Houdini 16.5 Compositing

Modify color output with look-up tables (LUTs)

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Display look-up tables (LUTs) modify the colors of an image before displaying it in the viewer. You apply the look-up table to a viewport, and the viewport uses the table to modify the colors of the images the viewport displays.

The most common use of look-up tables is to see how images will look in different media, such as TV/video and film, that have different color capabilities. Using a look-up table designed to mimic a certain display medium will give you a much better idea what your current work will look like after it’s transferred to that medium.

To...Do this

Use a look-up table to modify the colors of the viewer

  • Click the icon on the compositor control toolbar to open a pop-up menu. You can choose to load a LUT or enable/disable the current LUT.

Apply a look-up table as part of a compositing network

  • Use the Lookup COP to filter an image sequence through a look-up table.

Add a new look-up format to Houdini

  • See "The Houdini LUT format and adding new LUT formats" in the HDK documentation (Getting Data In and Out of Houdini, Lookup Tables: hdk_io_lut.html).

How to create a look-up table

  1. In a compositing network, create a chain of color-correction nodes that do the color modification you want. Color correction nodes appear blue when placed by default.

  2. Press on the last node in the chain and choose Save LUT.

  3. In the LUT File field, type the name of the LUT file, or click the + icon to use the save dialog.

    Open the arrow pop-up menu at the right end of the field to choose the format to save the LUT in. .lut is Houdini’s native ASCII look-up table format. .blut is the binary form of the .lut format, which is smaller but cannot easily be edited after saving.

  4. Depending on the operators in the color-correction chain, the result LUT may be channel independent (it can operate on single channels without having to look at the others), or channel dependent (it must look at the values of all channels to work). Channel-independent operations produce a 1-dimensional look-up table. Channel-dependent operations produce a 3-dimensional "look-up cube".

    If your node chain produced a channel-independent, 1D LUT, you can set the Length field. Higher numbers give more accurate color transformations, but large LUT lengths (> 1000) are slower.

    If your node chain produced a channel-dependent, 3D LUT, you can set the Size fields, from 2×2×2 to 256×256×256. Higher numbers give more accurate color transformations, but large cubes are slower.

    For 3D LUTs, you should use the .blut format to save space. 1D LUTs are typically 1-2 KB, while 3D LUTs can be several megabytes.

  5. Choose whether the sampling of the LUT should be linear or logarithmic (log). Log is used with high dynamic range images (HDR images) for more accurate color correction, but log LUTs are slower to apply than linear LUTs. When dealing with 8b images, linear is preferred.

  6. Set the Range. This is the brightness range within which the look-up table will apply (0 = black, 1 = white). Pixels outside this range are clamped. Logarithmic LUTs can only have positive values for both the start and end range values.

  7. You can turn on Clamp LUT to clamp the look-up table’s output to a given range. Clamping to 0,1 will exclude any values (black less than 0, white greater than 1) that cannot be displayed on a monitor.

How to set up a LUT

The easiest way to set up the LUTs in all the spots you want it is from the Edit > Color Settings > Color Correction tab in Houdini. You can apply the LUT to as many of the contexts as you wish, including gamma. To preserve this between sessions, choose Save As Default.

For just a straight MPlay session loading files, you can use a variety of methods:

  1. Use the following command.

    setenv HOUDINI_IMAGE_DISPLAY_LUT /show/job/display.lut
    
  2. Use the -d <lutfile> command line option and alias MPlay to mplay -d <lut>.

  3. Open the display options in MPlay (d), switch to the Correction tab, enter the LUT in the Display LUT, and click Save as Default.

Finally, to set the input range of the LUT, change the From: line to your range (ie, 0-13.5).

Note

The To: range should stay 0-1 for a display LUT. It is only used as a post-scale to compress HDR images into the 0-1 range if not set to 0-1.

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