Windows Media Photo

masonchung · 發表於 2007-1-13 18:57:53

微軟再出新招　Windows Media Photo欲取JPEG江山而代之
http://tech.digitimes.com.tw/ShowNews.aspx?zCatId=414&zNotesDocId=0000034332_B4B3HW4LH128MEI3CZKH9

Windows Media Photo (WMPhoto) 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. Note, this format has now been renamed from Windows Media Photo to HD Photo, although some already released products will continue to use the old name until updated.

Description
Windows Media 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 veritical 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.

Compression algorithm
Windows Media 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. Windows Media Photo processes images in 16x16 macroblocks.

WMPhoto 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 Windows Media 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".

Container format
WMPhoto uses a TIFF-like file container to store image data in a table of Image File Directory (IFD) tags. A WMPhoto file contains image data, an optional alpha channel data, WMPhoto 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 WMPhoto specification[1] "will be addressed in a future update".

[ 本帖最後由 masonchung 於 2007-1-20 02:42 PM 編輯 ]

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