This gives the best of both worlds: dithering in a linear space, with
good (and fast) perceptual error differences
TBD: would linear RGB work as well as XYZ?
can't figure out why contrast=2 is needed (openemulator uses a default
value of 1, so there must be a factor of 2x somewhere), or where the
slight hue rotation comes from - perhaps this is somehow introduced by
the more complex band-pass filtering that openemulator does?
I needed to also account for the DHGR timing difference that
introduces a phase offset of 1 pixel between the memory values and
displayed pixel timings.
Fix a last-minute bug with palette precomputation.
Use this to precompute a new ntsc palette with 256 entries (though
only 84 unique colours) that are available by appropriate pixel
sequences. Unfortunately the precomputed distance matrix for this
palette is 4GB!
Optimize the precomputation to be less memory hungry, while also
making efficient use of the mmapped output file.
Add support for dithering images using this 8-bit palette depth,
i.e. to optimize for NTSC rendering. This often gives better image
quality since more colours are available, especially when modulating
areas of similar colour.
Fix 140 pixel dithering and render the output including NTSC fringing
instead of the unrealistic 140px output that doesn't include it.
Add support for rendering output image using any target palette, which
is useful e.g. for comparing how an 8-pixel NTSC rendered image will
be displayed on an emulator using 4-pixel ntsc emulation (there is
usually some colour bias, because the 8 pixel chroma blending tends to
average away colours).
Switch the output binary format to write AUX memory first, which
matches the image format of other utilities.
