= Modify color output with look-up tables (LUTs) =

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.

tasks>>
    Use a look-up table to modify the colors of the viewer:
        * Click ((RMB)) the [Icon:IMAGE/lut_off] 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|Node:cop2/lookup] to filter an image
          sequence through a look-up table.

== How to create a look-up table ==

# In a compositing network, create a chain of color-correction
  nodes that do the color modification you want.

# Press ((RMB)) on the last node in the chain and choose __Save
  LUT__.

# 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.

# 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 2x2x2 to 256x256x256. 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.

# 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.

# 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.