSB files are partially-documented proprietary formats.
    Documentation

Proprietary format specification information is available for every Photoshop version. The current version of Photoshop, released in November 2019, is available from Adobe.

Adoption

As stated in Wikipedia, Photoshop has "...become the industry standard not only in raster graphics editing, but in digital art as a whole." Market data showcases that Adobe Photoshop comprises a significant share of the graphics industry, bolstering the claim that it is one of the most popular and widely used graphics editing software applications out there. Photoshop has become a tool for a wide variety of users including; web developers and designers, graphic artists, photographers, and other creative professionals. This has led to a wide range of support for PSD files. There is substantive support to convert PSD or PSB files to other traditional formats including: TIFF, JPEG, PNG, EPS, GIF using Photoshop as well as a variety of free web conversion applications. Compared to other commonly used raster formats, the PSD format keeps layers separate so converting to other image formats flattens the image into a single layer. Both Windows and Mac operating systems support a variety of applications both open source and proprietary. The following is not an exhaustive list of compatible software that can open and edit PSD and PSB files.

Software that can open and/or convert PSD and PSB files in Windows operating systems. The following software applications do not offer editing support:

  • Free File Viewer
  • IrfanView
  • ImageMagick
  • Pixillion Image Converter
  • PSD Viewer

Software that can open and edit PSD and PSB files in Windows operating systems:

  • Series of Adobe products including Adobe Photoshop, Illustrator, InDesign
  • CorelDraw Graphics Suite 2020
  • Corel PaintShop Pro 2020
  • ACD Systems Canvas X 2019
  • GIMP
  • MAGIX Xara Designer Pro X
  • Paint.NET (with accompanying PSD plugin)
  • Affinity Photo (does not support Adobe Photoshop smart objects and other elements)
  • Photopea Online Editor

Software that can open and/or convert PSD and PSB files in Mac operating systems. The following software applications do not offer editing support:

  • Snap Converter

Software that can open and edit PSD and PSB files in Mac operating systems:

  • Apple Preview (images are flattened, which does not allow for layer manipulation)
  • Series of Adobe products including Adobe Photoshop, Illustrator, InDesign
  • Snap Converter
  • Corel Painter 2021
  • GIMP
  • Pixelmator
  • PhotoLine
  • Krita

Comments welcome.

    Licensing and patents The format specification states, "This document is provided for 3rd parties to read and write the Photoshop native file format. This document does not explain how to interpret the data. This document describes the format of the data only."
Transparency PSD and PSB files are not designed to be human readable as they are binary encoded which cannot be completed rendered in a text or hex editor without unrecognizable characters.
Self-documentation Adobe provides documentation about adding metadata to PSD files, which are compatible across their other software platforms including InDesign, Photoshop, and Bridge. As stated by Adobe, "Metadata information is stored using the Extensible Metadata Platform (XMP) standard." Most of Adobe’s products including Illustrator, InDesign, and Photoshop are built using this standard, which is built on XML. The listed Adobe products also support bulk metadata entry for digital assets using tab-delimited or XML files. Further documentation on metadata import is provided by Adobe. Comments welcome.
External dependencies None
Technical protection considerations None

Quality and functionality factors Explanation of format description terms

Still Image
Normal rendering Good support.
Clarity (high image resolution) Excellent support, accommodating images with large picture sizes, high spatial resolution, and high bit depth. Photoshop allows users to save 16-bits-per-channel and HDR (high dynamic range) 32-bits-per-channel images as PSD files.
Color maintenance

Photoshop supports a multitude of color modes including: RGB, CMYK, Grayscale, Lab Color, Bitmap, and Duotone and provides for the ability for convert PSD files from one color mode to another. According to the file format specifications, data about the image’s color information is stored in the file header. Color information stored in the header includes information about the color mode, the color depth, and the number of channels. Color mode is generally indicated by a supported numeric value, which is defined as follows:

  • 0 = Bitmap
  • 1= Grayscale
  • 2 = Indexed
  • 3 = RG
  • 4 = CMYK
  • 7 = Multichannel
  • 8 = Duotone
  • 9 = Lab

The PSD file format specification makes it clear that ICC Profiles can be incorporated into PSD files. ICC profile information is located in the Image Resource ID section of the file format. ICC profiles in PSD files are also mentioned briefly in ICC SPEC:1:2010 version 4.3.0.0 in Annex B. Adobe provides ample documentation for manipulating color profiles for PSD files as well as across other Adobe platforms. This documentation states, "To embed a color profile in a document you created in Illustrator, InDesign, or Photoshop, you must save or export the document in a format that supports ICC profiles," listing PSD and PSB formats as ICC profile supported formats.

