LogLuv
TIFF
Tag Image File Format or Tagged Image File Format, commonly known by the abbreviations TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is w ...
is an encoding used for storing
high-dynamic-range imaging
High dynamic range (HDR), also known as wide dynamic range, extended dynamic range, or expanded dynamic range, is a signal with a higher dynamic range than usual.
The term is often used in discussing the dynamic ranges of images, videos, audio or ...
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 ...
physically-based rendering system 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
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls wit ...
and the
CIELUV
In colorimetry, the CIE 1976 ''L''*, ''u''*, ''v''* color space, commonly known by its abbreviation CIELUV, is a color space adopted by the International Commission on Illumination (CIE) in 1976, as a simple-to-compute transformation of the 1931 ...
(''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 alte ...
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 and colorimetry, a gamut, or color gamut , is a convex set containing the colors that can be accurately represented, i.e. reproduced by an output device (e.g. printer or display) or measured by an input device (e.g. cam ...
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
Lightness is a visual perception of the luminance (L) of an object. It is often judged relative to a similarly lit object. In colorimetry and color appearance models, lightness is a prediction of how an illuminated color will appear to a stand ...
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. Sample images are linked at the end of Greg Ward's paper
on the encoding.
See also
*
Tag Image File Format
Tag Image File Format or Tagged Image File Format, commonly known by the abbreviations TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is w ...
*
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 pix ...
*
JPEG-HDR
*
CIELUV
In colorimetry, the CIE 1976 ''L''*, ''u''*, ''v''* color space, commonly known by its abbreviation CIELUV, is a color space adopted by the International Commission on Illumination (CIE) in 1976, as a simple-to-compute transformation of the 1931 ...
Footnotes
References LibTIFF test imagesArchived
at archive.org.
Unlocked version of ''Journal of Graphics Tools'' article.
{{DEFAULTSORT:Logluv Tiff
Raster graphics file formats
High dynamic range file formats
Photographic techniques