tools.realwallpaper: real colour imagery on the day side
The day side now shows what the geostationary satellites see, like
zoom.earth, instead of the base map under white clouds synthesised from
infrared:
- Europe/Africa/Atlantic: MTG (Meteosat-12) rgb_truecolour from EUMETSAT.
rgb_geocolour looks the same by day but renders ~4x slower on the
server and timed out (502) for full 2048 px tiles.
- Americas/Pacific: GOES-East and GOES-West GeoColor from GIBS.
- Asia/Australia: GIBS has no Himawari colour product. Himawari band 3
(red, 10 min) is turned into white cloud over the base map, with the
cloud opacity being the reflectance beyond the ground's own albedo.
The daily VIIRS true colour mosaics were the alternative, but their
clouds are up to a day old, have swath seams of different times and
would not match the IR clouds next to them.
Each source is weighted by how well its satellite sees the point: full
trust up to 62° view zenith, none from 78° (the limb is smeared), and
the more nadir satellite wins in overlaps with cos^8 of the zenith
angle, so seams are soft. Trust also needs the sun not to be in front
of the satellite: looking towards a low sun both MTG (mornings over the
Indian Ocean) and Himawari (evenings) are washed out to a pale band
with straight edges, while a long path with the sun behind the
satellite is fine (Berlin at winter noon). A plain path-length limit
would have dropped the latter. Near the terminator (cosz < 0.2)
GeoColor fades into its purple night blend, so the live imagery fades
out there and the IR clouds over the base map take over as before.
Where it is shown, the synthetic clouds and so their relief shading and
shadows are scaled down, so the real image is not shaded twice.
All live layers (including the IR clouds) now request an explicit TIME,
the render time floored to 10 min: without TIME GIBS served tiles of
one layer from different frames (sun glint cut at a tile edge), and it
makes --time render the imagery of that time, not only the sun. The
TIME is capped at the newest frame the server lists (GIBS
DescribeDomains, EUMETSAT per-layer capabilities), since GIBS answers
an unpublished time with an empty image and EUMETSAT with an error;
MTG true colour stops producing frames while its disk is dark.
GIBS also now and then answers with an empty 1 kB image for a frame it
has (the same request succeeds when repeated), which left a quarter of
the Himawari disk missing, so empty tiles are asked for up to 3 times.