Color Management – Tone Mapping: AgX versus ACES
This is a simple test using #Blender and #Cycles to illustrate why #ACES has a fundamental problem with the natural transformation of colors and light intensity. The results are not color corrected. #AgX (developed by Troy Sobotka) is a bit washed out and robust while #ACES has more contrast with collapsing #HUE colors.
The reference for this test is a classic #ARRI film camera and how that camera records color and light intensity. Using what is called a color sweep, we get a visual result of how the #HUE colors change irregularly over the light intensity until they become desaturated and white due to overexposure on the #film material.
We use a simple scene with a plane, a sphere and an area light. The plane uses a white Principled #PBR material with 100% roughness. The sphere with 0% roughness reflects the light source in the #specular. The light is animated once through the entire #HUE spectrum. The intensity of the light source is 1,000 watts.
#AgX is rendered in linear BT.709 I-D65 color space.
#ACES is rendered in linear #ACEScg color space.
#Blender uses the corresponding #OCIO profile for each:
#AgX forked from sobotka/agx: https://github.com/EaryChow/AgX (23-02-17)
#ACES 1.2: https://github.com/colour-science/Ope...
It is important to understand how the human eye perceives colors relatively and how a #film #camera records colors and light. The goal of a color management system is to mathematically capture the entire color spectrum and convert it for the limited color representation of a display device. This process is called tone mapping.
"Tone mapping is a technique used in image processing and computer graphics to map one set of colors to another to approximate the appearance of high-dynamic-range images in a medium that has a more limited dynamic range." (https://en.wikipedia.org/wiki/Tone_ma...)
When a light source with a primary blue illuminates a scene with white objects, most "#CGI people" expect a corresponding blue coloration. It gets interesting when we increase the intensity of the light source. In a natural environment, we would still get the relative impression of blue coloration, but the light simultaneously brightens that environment. The higher the intensity, the brighter the environment becomes. But blue cannot become bluer, it must become brighter. And brighter means whiter.
The color profile of #ACES in this case gets the color and only from extreme values do we get a similar result to #AgX but then because of the extreme values with wrong colorings and on a display with additional unpleasant color bandings.
The color sweep of #AgX is closer to the color sweep of an #ARRI film camera and the tone mapping turns out correspondingly more natural because the natural light intensity is taken into account in addition to the color. This makes the image brighter and the colors are naturally desaturated due to overexposure, as happens when shooting on #film.
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