POV-Ray : Newsgroups : povray.general : 16 bit gray scale Server Time
14 May 2021 02:44:41 EDT (-0400)
  16 bit gray scale (Message 1 to 4 of 4)  
From: ingo
Subject: 16 bit gray scale
Date: 16 Mar 2021 13:47:05
Message: <XnsACEFBF16C51Dseed7@news.povray.org>
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.


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From: William F Pokorny
Subject: Re: 16 bit gray scale
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

and

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

then:

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

--- 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 
applied."

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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From: ingo
Subject: Re: 16 bit gray scale
Date: 17 Mar 2021 10:27:01
Message: <XnsACF09D2828973seed7@news.povray.org>
in news:6051fb4b$1@news.povray.org William F Pokorny wrote:

> bits per color

Sorry, don't kow what I did wrong. Brain glitch? Seems to work fine.
You seem to be right on the gamma problem.
The wrong file extension is a minor nitpick.

Thanks,
Ingo


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From: William F Pokorny
Subject: Re: 16 bit gray scale
Date: 28 Mar 2021 10:41:28
Message: <60609598$1@news.povray.org>
On 3/17/21 8:51 AM, William F Pokorny wrote:
> On 3/16/21 1:47 PM, ingo wrote:
...
> 
> --- Bug
...
> 

Summary. Three issues found as a side effect of checking the v3.8 bit 
depth feature.

A link to a more detailed thread given the web forums/news interface is 
restored:

http://news.povray.org/povray.beta-test/thread/%3C60563168%241%40news.povray.org%3E/


---
1) In v3.8. Greyscale ppm/pgm output not respecting the File_Gamma 
setting. Not a feature of v3.7.

2) In v3.8. The png sRGB chunk is not output unless the File_Gamma is 
set to srgb though the default output uses srgb. Default png write and 
immediate read won't match exactly. Release v3.7 looks OK to quick tests.

3) In v3.7 and v3.8. Incorrect greyscale png file input where bit 
storage depth <= 8 and input is non-linear (gamma!=1). In v3.8 this the 
default png greyscale write and read.

Aside: On the web view I currently see a mistaken, partial 
povray.beta-test news post with the suffix part 1, which I immediately 
deleted from Thunderbird. Please ignore it for the more recent and 
complete thread.


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