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IDTs: What Do They Do?

An ACES Input Transform (or Input Device Transform [IDT]) processes original camera data in the form of non-color-rendered RGB image values from a captured scene lit by an assumed illumination source (the scene adopted white) and converts them to ACES RGB relative exposure values in a scene linear color space.

Because camera manufacturers tend to know their cameras best, they’re almost always responsible for developing the Input Transform for their devices. In some circumstances where a camera manufacturer has not released an Input Transform, user-derived Input Transforms and have helped fill the void. IDT (remember, this is the reverse engineering step to get back to scene referred linear data) and the IDT is written using CTL programming language.  In ACES, cameras have IDTs (and possibly multiple IDTs for different lighting – tungsten, daylight, etc)

Camera system vendors are recommended to provide two IDTs for each product, one optimized for CIE Illuminant D55 (daylight) and a second optimized for the ISO 7589 Studio Tungsten illuminant. However more accurate IDT’s under various common lighting conditions used in cinematographic practice is very beneficial to the development and implementation of ACES and is highly encouraged.

An IDT is expected to be applied after the following types of initial processing:

  • dark frame subtraction
  • noise reduction
  • flat fielding
  • demosaicing
  • conversion to a device-specific RGB color space and/or deblurring

If linearization or white balance takes place prior to one of the processes above, then that linearization or white balance shall be excluded from the IDT.

Once an ACES IDT is used, color must be rendered using the ACES RRT and ODT appropriate for the display used for critical color evaluation.