OpenEXR

Pixel data are stored as 16-bit or 32-bit floating-point numbers. With 16 bits, the representable dynamic range is significantly higher than the range of most image capture devices: 109 or 30 f-stops without loss of precision, and an additional 10 f-stops at the low end with some loss of precision. Most 8-bit file formats have around 7 to 10 stops. With 16-bit floating-point numbers, color resolution is 1024 steps per f-stop, as opposed to somewhere around 20 to 70 steps per f-stop for most 8-bit file formats. Even after significant processing (for example, extensive color correction) images tend to show no noticeable color banding. The 16-bit floating-point data format is fully compatible with the 16-bit frame-buffer data format used in some new graphics hardware. Images can be transferred back and forth between an OpenEXR file and a 16-bit floating-point frame buffer without losing data. Most of the data compression methods currently implemented in OpenEXR are lossless; repeatedly compressing and uncompressing an image does not change the image data. With the lossless compression methods, photographic images with significant amounts of film grain tend to shrink to somewhere between 35 and 55 percent of their uncompressed size. OpenEXR also supports lossy compression, which tends to shrink image files more than lossless compression, but doesn’t preserve the image data exactly. New lossless and lossy compression schemes can be added in the future. OpenEXR images can contain an arbitrary number and combination of image channels, for example red, green, blue, and alpha; luminance and sub-sampled chroma channels; depth, surface normal directions, or motion vectors. Multi-resolution images, often called “mipmaps” or “ripmaps”, are commonly used as texture maps in 3D rendering programs to accelerate filtering during texture lookup, or for operations like stereo image matching. Tiled multiresultion images are also useful for implementing fast zooming and panning in programs that interactively display very large images. Often it is necessary to annotate images with additional data; for example, color timing information, process tracking data, or camera position and view direction. OpenEXR allows storing of an arbitrary number of extra attributes, of arbitrary type, in an image file. Software that reads OpenEXR files ignores attributes it does not understand. In order to make writing and reading OpenEXR files easy, the file format was designed together with a C++ programming interface. Two levels of access to image files are provided: a fully general interface for writing and reading files with arbitrary sets of image channels, and a specialized interface for the most common case (red, green, blue, and alpha channels, or some subset of those). Additionally, a C-callable version of the programming interface supports reading and writing OpenEXR files from programs written in C. Many application programs expect image files to be scan line based. With the OpenEXR programming interface, applications that cannot handle tiled images can treat all OpenEXR files as if they were scan line based; the interface automatically converts tiles to scan lines. The C++ and C interfaces are implemented in the open-source OpenEXR library. The OpenEXR library supports multi-threaded reading or writing of an OpenEXR image file: while one thread performs low-level file input or output, multiple other threads simultaneously encode or decode individual pieces of the file. The OpenEXR file format is hardware and operating system independent. While implementing the C and C++ programming interfaces, an effort was made to use only language features and library functions that comply with the C and C++ ISO standards. Support for a new data type has been added: deep data. Deep images store an arbitrarily long list of data at each pixel location. This is different from multichannel or ‘deep channel images’ which can store a potentially large, but fixed, am