Could you please explain more about why applying ACES tone-mapping is good for shadow removal? Does it works also with other tone-mapping methods?

[zoezhou1999]

Hi authors!
Thank you for this amazing work!
Could you please explain more about why applying ACES tone-mapping is good for shadow removal? Does it works also with other tone-mapping methods? I am still quite confused after reading the paper. Thank you!

Best
Yuhongze

[ingra14m]

Hi, thanks for your interest. From our perspective, ACES has two advantages:

• Match the image formulation in Blender and other physical engine. In the scenes with strong light intensity, we usually employ HDR tone mapping to map the color from BRDF to the final image.
• Calculate PBR colors in a larger range. Tone mapping like sRGB map the color from $[0, 1]$ to $[0, 1]$, which does not change the value range. While ACES could map the PBR colors from $[0, +\infty]$ to $[0, 1]$, enhancing the contrast between the shadow and indirect illumination.

And more importantly, we add $\gamma^{-0.2}$ to stretch the ACES curve to match other common tone mapping distribution as shown below, which can make this work suitable for other diffuse scenes as well.

[zoezhou1999]

Hi @ingra14m thank you for the reply. so can I understand this way: this work tries to correct LDR indirect illumination to HDR for correct PBR rendering, then having better ability to remove shadow?

[ingra14m]

I think more basically, this work tries to compute PBR colors (through BRDF) in a wider range. It can lay the foundation for the shadow and indirect illumination removal due to the more refined contrast control.

And don't forget the visibility modeling. We use NeuS Octree and regularized visibility estimation to make the visibility modeling more accurate, ensuring the robust shadow removal with the help of scene-specific ACES tone mapping.

[zoezhou1999]

Thanks! I seem to understand better by seeing the ACES curve of r=0.2 vs srgb. 😂 Will this method still struggle if the shadow is quite hard?

[ingra14m]

You could see hotdog in the README figure. I think it is a scene that contains severe shadow, making it challenging to decouple from the PBR materials.

Although it may still remain some stubborn shadows in other scenes, such as real-world or self-captured scenes, this method has already outperformed the existing ir methods in handling these challenging high-illumination scenes.