Added earth surface texture

readme.md

@@ -21,9 +21,12 @@ on the CPU through Mesa's lavapipe (`vulkan-swrast` on Arch,
 ## Performance and fidelity
 
 Lighting and camera fidelity are chosen separately: lighting tiers L0 to L3,
-camera tiers C0 to C3. A row is added here as each tier lands, with its
-measured cost and a sample image from the same scene, so the cost of each step
-up is visible at a glance.
+camera tiers C0 to C3, set with `RendererOptions::fidelity` in the library and
+`--lighting` and `--camera` on the command line. A tier is a fixed bundle of
+features, and a tier that needs data (L1 needs an image of Earth's surface)
+refuses to start without it. A row is added here as each tier is complete, with
+its measured cost and a sample image from the same scene, so the cost of each
+step up is visible at a glance.
 
 | Tier | Lighting | Camera | RTX 3060 | lavapipe (CPU) | Sample |
 |---|---|---|---|---|---|
@@ -95,6 +98,10 @@ in the repository, and the rest use NASA spacecraft models (Landsat 8,
 ICESat-2, OSTM/Jason-2, an SSL-1300 and ACE) that `scripts/fetch-models.sh`
 downloads into `examples/models/nasa/`. That script needs Node (for `npx`),
 because most NASA models are Draco-compressed and are decoded on the way in.
+At lighting L1, Earth's surface comes from NASA's Blue Marble, which
+`scripts/fetch-earth.sh` downloads and resizes (with ImageMagick) into
+`data/earth/`, one image per month. The month comes from the scene's epoch, or
+from `[appearance] earth_month` in a scene without one.
 
 ```sh
 cargo run --release -p resolved-cli -- render \
@@ -108,6 +115,12 @@ cargo run --release -p resolved-cli -- render \
 - `--preview` writes an 8-bit sRGB image for looking at, with the bias as
   black and saturation as white.
 - `--timings N` renders N frames and reports setup and per-frame times.
+- `--lighting L0|L1` and `--camera C0` pick the fidelity tiers (default L0
+  and C0). A tier the scene or the data cannot support stops with an error
+  naming what is missing.
+- `--data DIR` says where the tiers' data is (`earth/bmng-2004-MM.jpg`).
+  Without it the CLI tries `RESOLVED_DATA`, then `./data`, then the
+  repository's `data/`.
 - `--adapter` takes `auto`, `gpu`, `cpu` or part of an adapter's name;
   `resolved adapters` lists them.
 - `--kernels DIR` says where astrokit's kernels are, for scenes given by