A lot is said about Input Transforms - both good and bad. However, one of the first published ACES-related Academy documents is P-2013-001 - Recommended Procedures for the Creation and Use of Digital Camera System Input Device Transforms (IDTs). Probably very few people have read this as its quite dense, but it explains many methods for creating Input Transforms to use with ACES.
I wanted to share the results of some in house experimentation I did a few months ago to assess the state of ACES Input Transforms. The full details are in the attached PDF, and this is by no means a “formal” journal-type writeup, but gives an idea of the procedure followed and the results.
IDT Report v4.pdf (7.2 MB)
The basic idea was to see how well cameras could be made to match if the procedures described in P-2013-001 were followed under optimum conditions (i.e. using reliable spectral sensitivitity measurements, proper exposure, etc.).
In this experiment, a monochromator was used to expose a camera with narrow bands of light centered at regular wavelength increments across the visible spectrum. Data was collected and processed to derive spectral sensitivity data. The validity of the spectral sensitivity data was evaluated independently by testing linearity of sensor response as well as color accuracy between theoretical calculated data and empirical photography. Once confidence in the linearity and spectral sensitivity data was established, the procedure specified in “Academy P-2013-001: Recommended Procedures for the Creation and Use of Digital Camera System Input Device Transforms (IDTs)” was followed. Finally, the results of applying the IDTs for four different cameras were compared to the ACES Reference Input Capture Device (RICD) and to each other.
There are many opportunities for other options and improvements, but if the procedure described in P-2013-001 is followed using quality spectral sensitivity data - good matches from camera to camera can be obtained. The most difficult part was having the default exposure of a properly exposed 18% gray consistently come out at ACES=[0.18 0.18 0.18], but a one-time correction grade to establish the adjustment needed to to attain this was enough to make all cameras fall into a match with each other.
I am hoping somebody finds this useful and/or it prompts a discussion about Input Transforms. I am happy to answer any questions about the experiment.