"Edward Coffey" <eco### [at] alphalinkcomau> wrote in message
news:3E9### [at] alphalinkcomau...

> display_gamma does not only affect what POV sends to the screen, it also
> affects what it sends to the saved image file.

Well yes, sort of. *If* assumed_gamma is used, then the display_gamma is one
of the factors used in determining the actual rgb values. I thought I had
said that, but it is hard to be clear. If assumed_gamma is turned off, then
display_gamma doesn't matter-- the file will be built with no gamma
correction at all. [see footnote]

>                                                                 I believe
the connection
> between 'assuming' your audience uses a certain gamma and the
> 'assumed'_gamma setting is completely unintended and misleading.


Well, you are reading much more into something I intended as a useful
mnemonic than I thought would happen.


>
When
> rendering (not while doing the lighting and texturing on your system,
> only during the final render) for an audience you believe to have a
> certain display gamma, you set display_gamma to match it.

Yes, that seems to be what the dang thing is there for.


> The way I understand it is you set assumed_gamma to 1 and leave it
> alone, as others have said, it may be less confusing if it were a
> boolean option to select whether gamma correction is applied to the
> output. You develop your scene using display_gamma set to 1.8, and do
> any renders designed to be viewed on similar systems the same way. When
> you do a render designed to be viewed on systems with display systems
> having a gamma value nearer 2.2, you change the display_gamma value to
> 2.2 for the final render. It looks bright on your monitor, but you send
> it off to a PC user and it looks spot on when they view it on their
> gamma 2.2 system.

That sounds like an alternate approach that could work. However it isn't the
 one that's implemented in POV-Ray.

There is another piece of this that isn't getting enough emphasis. The
design used in POV-Ray fits very well with image file formats like .png that
enable the client software to do the final adjustments. This isn't very
widely supported as yet, but browsers and image editors that use the png
gamma information are coming. POV-Ray is just ahead of the curve on this
one-- its method of handling gamma will make more sense as newer software is
adopted.

Yet another thing is that assuming that all PCs are 2.2 and all Macs are 1.8
is increasingly in error as time goes by. I'm not the only PC user with a
newfangled flatscreen whose gamma is now around 1.6 - 1.8.


[note] It occurred to me as I was rereading this that if I'm using POV to
prepare an image for a heightfield, I probably need to set assumed_gamma at
zero or the heightfield will be distorted. That might explain why some of my
experiments have been less than successful... hmmm.

--
Will Woodhull
Thornhenge, SW Oregon, USA
willl.at.thornhenge.net

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