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JPEG XR

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这是本页的一个历史版本，由Sterrys（留言 | 贡献）在2007年4月20日 (五) 08:21 （→许可）编辑。这可能和当前版本存在着巨大的差异。

HD Photo
扩展名
.wdp/.hdp
互联网媒体类型
image/vnd.ms-photo
开发者Microsoft
格式类型图形文件格式

HD Photo (旧称Windows Media Photo)是一种连续色调静止图像压缩算法和文件格式，由Microsoft开发，属于Windows Media家族的一部分。它支持有损数据压缩以及无损数据压缩，并且是微软的XPS文档的首选图像格式。它以前在内部被称为photon。目前其软件支持并不广泛，但是.NET Framework 3.0已经提供了它的官方编码解码器。

描述

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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 also supports multiple images per file. 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.

压缩算法

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HD Photo uses a reversible color space conversion, a reversible lapped biorthogonal transform and a non-arithmetic entropy encoding scheme, which is very efficient in preserving high frequency image data. This makes the algorithm retain a higher image quality at high compression ratios. The transform operation needs 3 multiply and add operations and 7 add or shift operations at the highest quality level and in the highest performance mode, 1 multiply and add operation and 4 add or shifts are required per pixel, giving the codec a high performance. HD Photo processes images in 16x16 macroblocks.

HD Photo supports various color formats and multiple fixed and floating point numerical representation, thus giving a wide range of compression options. To remain compatible across various devices, it differentiates Basic and Advanced formats. While digital photography needs are satisfied by the Basic format, 3D rendering or advanced image processing scenarios require the Advanced format.

Microsoft claims that HD Photo offers a "perceptible image quality comparable to JPEG 2000 with computational and memory performance more closely comparable to JPEG and delivers a lossy compressed image of better perceptive quality than JPEG at less than half the file size, and that the lossless compression compresses images 2.5 times".

容器格式

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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 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 inadequacies of the TIFF format including the 4GB file-size limit, which according to the HD Photo specification[1] "will be addressed in a future update".

许可

微软公司拥有 HD Photo 技术的专利，微软公司向外界提供一个附带授权协议的“HD Photo Device Porting Kit 1.0”以鼓励人们使用这种产品。但是，这个协议的条款特别注明禁止将其用于任何开放源代码的系统。[2]

引用

  1. ^ Windows Media Photo Specification. [2006-06-29]. 
  2. ^ HD Photo Device Porting Kit 1.0. Microsoft, Inc. 12/21/2006. 

参见

外部链接


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