HD Photo
From ProPHOTO WIKI
Contents |
Introduction
HD Photo is an image file format created by Microsoft to address many of the shortcomings inherent in existing formats. The intention of this standard is to provide a mainstream file format that can be used throughout the entire photographic workflow. With lossless compression and native HDR support, HD Photo can provide many of the advantages of proprietary RAW formats. At the same time, it's new compression mechanism allows smaller file sizes than JPEG so it is also suitable for transmission over limited bandwidth networks.
Compression
Improved Efficiency
Using a new compression engine, HD Photo provides compression that is approximately twice as efficient as the mechanism used in conventional JPEG files. As such, an image stored in this format will consume approximately half of the space of a JPEG file with similar quality. Naturally, this can also be used to save higher quality versions of the image using the same amount of space.
Less Noticable Artifacts
In addition to the inherent advantages that more efficient compression provides, HD Photo was explicitly designed to circumvent many of the more obvious artifacts generated by other lossy compression formats. As such, any artifacts produced during the compression stages will often be less noticeable to a person viewing the images.
Lossless Compression
In addition to conventional lossy compression, HD Photo offers the ability to save images in a lossless mode that does not have any impact on the quality of the image. While not nearly as compact as conventional compression, this allows the format to be used for intermediate processing steps where generation loss is a potential concern. File size will depend on the contents of the image, however performance with typical photographs will result in compression ratios of approximately 2.5:1.
Additionally, unlike other formats, the lossless mode used in HD Photo uses the same algorithm as the lossy modes. As such, support for losslessly compressed images will be available in all software capable of using the HD Photo format.
HDR Imaging
One of the key advantages to this file format is its ability to store image data using non-integer methods. Using these mechanisms allows the format to significantly increase the dynamic range and gamut that can be stored within the image file. As such, many common editing tasks can be performed on the images without degrading the quality of the data stored within them.
Images stored in these modes will be loaded as HDR (High Dynamic Range) images in many common image processing packages (including Photoshop CS2). This allows software to take full advantage of this capability and maintain the integrity of the image data stored in these files between editing. Please see the article on HDR for more information on this topic.
scRGB Colour Space
The HD Photo format makes use of the new scRGB colour space in its default configuration. Designed specifically for use with high bit-depth or HDR images, the scRGB space provides a gamut much larger than any conventional space (including AdobeRGB, ProPhotoRGB and sRGB). Further, as scRGB's gamut also exceeds the range of colours that the human eye can see it should provide sufficient headroom for any future uses.
Additionally, unlike other common options, scRGB is a linear colour space (ie using a gamma of 1.0) so it matches up well with data generated by camera RAW files. This allows gamma correction to be left until the final stages of the imaging workflow and can prevent artifacts generated by multiple conversions.
Naturally, HD Photo does allow ICC profiles to be embedded in image files so users can elect to use other colour spaces if desired. As such, while scRGB is the default photographers can elect to use other colour spaces if desired.
Comparison to RAW Formats
As HD Photo was designed as an end-to-end format for storing image files, it provides some of the advantages traditionally the soul domain of RAW files. The combination of lossless compression, high bit-depth/HDR modes and a linear colour space allows the same sort of leeway with white balance and exposure correction in post processing. As such, for many photographers this format may be a suitable alternative to shooting proprietary RAW files.
Naturally, as the image data must be run through a demosaicing process prior to being stored in this format it isn't a direct replacement for RAW files. With that said, however, HD Photo does offer a suitable middle ground that may be more than sufficient for many people. Of course, this is contingent on support for the HD Photo format within digital cameras themselves.
Metadata
As with most other image formats, HD Photo supports a variety of different mechanisms for storing metadata within the images. In addition to the basic EXIF data used by cameras, advanced metadata frameworks such as XMP are part of the basic standard. As such, HD Photo contains the resources necessary to work within sophisticated workflow systems that depend on this data.
Availability
Native Support in Windows Vista
Windows Vista provides native support for the HD Photo format, so many of the programs provided with the OS can open images in this format. Further, Windows Vista ships with the necessary APIs to load and process these images so it is relatively easy for third party software developers to add support for this format.
Photoshop Plug-ins
Plug-ins will be made available in the near future that will provide users with the ability to load and save these files in Photoshop CS2 and CS3 on both Windows (XP and Vista) and OS X (Universal Binary) platforms. These plug-ins provide full access to the functionality provided by this file format, including the ability to save HDR images from within Photoshop itself.
Device Porting Kit
The specifications for this file format are available on a royalty free basis so any developer can make use of this format on any platform they would like. In addition, reference source code and developer tools are also provided in this kit in order to help simplify the process of adding support.
Digital Cameras
While no cameras currently support this format, HD Photo was explicitly designed with this type of device in mind. The compression algorithm used requires only limited processing power and buffer capacity, helping to simplify the design of ASICs supporting it. Additionally, much of the groundwork has been done for vendors so if they elect to add support for the format it should be relatively simple to do so. Support for HD Photo inside of cameras will be key to its success, so this will be something to watch over the coming months.
JPEG XR
In late 2007, the Joint Photographic Experts Group approved the creation of a new format called JPEG XR that is based on Microsoft's HD Photo specification. Further work is required to formalize the new standard, however it will ultimately be nearly identical to the HD Photo format discussed here.
See Also
- JPEG - The most commonly used file format for digital imaging.
- scRGB - Default colour space used by the HD Photo format.
External Links
- HD Photo Information - Information about HD Photo on Microsoft's website.
- HD Photo Specification - Detailed specifications for the HD Photo format.
- HD Photo Device Porting Kit - Kit containing resources required to add support for the HD Photo format on other devices.
- Press Release - Press release covering the formal release of the HD Photo format.
- Bill Crow's HD Photo Blog - Blog written by Microsoft's program manager for HD Photo.
