Logluv TIFF

Logluv TIFF is an encoding used for storing high dynamic range imaging data inside a TIFF image. It was originaly developed by Greg Ward for storing HDR-output of his Radiance-photonmapper in a time, where storage-space was a crucial factor. It's implementation in TIFF also allowed the combination with image-compression algorithms without great programming effort. As such it has to be considered a smart compromise between the imposed limitations. It is slightly related to RGBE, the most successfull HDRI storage format, an earlier invention of Greg Ward.

Details

Logluv TIFF's design solves two specific problems: storing high dynamic image data and doing so within a reasonable amount of space. Traditional image format generally stores pixel data in RGB-space occupying 24 bits, with 8 bits for each color component. This limits the representable colors to a subset of all visible and distinguishable colors, introducing quantization and clamping artifacts clearly visible to human observers. Using a triplet of floats to represent RGB would be a viable a solution, but it'd quadrupple the size of the file (occupying 32 bits for each color-component, as opposed to 8 bits).

Instead of using RGB, Logluv uses the CIELuv colorspace (with D65 whitepoint by default), which promises to distribute distinct colors (independent of its brightness or human observability) uniformly over two Chrominance components. As humans can't distinguish color in a very wide spectrum of possible colors, Logluv satisfies human observers with 8 bit on each of the U/V components. The Lightness component is then the most critical information-carrier — it has to suffice the requirements to store the high range offered by input-data, and is the component for which humans are the most sensitive. Logluv chooses a 16 bit presentation with base2-logarithmic scaling of the component (hence "Log"luv) enabling the representation of lightness values in the range of 38 aparture widths. The space occupied by one pixel is thus 32 bits (L16 + U8 + V8), marginally bigger than a standard 8 bit RGB-image.

Extension

In an attempt to prevent the expansion of data-size, Logluv comes in a 24 bit flavour, which in a rather complicated way quantizes Lightness to 10 bit and merges U/V into a 14 bit look-up based value.

Usage

Logluv TIFF has widespread use in HDRI applications such as IBL, image based lighting.

Reading and writing of Logluv TIFF images can be handled via LibTIFF. [ [http://www.remotesensing.org/libtiff/ LibTIFF Homepage] ] LibTIFF is freely available in both source and various binary packages for different platforms.

Resources

*HDRI, by Reinhard et. al has a discussion regarding Logluv Tiff in the 3rd chapter. [ [http://www.amazon.com/gp/product/0125852630 (Amazon Link) High Dynamic Range Imaging by Erik Reinhard, Greg Ward, Sumanta Pattanaik, Paul Debevec] ]
*For those looking for Logluv images, there are numerous example on Greg Ward Larson's page. [ [http://www.anyhere.com/gward/pixformat/tiffluv.html Greg Ward Larson on LogLuv Encoding for TIFF Images] ] [ [http://www.anyhere.com/gward/papers/jgtpap1.pdf LogLuv encoding for full-gamut, high-dynamic range images] ] [ [http://www.anyhere.com/gward/hdrenc/hdr_encodings.html A comparison of different HDR image encoding formats] ]

See also

*HDRI Image Format Comparison.
*Tagged Image File Format
*CIELUV color space

References