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How is color lookup table used to represent color?

By: Author Color With Leo

Color lookup tables (LUTs) are commonly used in computer graphics and image processing to map pixel values to desired output colors. LUTs provide a way to transform input values into specific color outputs and are a fundamental tool for controlling and adjusting color in digital images.

What is a Color Lookup Table (LUT)?

A color lookup table, or LUT, is essentially a mapping table that relates input values to output colors. The table consists of a predefined array that stores RGB or other color values that are accessed using an index. The input value serves as the index to the table, which then returns the corresponding color value from the table.

For example, a simple 8-bit color LUT could have 256 entries, with each entry containing a RGB color value. An input pixel value from 0 to 255 would be used to index into the table and retrieve the associated RGB color. This allows for rapid mapping of pixel values to colors using simple array indexing.

Why Use a Color LUT?

There are several reasons color LUTs are commonly used in graphics and imaging applications:

  • Speed – LUTs provide extremely fast color transformations since they only require array indexing instead of complex calculations.
  • Control – LUTs offer fine-grained control over color mapping, allowing arbitrary input-to-output color mappings.
  • Color correction – LUTs can be used to implement complex color grading and correction by modifying the mapping from pixel values to output colors.
  • Standardization – LUTs provide a standardized way of defining color transformations across devices and software.
  • Compact representation – Large color mappings can be represented in compact LUT data structures.

In summary, LUTs allow fast color mappings, provide color control, enable color corrections, offer standardization, and compactly define complex color transformations. This makes them a versatile tool for color management in graphics applications.

LUT Terminology

There are some common terms used when working with color LUTs:

  • Dimensions – The number of input and output channels in a LUT. For example, a 3D LUT has red, green, and blue input channels.
  • Grid size – The number of entries along each LUT axis. A 17x17x17 3D LUT has 17 entries for R, G, and B channels.
  • Addressing – The method used to index into the LUT array using the input values.
  • Interpolation – Estimating in-between LUT values by interpolating from known entries.
  • LUT format – The data format used to store the LUT data, such as .CUBE or .ICC profile.

Types of Color LUTs

There are several common types and formats of color LUTs:

1D LUT

  • Maps a single input channel to an output channel
  • Often used for gamma correction or tonal adjustments
  • Example: 256 entry grayscale pixel value to RGB mapping

3D LUT

  • Maps a 3-channel input (usually RGB) to a 3-channel output
  • Allow full color transformations and adjustments
  • Commonly 33x33x33 or 17x17x17 grids
  • Stored in formats like .CUBE or ICC profiles

.CUBE LUT

  • Standard 3D LUT format developed by Kodak
  • Text-based format with header and table data
  • Widely supported in apps like Photoshop, After Effects, Premiere

ICC Profile

  • Industry standard color management system format
  • Contains profiles for input devices, display devices, color spaces
  • Can contain multi-dimensional LUTs
  • Used to match colors across different devices/software

Applying Color LUTs

Color LUTs can be applied in several ways:

Direct Indexing

  • Use input pixel value directly as index into LUT array
  • No interpolation, fast operation suitable for real-time use
  • Can result in color banding if LUT resolution is low

Interpolated Indexing

  • Input values are interpolated between LUT entries
  • Higher quality but requires more computation
  • Reduces color banding artifacts
  • Trilinear interpolation is commonly used for 3D LUTs

Shader LUT

  • LUT is converted into a shader code fragment
  • Allows GPU hardware acceleration of LUT transformations
  • Used in real-time rendering to apply LUTs efficiently

Post-processing

  • LUT is applied as a post-process after image rendering
  • Flexible but can be slower if not hardware accelerated
  • Allows LUTs to be applied optionally in workflows

The optimal LUT application depends on the specific requirements of the workflow and application.

Workflow Uses of LUTs

Some common uses of LUTs in media production workflows include:

Camera Log to Display Mapping

  • Map camera log color values to display output color space
  • Match on-set look with post-production editing/grading
  • Standard LUTs used for common camera formats like ARRI LogC, RED LogFilm, etc.

Look Up Tables (LUTs)

  • Creative color transformations for video grading
  • Emulate the “look” of different films and camera formats
  • Quickly test different color treatments in editing

Technical LUTs

  • Transform between different color spaces like sRGB, Rec.709, DCI-P3, etc.
  • Match color spaces for compositing CGI and live footage
  • Standardize image colorimetry across a workflow

HDR LUTs

  • Map HDR video to standard dynamic range for monitoring
  • Tone mapping to avoid clipping bright highlights
  • Convert between HDR standards like PQ, HLG, HDR10, etc.

Advantages of LUTs

Some key advantages of using LUTs include:

  • Speed – LUTs provide fast color transformations, ideal for real-time processing
  • Creative control – Enable detailed color adjustments and stylized looks
  • Consistency – Help standardize color across devices and software
  • Portability – LUTs can be shared and applied across workflows
  • Compact format – Large color mappings defined with small data sizes
  • Reusable – LUTs can be reused and modified for new projects

By leveraging the color manipulation capabilities and speed of LUTs, workflows can achieve greater creative flexibility with optimized color processing.

Limitations of LUTs

However, there are some limitations to consider when using LUTs:

  • Information loss – Converting complex color data into compact LUTs can discard original data
  • Banding artifacts – Insufficient LUT resolution can introduce visible color banding
  • Unrealistic transformations – Extreme LUT adjustments may produce unrealistic looking results
  • Workflow mismatches – LUTs designed for a different workflow may not give ideal results
  • Rigidity – LUT transforms all colors uniformly, limiting selective adjustments

Understanding these limitations helps ensure LUTs are used properly within a color workflow and their potential downsides are mitigated.

Key Considerations When Using LUTs

Some best practices when working with LUTs include:

  • Use the highest LUT resolution practical – At least 33x33x33 for 3D LUTs
  • Only apply LUTs designed specifically for your camera, color space, etc.
  • Try to limit LUT adjustments to less than ±10% for realistic results
  • Use LUTs to establish an initial baseline look, refine further afterwards
  • For HDR, use techniques like dual LUTs to retain highlight details
  • Test LUTs across a range of challenging colors and luminance values

Following these guidelines helps maximize color quality and minimize artifacts when utilizing LUT based transforms in a workflow.

The Future of LUTs

Some evolving uses of LUT technology include:

  • AI-powered LUT optimization – Using AI to analyze images and videos to generate optimized LUT mappings specific to content
  • HDR and wide gamut enhancements – Larger LUTs to map emerging 10, 12, 16-bit HDR color spaces
  • Real-time video LUTs – Dedicated hardware LUT mapping for high resolution video processing
  • Adaptive/dynamic LUTs – LUT mappings that dynamically change based on image statistics
  • Cloud-based LUT delivery – LUTs generated remotely via the cloud for flexible access

As media workflows continue evolving, especially with expanded HDR and color gamut capabilities, LUTs remain a core, flexible tool for managing color transformations.

Conclusion

In summary, color lookup tables (LUTs) offer an efficient and versatile method for mapping pixel values to colors in graphics and video applications. LUTs allow rapid color transformations through simple array indexing, enabling real-time color processing critical for workflows. They also provide extensive creative control over color adjustments and effects. The standardized LUT format enables portability and reuse across devices and software. While limitations like color banding can exist, careful LUT generation and application mitigates these downsides. As color and dynamic range continue to advance, LUTs will remain an indispensable tool for color management, correction and stylization in modern media pipelines.