Demystify - OpenEXR & ACES

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OpenEXR in brief :

Color management and color workflows have evolved over many years. Cineon is such a backbone of the industry but it is now over 18 years old and it is time we moved to a more robust, flexible and accurate system.

While Cineon and DPX are successfully used daily in production, a new format appeared at the start of the new millennium, OpenEXR. 

OpenEXR was created by Industrial Light and Magic (ILM) in 1999 and released to the public in 2003. ILM developed the OpenEXR format in response to the demand for higher color accuracy and control in effects. OpenEXR is an open format, not tied to any one manufacturer or company and it is remarkable in several ways.

Key features :

  • Higher dynamic range and color precision than existing 8- and 10-bit image file formats.
  • Support for 16-bit floating-point, 32-bit floating-point, and 32-bit integer pixels. The 16-bit floating-point format, called "half", is compatible with the half data type in NVIDIA's Cg graphics language and is supported natively on their new GeForce FX and Quadro FX 3D graphics solutions.
  • Multiple image compression algorithms, both lossless and lossy. Some of the included codecs can achieve 2:1 lossless compression ratios on images with film grain. The lossy codecs have been tuned for visual quality and decoding performance.

The first movies to employ OpenEXR were Harry Potter and the Sorcerers Stone, Men in Black II, Gangs of New York, and Signs. Since then, OpenEXR has become ILM's main image file format.

ACES in Brief :

Academy Color Encoding System (ACES) is a color image encoding system proposed by the Academy of Motion Picture Arts and Sciences that will allow for a fully encompassing color accurate workflow, with "seamless interchange of high quality motion picture images regardless of source.

Key features :

  • ACES utilizes a file format that can encode the entire visible spectrum in 30 possible stops of dynamic range.
  • To create a future-proof “Digital Source Master” format in which the archive is as good as the source.
  • Color space greater than the gamut of the human eye
  • 16-bit color bit depth (floating point)

Each camera will be different, each camera has to be different, but it should be possible to publish a common high end target and then get each of the camera makers to allow you to render or convert your images into that one well understood common space. Each camera company will need to understand their own cameras to make the conversion, and clearly even in this common place some source material will be higher or lower resolution, noisier or quieter, with more or less dynamic range, but that once we are in this common place all files are scene linear. Their idea of the color blue is not influenced by how it might be projected, this color blue will not be clipped or restrained by a small gamut. In fact, the ACES / IIF colorspace gamut is enormous. And this 'ACES thing' will sit in a slightly restrained version of an OpenEXR file - so we don't all have to rebuild every piece of software we have.

Once we have graded, comped, matched or image processed our film in ACES we then need to get it out of this sort of utopian wonderland of flexibility and actually target it for a cinema, or HD monitor or whatever and thus the Academy have also drafted a spec for what is called a renderer which will convert the files back to destination referenced imagery - for example standard Rec 709.  It is important to note this final conversion is again standardized. Each company does not get to render its solution back to a slightly different creatively different version. One conversion will match another, even when done on different boxes.