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The Present and the Future of Digital Print Stock Design

July 14, 2022

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In the July issue of The SMPTE Motion Imaging Journal, Los Angeles-based senior color scientist and image scientist Cullen Kelly takes an enlightening look at “Digital Print Stock Design: A Framework for the Post-Film Era.” Kelly notes that the craft of color grading is currently at a crossroads. Every captured image requires processing and creative adjustment before it can be reproduced. For most of cinema history, the defining factor in the post-production “look” of capture imagery was the print stock – engineered to impart preferred color characteristics while serving the technical function of transforming scene-referred imagery for display. Yet today, as the physical medium of print stock has been retired from image mastering workflows, so too has the principle of a global creative transform. Colorists and their tools continue to evolve as they look into the future of bleeding-edge image capture and reproduction. 

With the ongoing proliferation of camera and display standards, unifying the imagery of content has become increasingly challenging. The industry-wide recognition of the need for world-class engineering to deliver world-class color grading has led to the development of systems addressing these technical aspects, most notably the Academy Color Encoding System (ACES). Yet there is no current initiative aimed at supporting the creative aspects of unifying motion imagery at the macro-level. In his article, Kelly outlines a proposed design of Digital Print Stocks (DSPs) within a Digital Print Stock Framework (DPSF)

These digital print stocks comprise a set of manipulations whose parameters can be intuitively modified by artists, offering a high degree of control with minimal demand for specialized knowledge of image science, and resulting in a global look that can be deployed within any imaging pipeline. Kelly proposes that the architecture of this framework should be open source, modular, lightweight, platform-agnostic, and device-independent. In addition, the framework’s manipulations should be precise, relying on per-pixel mathematical operations rather than interpolation of values, and as broad and soft as possible, relying on techniques such as tone mapping, matrix transformation, gamut mapping, and curve adjustments to sculpt a “creative color space.” Finally, the framework should offer an interface that includes visual feedback to live-preview the application of the transform, with intuitive parameters and controls, and sensible guides and constraints.

Kelly presents a list of the most common tools currently used by colorists for macro-level look design and proposes the components of a new proposed Digital Print Stock Framework. 

Current tools include: 

  • Software-Specific Tools. Every professional-grade color correction software platform boasts advanced tools that go beyond traditional color correction manipulations and offer unique creative options. Drawbacks include that the tools are still not purpose-built for macro-level look development, they’re closed-source with little window into their underlying functionality, and they’re confined to use within the software for which they were created.
  • Third-Party Plug-ins. Colourlab Look Designer, Filmbox, Filmconverts and others can produce unique and aesthetically desirable manipulations which would otherwise not be available to the artist. However, these plug-ins are proprietary and closed-source, forcing their user into the role of consumer rather than creator. 
  • Color Transform Language/DaVinci CTL. The color transform language and DaVinci are powerful tools for image manipulation. However, they are platform-dependent and their use requires intermediate to advanced knowledge of programming and image science.

Kelly’s  proposed new framework is built upon the ability to add, revise, and/or remove image transform components as needed. 

Proposed core transforms should include:

  • Creative Contrast Curve. This should be the first consideration of any digital print stock, as it has a greater visual impact on the image than any other single component and also exerts significant influence on the behavior of subsequent components. It must globally define the creative black point, white point, and separation or compression of the tones in between. It should be presented to the artist in a form that offers intuitive parameters, precise control, and sensible constraint. Parameters include black point, toe length, toe strength, shoulder length, shoulder strength, ceiling, split-tone strength, split-tone toe color, split-tone shoulder color, and exposure lock.
  • RGB Matrix. Upstream and/or downstream of the application of a creative contrast curve, an n by 3 RGB matrix applied in a linear tone curve can be used to broadly define the outer boundaries of the digital print stock’s “creative gamut.”
  • RGB Tetrahedral Transform. The digital print stock’s creative gamut can be further refined by the use of a tetrahedral interpolation transform, an algorithm for converting one input color space to another. The DaVinci Colorspace Transform subdivides the RGB cube into six tetrahedra, transforms their vertices via user-provided parameters, and then interpolates the values in between.
  • Global Saturation Curve. Following linear adjustments of the RGB cube using matrices and tetrahedral transformation, the application of a nonlinear curve adjustment to an “S” saturation channel (derived using color models such as hue/saturation/luminance, hue/saturation/value, or luminance/color/hue) can smoothly increase overall saturation levels without pushing the creative gamut beyond its defined edges.
  • Secondary Curves Manipulations. The serial application of the above manipulations can quickly author a vast range of clean, versatile, creative transforms. However, the inclusion of narrower or more specialized manipulations may be needed, especially if the goal is to reproduce the finer idiosyncrasies of a file negative and/or print stock.
  • Additional Components. These many include spatial or textural manipulations, as well as manipulations whose behavior changes based on analysis of the incoming images.

Kelly considers the design and application of scene-referred creative looks to be a unique challenge, with subjective goals that can make the task more complex in many regards. Establishing a well-selected and well-implemented set of creative transforms that offer maximum aesthetic latitude and intuitive adjustment for a wide variety of artists will require the participation and collaboration of creatives and imaging scientists alike. Digital print stock design will be an ever-evolving framework as the industry continues to refine its creative practices.

Learn more about the future of Digital Print Stock Design. Read Cullen Kelly’s complete article in the July issue of The SMPTE Motion Imaging Journal.

Keywords:

Academy Color Encoding System (ACES), color grading, color management, digital cinema, film print, HDR, look development

 

 

 

Tag(s): Featured , News , RGB , Colorist

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