Prometheus – good reference to check for issues in low level greyscale/gamma
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Wanted to pass on something that I discovered lately.
I was calibrating a JVC RS55 projector recently. Had issues with the ISF interface and could not get it to communicate as well as I wanted. Switched over to Calman autocal interface and was able to connect and communicate with the PJ (same cable and all). Started calibrating the ISF modes. Calman uses the expanded settings (0-255) by default but it can be changed if necessary. I select the basic settings in the DDC panel and started calibrating the projector. I believe I set a target of 2.30 for gamma and let autocal do its stuff (for greyscale and gamma). Post calibration charts were pretty good, but I got a spike at 20% white (Delta E 1976 was about 2). Realizing that I would hv to explain why I did not want to flatten the gamma and the greyscale, I pulled up the DDC panel and made some manual adjustments to flatten them (I had already explained that a Delta E of 3 and below was good enuf, still with the client peering over my shoulder, I felt I shd improve further). I had to make some major changes to get the greyscale and gamma to behave at 20% white. I then completed colour which was pretty good, did a post cal reading and all looked ok (great actually on charts).
I pulled up my normal test scenes. First, a greyscale ramp (I believe a 21point), did not see anything major. I pulled up the Video Forge in built proofing test pattern that shows the orange, lemon, line, baby/kid skin tones & the continuous greyscale ramp on the left. Here I noticed that the greyscale ramp showed some colour shift in the low end (very minor) and it seemed to show some minor posterization as well in the lower greyscale. (I found this later when I was troubleshooting the issue).
I then tested the THX calibrator disc skin tone (kids again) scene and all looked well.
I was about to test with my normal disc (Casino Royale) before the client interjected and suggested I check out Prometheus instead. I had not watched this but decided to oblige (thinking I can check Casino Royale later).
Prometheus opening scene has a very slow camera panning scene of another world that has light hues of green, red and blue. Here I saw massive posterization. On all three colours but worse on the green. Not knowing what was going on, I checked my other reference discs. I could not see any major issues as none of my test scenes showed them as clearly as this scene. I went back to Calman autocal and redid the calibration. The same spike at 20% was there. This time, I did not alter it and let it be.
Testing the Prometheus again, showed no more posterization. I was actually relieved that the customer asked me to check this out as I realized that none of my test scenes actually would have caught this. Casino Royale 1st chapter has a B/W scene that is sensitive to colour shifts in greyscale but it did not reveal the issue. Now, to be fair the Video Forge proofing scene showed it a little but not to the extent that Prometheus showed it. I then went in and change the gamma a little so that the corrections were not as extreme.
I believe that using the autocal actually brought the corrections close to the edge of the limits where the projector relative to the initial mode I started in.
I now use this scene especially when I do 10point gamma/greyscale to see if anything I am doing is extreme.
Hope this helps if anyone is looking for a test scene to check low level greyscale issues.
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