On 7/11/20 12:17 AM, Kima wrote:
...
> As an update, if it might be beneficial to others.
> 
> In my experience, the slowest format to save is PNG. Of course, PNG is the best
> and common format in terms of quality and size.
> 
> Saving in JPEG is the fastest though it is highly compressed. Unfortunately, a
> heavily lossy format like JPEG is not suitable for the purpose of ray tracing.
> 
> JPEG with 100% quality is close to PNG (POV-Ray see 85% by default). The
> official document emphasises that even with 100% quality, there might be some
> artefacts. I am inspecting this failure but haven't found any yet. JPEG with
> 100% quality is about 40% of PNG size.
> 
> As a side note, I believe the default value of compression in POV-Ray (85%) is
> too low. The default value in ImageMagick for conversion to JPEG is 92%. It does
> not make sense to use many resources to ray trace a scene and save it as a
> heavily lossy format.
> 

Thanks for the information. I found myself thinking about your post... 
Where did exr output come out size and performance wise?

There are formats other than exr which don't compress on output - ppm is 
another I think, and you can set the depth there to 16 bits per color 
channel too. If the uncompressed output is much faster, perhaps use one 
of those formats and compress/convert with external scripts?

I'm no image format expert (Christoph was the guy there) - and I haven't 
spent much time in the image file handling code. Still, I'll risk adding 
a few POV-Ray code related comments. :-)

- The jpeg output doesn't dither which may be part of the speed 
difference relative to png.

- The jpeg library itself looks to support a no-compression jpeg output 
by setting the quality to 0, but the current POV-Ray wrap maps <=1 
values (the default compression value is -1) to 85%.

- I'd say 85% jpeg compression is sometimes suitable. :-) Just depends 
on what one is doing with POV-Ray. Aside: IIRC 85% was chosen to line up 
with the effective compression relative to some previous jpeg output, 
not because it was considered the best quality choice.

- POV-Ray mostly uses standard external libraries when reading and 
writing images so the compression de-compression isn't POV-Ray per sa.
The compression approaches are not all the same - with higher quality 
and lossless compression tending to be the most expensive. Which you 
probably knew...

Aside:

The jpeg library efforts are somewhat fractured. Most support no 
compression option I believe, some lossless too, etc. There is though 
one library better optimized for modern CPUs (turbo something?) and it's 
not currently the library used by POV-Ray. I think google did much of 
the work IIRC. A bit of a tangle in any case.

Somewhere (github?) I ran across a highly optimized for simd 
architectures png library with benchmarks that looked great compared to 
the common png library. But, during my glance, it wasn't clear how much 
real world use of it there was.

Anyway... I guess, it's somewhat meaningless practically, but there are 
image libraries out there which - perhaps - could enable better image 
input/output performance in POV-Ray.

Bill P.

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