Sustainability of Digital Formats
Planning for Library of Congress Collections
Sustainability of Digital Formats
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | Digital Moving-Picture Exchange (DPX) Format |
| Description | File format for the exchange of resolution-independent, pixel-based (bitmapped) images, intended for very high quality moving image content for theatrical distribution; DPX masters provide the input for film recording (digital images back to film for projection) or D-Cinema digital projection systems. Each DPX file represents a single image with a single component, e.g., luma, or multiple components, e.g., red, green, blue; or Cb, Y, Cr (chrominance-luminance data). Many variations in multiple component data are supported. DPX images may be produced by scanning film or by using a camera that produces a DPX output. Sound information is not incorporated in DPX and producers must manage soundtracks separately. |
| Production phase | Typically a middle-state format for material exchange, "post-production" in movie industry parlance, or archiving; in some circumstances may be a initial state format, i.e., "production," when digital cameras are employed. |
| Relationship to other formats | |
| Has earlier version | DPX specification, 1994, not documented at this time |
| LC experience or existing holdings | Trial use beginning in 2004, with a motion picture scanner at the Motion Picture, Broadcasting, and Recorded Sound division's Film Preservation Laboratory, Dayton, Ohio. |
| LC preference | None at this time. |
| Disclosure | Open standard. Developed by the Society of Motion Picture and Television Engineers (SMPTE), a member of the American National Standards Institute (ANSI). |
| Documentation | SMPTE 268M-2003, SMPTE Standard for File Format for Digital Moving-Picture Exchange (DPX), Version 2.0 |
| Adoption | Some adoption. Several film scanners support DPX, and it is offered as an output from the Origin camera from the Canadian DALSA company. Some of the drawbacks of the format, such as the absence of a standardized approach for managing accompanying sound, and limits on bit depth (see section on clarity below), may lead potential users to consider alternate formats, e.g., MXF. |
| Licensing and patent claims | None. |
| Transparency | Wrapper is transparent; overall transparency depends upon the essence encoding. |
| Self-documentation | DPX files contain three sections in addition to picture data: (1) generic file information including data format, (2) motion picture and television industry specific information such as film perforation edge number or video timecode information, and (3) user-defined information which may include ASCII data. Since each DPX frame is "one of tens of thousands" in a motion picture, users will track intellectual/bibliographic metadata separately from the set of DPX files. |
| External dependencies | None |
| Technical protection considerations | None |
| Video elements | |
| Normal rendering for video | DPX files are not be designed to play in the customary sense. Most applications in which DPX files may be played will be professional; this is not a format intended for desktop PC applications. |
| Clarity (support for high image resolution) | Depends upon picture size and bit depth; for uncompressed bitmaps. The format has no limit on picture size; many motion picture applications work at 2K (2048x1080 pixels) and 4K (4096x2160 pixels) resolution. Several color space implementations are supported. Regarding bit depth (necessary to encompass the wide dynamic or brightness range of actual scenes or images on film), many in the motion picture industry advocate having masters with 16-bit-per-channel linear data, whether RGB, YUV (chrominance-luminance), or some other, e.g., raw data from a Bayer array. The manufacturers of the DALSA Origin camera report "the DPX framework does not preclude 16-bit data, but 16-bit linear implementations have not been established yet [incorporated in the standard]." (p. 4) |
| Functionality beyond normal video rendering | Not investigated at this time. |
| Sound elements | |
| Normal rendering for sound | Not applicable |
| Fidelity (support for high audio resolution) | Not applicable |
| Support for multiple sound channels | Not applicable |
| Functionality beyond normal sound rendering | Not applicable |
| Tag type | Value | Note |
| Filename Extension | dpx | From the specification |
| Internet Media Type | none found | |
| Magic numbers | Hex: 0x53445058 ASCII: SDPX | If big-endian (most significant byte first), from the specification |
| Magic numbers | Hex: 0x58504453 ASCII: XPDS | If little-endian (least significant byte first), from the specification |
| General | The European PRESTO project supported efforts to preserve broadcast content via digitization. A PRESTO report on a file format conversion software issued in 2002 includes two statements that pertain to DPX: "Currently the DPX file format has become an industry standard for film production. It supports resolution independency (spatial and densitometric), but does not meet the requirements of broadcasters because, for instance, it does not contain sound data or metadata. Film is stored as a number of single uncompressed image files. The MXF format intends to overcome these limitations by, for instance, support of metadata and by support of film structure information (shot information)." (p. 2) The PRESTO report also includes a paragraph that indicates one application's approach to filenaming with one-frame-at-a-time formats like DPX, a critical issue since 30 minutes of film contains 43,200 frames and 30 minutes of video contains about 54,000: "Both input and output image sequences are expected to be stored as a number of files in the same directory in one of the following image formats: TIFF, PNM, SGI, RGB, DPX. Interlaced image sequences [video fields] are expected to be stored as a sequence of images containing two woven fields. The filename is assumed to be in the format name.n.ext, where n is the frame number with up to 8 digits and ext is the file extension. The number of digits in the frame number has to be constant for all frames of one sequence." (p. 13) Frank Wylie of the Library of Congress Film Preservation Laboratory confirms that most DPX-capable applications provide this type of filenaming as the default. |
| History | Stream and File Formats: Where Are We Now? states that DPX "was developed several years ago to support the transfer of uncompressed images between telecine machines. It was later used for synthetic image file transfers." The Wikipedia reports: "The DPX file format is derived from the output file format of the Kodak Cineon film scanner." This format was called Cineon. |
URLs
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• SMPTE 268M-2003, SMPTE Standard for File Format for Digital Moving-Picture Exchange (DPX), Version 2.0
URLs
• http://www.cineon.com/ff_draft.php
• DALSA press release on the Origin camera (http://www.dalsa.com/dc/news/news.asp?itemID=172)
• Output specifications for the DALSA Origin camera (http://www.dalsa.com/dc/design/output.asp)
• 4K Digital Capture and Postproduction Workflow (http://www.dalsa.com/dc/documents/4K_Workflow_Whitepaper_03-70-00209-00.pdf)
• Stream and File Formats: Where Are We Now? (http://www.broadcastpapers.com/asset/GVGStream+FileFormats.pdf)
• PRESTO report on file format conversion software (http://presto.joanneum.ac.at/Public/D5_3.pdf)
• Wikipedia entry on DPX (http://en.wikipedia.org/wiki/DPX)
• DPX and aspect ratio issues (http://www.geocities.com/Hollywood/Makeup/4303/dpx.html)
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