[argyllcms] Re: Slightly wrong black levels on projector

  • From: Antonio Marcheselli <antonio.marcheselli@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sun, 22 Nov 2015 22:06:24 +0000

I'll take the lack of interest as a "get a life" reply? :)

Typically no reply means that you've asked a questions to which
it's difficult to frame a useful response.
Hi Graeme,
I was just joking, no worries! :)

Sorry - what exactly are you asking ? - it's not clear. i.e. what are you expecting here that's different from what you are getting ? Note that there is nothing magical about a step wedge - i.e. there is no reason to think that every step of some wedge should be visible when correctly calibrated, since visibility depends on the step size and the target curve shape. A sufficiently high pure gamma curve on top of a high black offset + 256 step wedge will be expected to have several steps that are not visibly different.

I may say a blasphemy here but I used BT1886 and I thought the purpose of that standard is to avoid black crushing.
I may be happy with 17 and 18 looking the same, but my point is that 17 is not visible at all. To my limited experience that should not happen with a BT1886. But I may be mistaken.

for example of a case where the first steps are expected
to be invisible.

So the best way would be to measure your step levels and compare to the expected
value, to assure yourself that the calibration is sufficiently accurate
your instrument is capable of the necessary accuracy). Easiest approach would
probably be to run a verification chart with the step wedge values as the
[ You could do this one step at a time using ColorMeter too. ]

I see. However if I use BT1886 the gamma at 10% should be about 1.8. I would definitely expect some black to be crushed if I used a pure gamma 2.4 - particularly on my awful projector - but I thought the point of the whole BT1886 was to avoid that?

I will try to create the verification chart you suggest and I'll report back.

Thanks for you time! :)

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