From VFXWiki
[edit] File LUT [FLU]
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The File LUT tool applies a look up table (LUT) to the image. Unlike the Color Curves tool it does not use a spline-based LUT. Instead it loads the LUT from a file stored on the system or network.
This approach has two advantages. The first is that the only part of the LUT stored in the composition is the path to the file. Since LUT files can be very large, this can dramatically reduce the file size of a composition when several LUTs are present. The second advantage is that it becomes possible to adjust all File LUT tools using the same file at the same time, simply by changing the contents of the LUT. This can be useful when the same LUT-based color correction is applied in many different compositions.
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[edit] Controls Tab
LUT File
Use this control to select the path to the file describing the LUT. Currently this tool supports LUTs exported from Fusion using the ALUT format. The tool will fail if it is unable to find or load the specified file.
Color Space
Use this control to change the color space the LUT is applied in. The default is to apply the curves described in the LUT to the RGB color space, but options for YUV, HLS, HSV and others are also available.
Pre-Divide/Post Multiply
Selecting the Pre-Divide/Post-Multiply checkbox will cause the image pixel values to be divided by the alpha values prior to applying the LUT, and then re-multiplied by the alpha value after the correction.
This helps to prevent the creation of illegally additive images, particularly around the edges of a blue/green key or when working with 3D rendered objects.
