Readme update

docs/optics.md

@@ -448,7 +448,8 @@ This gets the overall brightness and the phase (how much of the disk is
 lit) right. It misses some things that are visible in real images:
 
 - Earth's oceans, land and clouds differ in brightness by a factor of
-  several. An albedo map comes in M2.
+  several. From lighting L1, Earth's surface comes from an image (next
+  section); clouds come in M4.
 - The full Moon looks almost uniformly bright out to its edge. A Lambertian
   sphere instead darkens toward the edge, because the light there arrives
   and leaves at grazing angles. Lunar soil scatters light back toward its
@@ -658,7 +659,7 @@ Limitations:
   shades (a few mm on Landsat 8) lets light through, because of the acne
   margins above.
 - **One sample per pixel.** Shadows are sampled at the pixel center, like
-  everything else, until M2's pixel-area sampling.
+  everything else, until M3's pixel-area sampling.
 - **Bodies.** Earth and the Moon do not use shadow maps. Their shadows on
   the target are the eclipse factor below, applied to the whole target.
 
@@ -721,10 +722,10 @@ velocity as an input, which the scene does not carry yet.
 | Effect | Where |
 |---|---|
 | Light from Earth and the Moon illuminating the target (Earthshine, Moonshine) | M2 |
-| Earth's shadow on the Moon (lunar eclipses) | M2 |
+| Eclipses seen on the bodies: Earth's shadow on the Moon, the Moon's on Earth | M2 |
 | Earthshine on the Moon's night side | M4 |
 | The Sun as a disk that is darker toward its edge (limb darkening) | M2 |
-| Pixel-area sampling (anti-aliasing): graded edges, thin parts | M2 |
+| Pixel-area sampling (anti-aliasing): graded edges, thin parts | M3 |
 | 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 |