PSD files also include a Color Mode Data Section, but only Indexed color and Duotone color modes have color mode data. For other modes, this section only contains a 4-byte length field that is set to zero. For Indexed color and Duotone images, as stated in the Adobe file format specifications,

  • Indexed color images: length is 768; color data contains the color table for the image, in non-interleaved order.
  • Duotone images: color data contains the Duotone specification (the format of which is not documented). Other applications that read Photoshop files can treat a Duotone image as a gray image, and just preserve the contents of the Duotone information when reading and writing the file.

Adobe's Color Mode documentation details the "Lab Color" mode which is based on the human perception of color. Adobe's color profile documentation indicates that Lab images can be saved in Photoshop (PSD) and Large Document Formats (PSB), particularly 16-bits-per-channel images.

Support for vector graphics, including graphic effects and typography Good support. Adobe Photoshop supports integrating vector graphics and textual elements. One of Photoshop's documented key concepts is the definition and comparison of raster and vector graphics. Here, the documentation offers links for "How To" guides for creating vector graphics using a variety of tools including Shape tools and Pen tools. The shape tool specifically adds functionality that allows users to add vector design elements. Type layers in Photoshop, "consists of vector-based type outlines-mathematically defined shapes that describe the letter, numbers, and symbols of a typeface. Photoshop preserves vector-based type outlines and uses them when you scale or resize type."
Support for multispectral bands No support for multi-spectral imaging. Because of the merge channel function, Photoshop users have the ability to create a false-colour composite image. This functionality, along with Photoshop's blending tools, have led to further studies about Photoshop's applicability in art examination and conservation.
Functionality beyond normal rendering PSD and PSB files can contain multiple manipulated layers created in Photoshop. Section four of the PSD file contains layer and mask information. To maximize compatibility, users have the option to incorporate a composite version of the layered image to be read by other applications. See Identification and Description.

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension psd
PSD (Adobe Photoshop Document).
Internet Media Type image/vnd.adobe.photoshop
See registration at IANA.
Magic numbers Hex: 38 42 50 53 00 01
ASCII: 8BPS
The Hex value ending 00 01 indicates version 1. See PRONOM PUID x-fmt/92.
Mac OS file type 8BPS
See registration at IANA.
Pronom PUID x-fmt/92
See https://www.nationalarchives.gov.uk/PRONOM/x-fmt/92 for PSD (Adobe Photoshop Document). This PUID also covers PDD (Adobe PhotoDeluxe).
Wikidata Title ID Q2141903
See https://www.wikidata.org/wiki/Q2141903 for PSD (Adobe Photoshop Document).
Tag Value Note
Filename extension psb
PSB (Adobe Photoshop Large Document).
Internet Media Type image/vnd.adobe.photoshop
See registration at IANA.
Magic numbers Hex: 38 42 50 53 00 02
ASCII: 8BPS
The Hex value ending 00 02 indicates version 2. See PRONOM PUID fmt/996.
Pronom PUID fmt/996
See https://www.nationalarchives.gov.uk/PRONOM/fmt/996 for PSB (Photoshop Large Document).
Wikidata Title ID Q1437034
See https://www.wikidata.org/wiki/Q1437034 for PSB (Photoshop Big).

Notes Explanation of format description terms

General

History Adobe released its first version of Photoshop to the public in 1990 with Photoshop 1.0 for Macintosh operating systems. Photoshop was not released for Windows operating systems until Version 2.5 in 1993. Each Adobe Photoshop version adds new functionality for the software, thus increasing the functionality and complexity of the file format. In 1994, Photoshop's 3.0 release included layers, one of the most distinct features of PSD files. 2000 saw the release of Photoshop 6.0 which featured vector shapes and the type tool. Adobe's release of Photoshop CS in October 2003 saw added file size capability with the release of Photoshop Large Document (PSB) files which allow documents up to 300,000 pixels in either dimension. See Wikipedia's summary of Photoshop's version history. Other illustrated sources and summaries of Photoshop version history are documented in Useful References.

Format specifications Explanation of format description terms


Useful references

URLs


Last Updated: 06/17/2021