Careful mapping of HDR values to LDR is an important part of a modern game rendering pipeline. One of the goals of our new renderer was to replace Reinhard‘s tone mapping curve with some kind of a filmic tone mapping curve. We tried one from Ucharted 2 and tried rolling our own, but weren’t happy with either of this solutions. Finally, we settled on the one from ACES, which is currently a default tone mapping curve in Unreal Engine 4.
ACES color encoding system was designed for seamless working with color images regardless of input or output color space. It also features a carefully crafted filmic curve for displaying HDR images on LDR output devices. Full ACES integration is a bit of overkill for games, but we can just sample ODT( RRT( x ) ) transform and fit a simple curve to this data. We don’t even need to run any ACES code at all, as ACES provides reference images for all transforms. Although there is no linear RGB D65 ODT transform, but we can just use REC709 D65 and remove 2.4 gamma from it.
Curve was manually fitted (max fit error: 0.0138) to be more precise in the blacks – after all we will be applying some kind gamma afterwards. Additionally, data was pre-exposed, so 1 on input maps to ~0.8 on output and resulting image’s brightness is more consistent with the one without any tone mapping curve at all. For the original ACES curve just multiply input (x) by 0.6.
Fitted curve’s HLSL source code (free to use under public domain CC0 or MIT license):
float3 ACESFilm(float3 x)
{
float a = 2.51f;
float b = 0.03f;
float c = 2.43f;
float d = 0.59f;
float e = 0.14f;
return saturate((x*(a*x+b))/(x*(c*x+d)+e));
}
Fitted curve plotted against source data’s sample points:
UPDATE: This is a very simple luminance only fit, which over saturates brights. This was actually something consistent with our art direction, but for a more realistic rendering you may want a more complex fit like this one from Stephen Hill.