Logluv TIFF
Encyclopedia
Logluv TIFF is an encoding used for storing high dynamic range imaging
data inside a TIFF image. It was originally developed by Greg Ward for storing HDR-output of his Radiance
-photonmapper in 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.
and clamping
artifacts clearly visible to human observers. Using a triplet of floats to represent RGB would be a viable a 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 CIELUV color space (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 Logluv) enabling the representation of lightness values in the range of 38 aperture widths. The space occupied by one pixel is thus 32 bits (L16 + U8 + V8), marginally bigger than a standard 8 bit RGB-image.
applications such as IBL, image based lighting
.
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.
High dynamic range imaging
In image processing, computer graphics, and photography, high dynamic range imaging is a set of techniques that allows a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods...
data inside a TIFF image. It was originally developed by Greg Ward for storing HDR-output of his Radiance
Radiance (software)
Radiance is a suite of tools for performing lighting simulation originally written by Greg Ward. It includes a renderer as well as many other tools for measuring the simulated light levels. It uses ray tracing to perform all lighting calculations, accelerated by the use of an octree data structure...
-photonmapper in 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
RGBE image format
RGBE is an image format invented by Gregory Ward Larson. It stores pixels as one byte each for RGB values with a one byte shared exponent. Thus it stores four bytes per pixel....
, 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 quantizationQuantization (image processing)
Quantization, involved in image processing, is a lossy compression technique achieved by compressing a range of values to a single quantum value. When the number of discrete symbols in a given stream is reduced, the stream becomes more compressible. For example, reducing the number of colors...
and clamping
Clamping (graphics)
In computer graphics, clamping is the process of limiting a position to an area. Unlike wrapping, clamping merely moves the point to the nearest available value.To put clamping into perspective, pseudocode for clamping is:...
artifacts clearly visible to human observers. Using a triplet of floats to represent RGB would be a viable a 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 CIELUV color space (with D65 whitepoint
Color temperature
Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, and other fields. The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of...
by default), which promises to distribute distinct colors (independent of its brightness or human observability) uniformly over two Chrominance
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
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
Lightness (color)
Lightness is a property of a color, or a dimension of a color space, that is defined in a way to reflect the subjective brightness perception of a color for humans along a lightness–darkness axis. A color's lightness also corresponds to its amplitude.Various color models have an explicit term for...
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 Logluv) enabling the representation of lightness values in the range of 38 aperture 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 HDRIHigh dynamic range imaging
In image processing, computer graphics, and photography, high dynamic range imaging is a set of techniques that allows a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods...
applications such as IBL, image based lighting
Image based lighting
Image-based lighting is a 3D rendering technique which involves plotting an image onto a dome or sphere that contains the primary subject. The lighting characteristics of the surrounding surface are then taken into account when rendering the scene, using the modeling techniques of global...
.
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.