POV-Ray : Newsgroups : povray.general : ambient 0.3 triggers 'suspiciously high ambient value found' warning? : Re: ambient 0.3 triggers 'suspiciously high ambient value found' warning? Server Time
27 Nov 2022 19:33:22 EST (-0500)
  Re: ambient 0.3 triggers 'suspiciously high ambient value found' warning?  
From: Cousin Ricky
Date: 9 Feb 2022 08:31:28
Message: <6203c230$1@news.povray.org>
On 2022-02-09 07:38 (-4), tutki wrote:
> Cousin Ricky <ric### [at] yahoocom> wrote:
> [...]
>> Ambient 0.3 does seem rather high to me, but it's not for me or POV-Ray
>> to tell you, as an artist, not to use it that high.  The 'emission 0'
>> tells POV-Ray 3.8 that you have consciously made that choice.
> What are typical values for ambient that aren't considered high?
> I'm noticing that 3.7 seems to be producing a much brighter image than 3.6. Is
> there some global setting that I should be using to make it equivalent to 3.6?
> Besides #version, that is. I want to know what's the difference between 3.6 and
> 3.7 lighting that's causing it to produce a much brighter (almost oversaturated)
> image.

Do you have an assumed_gamma in your global_settings?  In 3.6, there was
no default value, so the image defaulted to whatever gamma was used by
your monitor, typically 2.2 or sRGB.  In 3.7+ a warning is given about
the missing assumed_gamma, and it defaults to 1.0.  This causes scenes
with no assumed_gamma to look lighter and washed out when rendered in
3.7+, compared to earlier versions.

The reason for all this is explained in the gamma handling section of
the documentation.

This makes me think that this is the reason 0.3 doesn't seem high to
you.  In older versions of POV-Ray, due to the implicit gamma
conversion, a value of 0.3 translated to a physical brightness of 0.07.
 We are typically unaware of this because our perceptions are nonlinear.

The quickest way to solve this problem is to add global_settings {
assumed_gamma srgb } near the top of your scene file, but this is just a
short term solution.

Longer term, you can replace all your rgb keywords with srgb, which
lowers the brightness of the colors so that they look correct with
assumed_gamma 1.  If you use the named colors from colors.inc, prefix
those names with srgbft.  But you will also have to adjust your lighting
conditions downward.  Adjusting older scene files will be a pain, but
for new scene files, setting up your lighting with assumed_gamma 1
should be simpler than with the old unregulated and nonlinear
2.2/sRGB/1.8 gamma, or whatever.

(Be aware that the srgb series of keywords is not a cure-all, and has
its own traps.  See thread "strange problem with srgb color in
light_source" in povray.general from last March.)

Post a reply to this message

Copyright 2003-2021 Persistence of Vision Raytracer Pty. Ltd.