a couple of interesting points has been made elsewhere and we thought it we would be good for the VWG to do a summary on ACESCentral.
@Troy_James_Sobotka started the thread this way :
I am unsure that the volume compression is even remotely as “creative subjective” as some would believe; it’s a finite analytical / technical compression of the amplitude that should maintain the tonality distribution.
To which @nick replied :
Isn’t a compression always subjective? Where the compression curve begins and how far out gets compressed to the boundary are choices, and there are no “correct” answers.
Followed by some interesting examples, provided by Troy :
Whether or not aforementioned aesthetic compression function is acceptable is not as wide open creative either; there are finite boundaries that even casual observers will deem unacceptable. Over compression is a very real problem at 8 bit consumption levels, and yields rather garish imagery as demonstrated.
But the primary point I would make is that if we concretely identify what “tone” is, and I’d hope most here would agree more or less on a definition, there is precisely one correct compression target.
The curve has a direct relation to effected luminance. But it’s actually affecting radiometric-like energy. Feeding an achromatic sweep pattern yields one luminance series.
That may or may not be acceptable; too much compression will flatten tonality. Again, this is not an openly creative / subjective thing. I mean I guess it is, if one accepts “Wow that looks woeful” as potentially a desired target.
When we feed a chromatic series to the curve, the input energy is the mechanic, and the sensation is luminance. The affected value is energy-like, the effected value is luminance.
Therefore we are able to take any chromatic series and, subject to the nature of the signal compression, derive the luminance. And for any given curve, there is again, exactly one luminance series.
Example with an asymptote curve; the values “s” curve to an asymptote. This means that all energy-like values in the triplet have an input of open radiometric-like domain X to output closed domain display / optically linear Y. Here’s what happens…
We can already see that we have a huge problem here. The inverse falloff of her cheek should be far greater. Yet all S curves will do this if we follow the general principles of energy transport.
Here, as X approaches infinity, Y approach 100% optically linear. Hence that tonality collapse.
Now let’s see what happens if we loosen the asymptote and let the curve “shoot upwards” beyond the limited domain we specify with a constant slope. This is via Colour’s awesome little
extrapolatetool I might add, so thanks to Thomas (yet again)…
Note how we are now recovering some of the inverse square law of the energy because we are now not plateauing, and instead letting luminance “carry on” up the optically linear domain output. Also note that we still have problems. I’d suspect most folks would agree that the inverse square off of her cheek is not yet appropriate of the luminance we would expect on perceptual ingestion of the chromatic scene. Also note that there are heavy “cusps” where the curve is still not ascending enough to reveal tonality. This is the tonality posterization / collapse effect here.
Finally, let’s see if we loosen the upper end of the S curve such that the angle carries on with a more “reasonable” slope.
Presto. Sure we can all bicker about which is more acceptable here, but I’d be rather confident that the latter is closer to the estimated apparent luminance than the first two if we canvassed most folks here.
So what can we glean from this? The answer is that again, for any given aesthetic compression of the signal amplitude, there is precisely one luminance series. That’s critical. It also permits further inferences.
A few observations:
- That the notion of a perfectly analogous S curve as per the Hurter Driffield density curves, under variable chromaticities due to film dye density in juxtaposition to the notion of constant chromaticity digital RGB emissions is problematic in a fundamental sense.
- That we can clearly identify posterization based off of the feedback from a luminance calculation for any given signal compression curve long before we add in chroma.
- That based on the difference between the idealized luminance feedback for the compression curve, we can simultaneously deduce the optimal volume compression using a chromaticity linear mechanic; it’s a minimization problem essentially where we traverse the chromaticity linear lines to push back luminance to the appropriate levels where the displays are incapable of producing fully chromatic variants.
Feel free to join the conversation !