Logluv
TIFF
Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by scanning, faxing, word process ...
is an encoding used for storing
high-dynamic-range imaging
In photography and videography, multi-exposure HDR capture is a technique that creates extended or high dynamic range (HDR) images by taking and combining multiple exposures of the same subject matter at different exposure levels. Combining mu ...
data inside a TIFF image. It was originally developed by Greg Ward for storing HDR-output of his
Radiance
In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiati ...
-photonmapper at a time where storage space was a crucial factor. Its 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 successful 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 solution, but it would quadruple the size of the file (occupying 32 bits for each color-component, as opposed to 8 bits).
Instead of using RGB, LogLuv uses the logarithm of the
luminance and the
CIELUV (''u''’, ''v''’)
chromaticity
Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called ...
coordinates in order to provide a
perceptually uniform
In color science, color difference or color distance is the separation between two colors. This metric allows quantified examination of a notion that formerly could only be described with adjectives. Quantification of these properties is of great ...
color space. LogLuv allocates 8 bits for each of the ''u''’ and ''v''’ coordinates, which allows encoding the full visible
gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain ''complete subset'' of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circ ...
with imperceptible step sizes. In order to provide the required high dynamic range with imperceptible luminance steps, LogLuv uses 16 bits to encode a
fixed-point base 2 logarithm of the luminance, which allows an
EV range of nearly 128 stops. 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 Image-based lighting (IBL) is a 3D rendering technique which involves capturing an omnidirectional representation of real-world light information as an image, typically using a 360° camera. This image is then projected onto a dome or sphere analog ...
.
Reading and writing of Logluv TIFF images can be handled via LibTIFF. 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.
* For those looking for Logluv images, there are numerous example on Greg Ward Larson's page.
A comparison of different HDR image encoding formats
/ref>
See also
* Tagged Image File Format
Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by scanning, faxing, word process ...
* RGBE image format
RGBE or Radiance HDR is an image format invented by Gregory Ward Larson for the Radiance rendering system. It stores pixels as one byte each for RGB (red, green, and blue) values with a one byte shared exponent. Thus it stores four bytes per pixe ...
* JPEG-HDR
JPEG XT (ISO/IEC 18477) is an image compression standard which specifies backward-compatible extensions of the base JPEG standard (ISO/IEC 10918-1 and ITU Rec. T.81).
JPEG XT extends JPEG with support for higher integer bit depths, high dynami ...
* CIELUV
References
{{DEFAULTSORT:Logluv Tiff
Raster graphics file formats
High dynamic range file formats
Photographic techniques