From Master File to Viewing Screen
After footage has been brought into the ACES color space via an Input Device Transform (IDT), all creative work—from color grading to complex visual effects—is performed in this large, scene-referred working space. The resulting ACES master file contains a huge amount of color and brightness information, far more than any current display device can actually reproduce.
To be viewed correctly, this master image must be transformed to match the specific characteristics and limitations of the viewing device. This is the job of the Output Device Transform (ODT). The ODT is the final, view-specific ‘rendering’ of the image, ensuring that the creative intent is preserved across different types of screens.
One Master, Many Deliverables
The power of the ACES workflow lies in its ‘one master, many deliverables’ approach. A post-production facility creates a single, high-fidelity ACES master file. From this one file, they can generate every required delivery format simply by applying the correct ODT. This is far more efficient and consistent than creating separate color grades for each type of deliverable.
Common ODTs include:
- Rec. 709: For standard dynamic range (SDR) content intended for HDTV broadcast and Blu-ray discs.
- DCI-P3: For digital cinema projectors in movie theaters. This standard has a wider color gamut than Rec. 709.
- Rec. 2020 (ST 2084 / PQ): For high dynamic range (HDR) content intended for HDR-capable televisions and streaming services.
By using this standardized output process, filmmakers can be confident that the film will look as intended, whether it’s being watched on an iPhone, a 4K HDR TV, or a giant cinema screen.
The RRT + ODT Combination
Technically, the output process is a two-step combination. First, the Reference Rendering Transform (RRT) is applied. This is a standardized step that maps the scene-referred data from the master file and applies a pleasing, film-like tone curve and aesthetic, creating a ‘master look.’ Then, the ODT is applied to this RRT-processed image to encode it for the specific gamut and brightness level of the target display. In most post-production software, these two steps are combined into a single selection, often just called the ‘Output Transform.’