The Floating Point Engine

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The Floating-Point Engine

RawTherapee performs all calculations in 32-bit floating point precision (in contrast to 16-bit integer as used in many other converters such as dcraw and also in RawTherapee up to version 3.0).

Classical converters work with 16-bit integer numbers. A pixel channel has values ranging from 0-65535 in 16-bit precision (to increase precision, converters usually multiply the 12- or 14-bit camera values to fill the 16-bit range). The numbers have no fractions, so for example there is no value between 102 and 103. In contrast, floating point numbers store a value at a far wider range with a precision of 6-7 significant digits. This helps especially in the highlights, where higher ranges can be recovered. It allows intermediate results in the processing chain to over- or undershoot temporarily without losing information. The fraction values possible also help to smooth color transitions to prevent color banding.

The downside is the amount of RAM that floating point numbers require, which is exactly twice that of 16-bit integer. Together with the ever-increasing megapixel count of digital cameras, a 32-bit operating system can quite easily run out of memory and cause RawTherapee to crash. Therefore a 64-bit operating system is highly recommended for stability.

We officially ended support for 32-bit versions of RawTherapee with release 5.0-r1 in February 2017. Do not file bug reports regarding issues on 32-bit systems.

If you nevertheless need to use RawTherapee on a 32-bit system, the following will help make the most of it:

  • Use 4-Gigabyte Tuning in Windows. See "4-Gigabyte Tuning: BCDEdit and Boot.ini" for an explanation of what it is, and find out how to do it by reading the guide "How to set the /3GB Startup Switch in Windows XP and Vista".
  • Close other programs while working in RawTherapee.
  • Use a single Editor tab.
  • Turn off "auto-start" in the Queue. Add photos to the Queue as usual. When ready to start processing them, restart RawTherapee to free up RAM (no image open in the Editor), and start the queue.
  • Ensure that RawTherapee does not load dark-frame or flat-field images if you do not use them.
  • Avoid having more than a few hundred photos per folder, as each photo requires a little RAM (thumbnail, embedded ICC profile, etc.).

Memory Requirements

To open an image in the Editor, RawTherapee 5.6 needs very roughly this much RAM, in bytes:

  • Non-raw
    • 8-bit: (width * height * 3) + (width * height * 4) + (previewWidth * previewHeight * 28)
    • 16-bit: (width * height * 3 * 2) + (width * height * 4) + (previewWidth * previewHeight * 28)
    • 32-bit: (width * height * 3 * 4) + (width * height * 4) + (previewWidth * previewHeight * 28)
  • Raw
    • (width * height * 4) + (width * height * 4) + (width * height * 12) + (previewWidth * previewHeight * 28)

Some overhead memory is additionally required, for example for generating thumbnails of other images which reside in the opened image's folder.

The memory requirement for processing and saving an image depends on what tools you use and can vary significantly from the above - the above pertains only to opening an image.