clipka <ano### [at] anonymousorg> wrote:
>
> Actually POV-Ray chooses to not use chroma subsampling in JPEG output.
>

Interesting! Why that?

> > I think implementing the dithering for JPEG would be labor lost.
>
> Not really. When I implemented dithering support, I concocted a nice
> little framework for it; enabling it for JPEG output would essentially
> be a one-liner.
>

That reason is good enough ;)

>
> Are you sure you weren't seeing JPEG/MPEG artifacts?
>
> POV-Ray's dithering noise is at a magnitude of 0.4% in output colour
> space (presuming 8 bit per colour channel). /Especially/ in the vicinity
> of high contrast edges this noise should be imperceptible by human vision.

Two things ... First, by accident I found out what I had experienced with the
Bayer dithering. (And it turned out it was _not_ in my head, but _real_.) See
the three images in the attachment, rendered without dithering and with the +th
and +thB4 methods. I am using the Gnome viewer (3.18.2), and if I have the
images full-screen (on a Fulll-HD display; but the images are only 1280x720!),
the dithered images look identical, while the undithered one looks worse.
However, if I change the window size of the viewer, causing the images to be
scaled internally (by whatever method), something interesting happens: the
undithered and the +th images do not change at all, while for the Bayer
dithering the concentric rings increase drastically at specific window sizes, --
even beyond the undithered images! It seems to be a characteristic of this
dithering when scaled. And the same has obviously happend when I played back an
MPEG rendered using the +thB4-images.

Second, I am quite sure that the artifacts on the text are from the dithering
(by POV-Ray)! I don't know your eye, but with my eyes I can see even the
slightest deviations at completely vertical/horizontal edges, as they occur with
text. I'd even say: if the dithering causes any real fluctuation at (straight)
text edges, it _will_ be noticable. Books are printed at 1200 DPI or even higher
for the same reasons. I cannot attach the video right now for because of the
size. If I have the time I'll post one, but you can believe me that the effect
is very real :) (It is absent if I render without dithering, ruling out the MPEG
artifacts).

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