Advanced DRT Q&A: Practical Implementation in Color Grading with ACES and DaVinci Resolve

Cullen Kelly

Introduction [0:00]

The speaker introduces a Q&A session focused on Display Rendering Transforms (DRTs), a crucial yet often misunderstood aspect of color grading, building on a previous conceptual video. He emphasizes the importance of understanding the concepts before practical application.

DRTs Analogy to Prints [4:10]

The concept of a "print" is central to understanding DRTs.

• A print is a set of macro-level manipulations that affect every single shot in the timeline uniformly.
• These manipulations happen after any individual shot grading.
• The analogy used is that if color grading is assembling ingredients for a cake, the print is the oven that bakes it, developing the raw data into a viewable image.
• DRTs are a core component of this print and are not changed per shot, scene, or to impart different looks at various points in a film.
  • Creative changes for specific scenes are handled through "sub-looks," which are distinct from the macro-level DRT.

Compressing Highlights vs. Contrast Boost [8:30]

A common question arises regarding DRTs reducing perceptual brightness in highlights due to tone mapping.

• Tone mapping is definitionally the compression of scene dynamic range into display dynamic range, which inherently reduces highlight brightness (also known as rollover or compression).
• This compression is necessary to prevent clipping and gracefully approach the display's luminance limits.
  • An example demonstrates how not using tone mapping leads to harsh, clipped highlights (e.g., like early 2000s films).
• While most standard DRTs don't offer explicit control over the shoulder shape (highlight compression), advanced look development tools (like Contour) allow for shaping this curve.
  • It's recommended to address highlight issues at a macro/system level through look development rather than by individually boosting contrast per shot, which is slow and inconsistent.

    

Tone Mapping Options [16:10]

DaVinci Resolve's Color Space Transform (CST) offers different tone mapping methods that affect how colors change as brightness increases.

• Options: Luminance Mapping, DaVinci, and Saturation Preserving.
  • The first three "simple" methods are generally undesirable as they lead to clipping.
• Evaluation: Tools like RGB chips and vector scopes are useful for observing how different tone mapping methods handle color and tone gradation.
  • The speaker prefers Luminance Mapping as it generally maintains better separation and gradation of tones.
• The chosen tone mapping method primarily controls how color changes along the tone scale, not the overall curve shape of the image.

ACES and JP2499 [23:48]

The discussion addresses the use of JP2499 (a specific DRT) within an ACES workflow.

• ACES Definition: ACES (Academy Color Encoding System) primarily defines the input transformations (IDTs) and the working color space (ACES CCT or ACES linear) for consistent color management.
• Creative Choice: The output rendering transform (DRT/ODT) in ACES is a creative decision, not rigidly bound to a single standard.
  • It is possible to grade in ACES CCT and then use a non-ACES DRT like JP2499 by converting to ACES linear before applying the desired output transform.
• This approach allows colorists to leverage the benefits of ACES color management while maintaining creative control over the final image rendering.
• Different DRTs produce aesthetically different results; neither is inherently "wrong" as long as they correctly map source to destination.

What DRT Should You Use [31:30]

There is no single "perfect" DRT for all projects.

• The choice of DRT is aesthetic and project-specific.
• The DRT is a crucial piece of the overall "print," but rarely the entire print itself.
• A chosen DRT almost always needs a "companion look" (further creative manipulations) to achieve the desired final aesthetic, as a standalone DRT typically only handles the technical transformation to display.

Types of DRTs: Per Channel vs. Chromaticity Linear [39:00]

Two main categories of DRTs are discussed:

• Chromaticity Linear:
  • Aims to prevent color distortion as colors pass through the DRT, maintaining scene chromaticity.
  • Often results in a less aesthetically appealing "neutral" image, requiring additional creative work to introduce desired color characteristics (e.g., saturation rolloff in highlights).
• Per Channel DRTs:
  • The most common type, including most Resolve CST options and ACES 1.x ODTs.
  • Inherently introduces color distortion, but this distortion is often aesthetically desired and necessary, especially for mimicking traditional image formation like film prints (which are per-channel).
  • These DRTs impart characteristics like desaturating colors in highlights as they get brighter.

How Your Look Interacts with the DRT [44:11]

The optimal placement of your "look" (creative grade) relative to the DRT is before the DRT (upstream).

• Reason 1: Multiple Display Targets:
  • If you need to deliver to multiple display targets (e.g., SDR Rec. 709 and HDR Rec. 2020), applying your look in a scene-referred working space (before the DRT) ensures that the creative intent is consistently preserved across different output formats.
  • Applying the look after the DRT, in a display-referred space, would require re-grading for each display target, which is inefficient and may not yield matching results.
• Reason 2: Latitude and Control:
  • Working in a display-referred space (after the DRT) is "confined" due to the dynamic range compression. This limits grading latitude, making it harder to manipulate the image without introducing undesirable clipping or artifacts.
  • Grading in a larger, less constrained scene-referred space (before the DRT) provides more flexibility to shape and sculpt the image.
  • Analogy: Mixing ingredients in a large bowl (scene-referred) versus a small jar (display-referred) where spills (clipping) are easily made.

Project Settings vs. Node Level DRTs [49:50]

The choice between managing color in project settings or using a node-based workflow has significant implications.

• Project Settings (DaVinci YRGB Color Managed):
  • Obscures the imaging pipeline, requiring users to navigate to project settings to see color management details.
  • Limits DRT choices to a few built-in options (e.g., Resolve DRT, Red IPP2).
  • Crucially, it is not possible to combine project-level color management with additional DRTs at the timeline level without causing a "double transform" and producing incorrect, "whackadoodle" images.
• Node-Based Workflow:
  • Provides explicit visibility of the entire imaging pipeline within the node graph, allowing for easy auditing and confirmation of each transform.
  • Offers greater flexibility to use custom or external DRTs (like JP2499 or Open DRT) by applying them as nodes.
  • This method is generally preferred for advanced color grading workflows due to its transparency and control.

JP2499 Output Settings [57:27]

Specific output settings for the JP2499 DRT are detailed.

• Input: Typically set to DaVinci Wide Gamut Intermediate, reflecting the working color space.
• Output (SDR): Usually Rec. 709 Gamma 2.4.
  • The conversion from Gamma 2.4 to Gamma 2.2 (often needed for web outputs) should be performed in a separate, explicit node rather than within the DRT itself.
• Output (HDR): The specific output color space depends on client deliverables (e.g., P3D65 ST2084 PQ, Rec. 2020 ST2084 PQ).
• HDR Tonal Mapping Options:
  • "Gaining the SDR": This option tries to maintain the tonal relationship across SDR and HDR, but can result in an image that is "too hot" and a gray point that is too high, in the speaker's opinion.
  • "Unroll the SDR" (or "Reform the image"): This is the preferred method, where the body of the image largely remains consistent with the SDR version, and highlights are "stretched out" above the middle gray point to leverage the increased dynamic range of HDR.
• HDR Peak White: The default of around 300 nits is often a good starting point. The speaker generally prefers to lower this to 200-250 nits for a more aesthetically pleasing highlight rolloff, as pushing much higher can be visually uncomfortable. Client specifications will ultimately dictate the required peak white level.