The following pictures show the same OpenEXR image; yet
the amount of detail you can see when comparing the three is
substantially different. The view out the window is obscured
by the bright sunlight, although there is, in fact, a
building next door with a rust-colored roof; the sky is
blue, and there are even power lines visible above the
building. Using the Viewer, you can adjust the amount of
exposure on the fly to see more details emerge, as the
following images show.
ILM developed the OpenEXR format in response to the demand for higher color
fidelity in the visual effects industry. When the project began in
2000, ILM evaluated existing file formats, but rejected them for
- 8- and 10-bit formats lack the dynamic range necessary to store
high-contrast images captured from HDR devices.
- 16-bit integer-based formats typically represent color component
values from 0 ("black") to 1 ("white"), but don't account for
over-range values (e.g., a chrome highlight) that can be captured by film
negative or other HDR devices. For images intended only for
display or print reproduction, clamping at "white" may be sufficient;
but for image processing in a visual effects house, highlights often
need to be preserved in the image data. Preserving over-range values in
the source image allows an artist to change the apparent
exposure of the image with minimal loss of data, for example.
- Conversely, 32-bit floating-point TIFF is often overkill for visual
effects work. 32-bit FP TIFF provides more than sufficient precision
and dynamic range for VFX images, but it comes at the cost of storage,
both on disk and in memory. When creating background plates for VFX
work, film is usually scanned in at 2k pixels wide or more, and
24p HD images are 1920x1080, so background plates are already quite
large relative to "typical" digital images.
ILM decided to develop a new HDR file format with
16-bit floating-point color component values. Since the IEEE-754
floating-point specification does not define a 16-bit format, ILM
created the "half" format. Half values have 1 sign bit, 5 exponent
bits, and 10 mantissa bits. For linear images, this format provides
1024 (210) values per color component per f-stop, and 30 f-stops (25 - 2),
with an additional 10 f-stops with reduced precision at the
low end (denormals).
The half format supports denormalized numbers, positive and negative
infinities, and NaNs. It is identical to the half data type in
NVIDIA's Cg graphics language, allowing a developer to process values
from an OpenEXR image directly on current NVIDIA GPUs such as the GeForce FX family.
In addition to the half data type, OpenEXR supports 32-bit unsigned
integer and 32-bit floating-point data types. OpenEXR images can have
an arbitrary number of channels, each with a different data type.
The current release of OpenEXR supports several lossless compression
methods, some of which can achieve compression ratios of about 2:1 for
images with film grain. OpenEXR is extensible, so developers can easily
add new compression methods (lossless or lossy).
OpenEXR images can be annotated with an arbitrary number of attributes, e.g., with color balance information from a camera.
OpenEXR images are able to support stereoscopic workflows; version 1.7.0 has support for multiple views resident in the image files.
OpenEXR v2 introduces support for storing "Deep Data" buffers wherein pixels are capable of holding variable number of data samples, particularly as pertaining to depth. The implementation is capable of both hard surface and volumetric representation requirements for deep compositing workflows. OpenEXR v2 also introduces the concept of "Multi-Part" files that contain a number of separate, but related, images in one file. Access to any part is independent of the others.
Please see the documentation pages for further detail.
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