CLI, along with several examples, plus model downloading

docs/optics.md

@@ -371,6 +371,28 @@ its triangles. That is what M5's ray tracer does, because following rays
 gives exact shadows and light bouncing between surfaces. Rasterization can
 only approximate those, for example with the shadow maps planned for M2.
 
+### One sample per pixel
+
+Both techniques decide each pixel's value from one point, the pixel's
+center. A real pixel collects light over its whole area, about 5.5 um
+square on the detector. Where a pixel straddles an edge, say half on a
+sunlit panel and half on dark space, the real pixel reads about half the
+panel's value. The renderer reads either the full value or none, depending
+on which side the center falls.
+
+This shows in two ways. Edges are stair-stepped instead of graded. And
+anything thinner than a pixel, like the booms of the example satellite seen
+from 40 m, breaks into dashes, because only some pixel centers land on it.
+For an algorithm that finds edges or measures brightness at sub-pixel
+precision, the graded edge is the signal, so this matters for limb
+navigation and pose estimation.
+
+The usual fix is supersampling: shading several points inside each pixel
+and averaging. The GPU's multisampling hardware does this cheaply for
+rasterized edges. Sampling the bodies' limbs needs it in the ray casting
+shader. The lens blur that comes with the point spread function (M3) also
+spreads every edge over a few pixels.
+
 ## Earth, Moon and Sun
 
 Code: `bodies.rs`, `gpu/bodies.rs`, `shaders/bodies.wgsl`.
@@ -453,8 +475,18 @@ or for anything in the near infrared, where the red value simply continues.
 For absolute radiometry of a real spacecraft, give the important materials
 measured reflectance spectra (M4 adds those, and BRDFs).
 
+Texture images are not used yet; each material renders with its base color
+value alone. The loader removes textures before reading a glTF file, so a
+model still loads when its textures are in a format or file the loader cannot
+read. A model that relies on textures for its colors (one material with a
+painted image) renders in that material's flat base color, often gray.
+
 STL has no units. CAD tools usually export millimeters, so loading a CAD
-model usually needs `scale: 0.001`. glTF is always meters.
+model usually needs `scale: 0.001`. glTF is meters by its specification, but
+models in the wild do not always follow it: of the NASA example models,
+Landsat 8 and ICESat-2 are in meters, while the SSL-1300 needs a scale of 0.1
+and ACE about 0.005. Checking a model's size against the real spacecraft's is
+worth the minute it takes.
 
 ## Drawing the target
 
@@ -535,6 +567,7 @@ velocity as an input, which the scene does not carry yet.
 | Earthshine on the Moon's night side | M4 |
 | Shadows the target casts on itself | M2 |
 | The Sun as a disk that is darker toward its edge (limb darkening) | M2 |
+| Pixel-area sampling (anti-aliasing): graded edges, thin parts | M2 |
 | Lens distortion | M3 |
 | Blur from diffraction and lens imperfections (point spread function) | M3 |
 | Noise: photon shot noise, read noise, dark current, pixel-to-pixel variation | M3 |
@@ -543,4 +576,4 @@ velocity as an input, which the scene does not carry yet.
 | Earth and Moon albedo maps | M2 |
 | Earth's atmosphere: scattering, the bright limb | M4 |
 | Motion blur, rolling shutter, stray light | M6 |
-| Apparent positions: light time and aberration | not yet planned |
+| Apparent positions: light time and aberration | M3 |