Why can a Hald CLUT be 1600×1600 instead of 4096×4096?
Asked 8/4/2022
2 views
2 answers
0
I thought an exhaustive 8-bit color lookup table would need to represent all 256×256×256 RGB combinations, which seems like it would require a 4096×4096 image. Why are some Hald CLUT files much smaller, around 1600×1600? How can they still work without storing every possible color explicitly?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
1
The lookup tables does not necessarily need to cover the whole colour space. According to RawPedia values missing in the LUT will simply be interpolated.
From the linked page:
Even if your HaldCLUT image contains colors in only 8-bit precision, missing values will be interpolated so that posterization will not occur in your photo. As such, since there is no visual loss of quality, we recommend using the level 12 identity file, or even an 8-level one
When creating a 12 level LUT I get 1728x1728 pixels which is close to the 1600x1600 of the ones you linked to.
Originally by user38328. Source · Licensed CC BY-SA 4.0
user38328
3y ago
0
Generated from our catalog & community — verify before relying on it.
A Hald CLUT does not have to contain every one of the 256×256×256 RGB values explicitly. Smaller Hald CLUTs sample the color space at a lower resolution, and the missing values are reconstructed by interpolation when the LUT is applied.
That means a smaller file can still produce smooth results without obvious posterization. As noted in RawPedia, even Hald CLUTs with only 8-bit precision can work well because intermediate colors are interpolated.
So your 4096×4096 estimate would apply only if you wanted to store every 8-bit RGB combination directly as individual entries. In practice, Hald CLUTs are often built at lower “levels,” which greatly reduces the image dimensions. For example, a level-12 Hald CLUT is 1728×1728 pixels, which is in the same range as the 1600×1600 files you saw.
In short: those smaller Hald CLUTs work because they are sampled LUTs, not exhaustive ones.
Recommended products
UniqueBot
AI3y ago