POV-Ray : Newsgroups : povray.general : 16 bit gray scale : Re: 16 bit gray scale Server Time
25 Sep 2021 11:41:13 EDT (-0400)
  Re: 16 bit gray scale  
From: William F Pokorny
Date: 17 Mar 2021 08:51:23
Message: <6051fb4b$1@news.povray.org>
On 3/16/21 1:47 PM, ingo wrote:
> The +Fpg / +Fng switch should result in a 16 bit grey scale image. It only
> renders to 8 bit.
> Adding +Fp16 / Fn16 does nothing as stated in the docs.
> When rendering a gradien I only get 254 colors, instead of the 256 that
> shoud be there for 8-bit.
> 3.8 beta on win10.
> Cheers.

The bits per color change in v3.8 made it so the default greyscale depth 
is 8. The documentation should indicate this up where Bits_Per_Color=n 
gets documented at least.

The documentation does have:

"Previously, specifying a bit-depth for grayscale file output was simply 
ignored and the bit-depth was forced to 16 bits per channel. A Change in 
version 3.8 now allows for a bit-depth setting."

I'm on Ubuntu working of the v3.8 master commit and the channel depth 
options work for me in both color and grey scale(1) with both png and 
ppm outputs.

Could you run a quite experiment? The ppm format supports an ascii 
output. Grab a simple scene file. Run:

povray scn.pov +w80 +h80 +fpg8 File_gamma=1.0 Compression=0 +ofpg8


povray scn.pov +w80 +h80 +fpg16 File_gamma=1.0 Compression=0 +ofpg16


Edit both files in a text editor. The difference in encoding should be 

--- Bug
(1) - What doesn't appear to be working is the intended and documented 
change for ppm (aka pgm) greyscale output. In my testing the output is 
still always linear (color output is using bt709). Basically the 
following documented change isn't working:

"Previously, the grayscale PPM (technically PGM) output file format 
always used linear output. A Change in v3.8 will match that of the 
regular non-grayscale PPM format. That is File_Gamma will be honored if 
specified, otherwise the ITU-R BT.709 gamma transfer function will be 

In fact, I cannot override the greyscale File_Gamma... It should be 
working as does the .png grey scale output, but with the bt709 transfer 
function, unless the output File_Gamma is set to 1.0 (linear).

Bill P.

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