HD Photo (HD Photo Microsoft, .wdp, .hdp)

HD Photo (formerly Windows Media Photo) is a still-image compression algorithm and file format for continuous tone photographic images, developed by Microsoft as a part of the Windows Media family. It supports lossy as well as lossless compression, and is the preferred image format for Microsoft's XPS documents. It was previously known internally as Photon. Software support for the format is not widespread as of early 2007; however, official managed code and unmanaged code implementations of the codec are available as part of .NET Framework 3.0 and Windows Imaging Component respectively. Both components are part of Windows Vista and are available for Windows XP.

HD Photo has been announced by Microsoft and the Joint Photographic Experts Group to be under consideration for a JPEG standard, tentatively titled JPEG XR.

HD Photo is an image codec that gives a high-dynamic-range image encoding while requiring only integer operations (with no divides) for both compression and decompression. It supports monochrome, RGB, CMYK and even n-channel color representation, using up to 16-bit unsigned integer representation, or up to 32-bit fixed point or floating point representation, and also supports RGBE Radiance. It may optionally include an embedded ICC color profile, to achieve consistent color representation across multiple devices. An alpha channel may be present for transparency, and Exif and XMP metadata formats are supported. The format allows decoding part of an image, without decoding the entire image. Full decoding is also unnecessary for certain operations such as cropping, downsampling, horizontal or vertical flips, or cardinal rotations.

All color representations are transformed to an internal color representation. The transformation is entirely reversible, so, by using appropriate quantizers, both lossy and lossless compression can be achieved.

HD Photo uses a TIFF-like file container to store image data in a table of Image File Directory (IFD) tags. An HD Photo file contains image data, an optional alpha channel data, HD Photo metadata, optional XMP metadata stored as RDF/XML, and optional Exif metadata, in IFD tags. The image data is a contiguous self-contained chunk of data. The optional alpha channel, if present, is compressed as a separate image record, enabling decoding of the image data independently of transparency data in applications which do not support transparency.

Being TIFF-based, this format inherits all of the limitations of the TIFF format including the 4 GB file-size limit, which according to the HD Photo specification "will be addressed in a future update".