Example and docs for blooming

docs/optics.md

@@ -588,6 +588,22 @@ half down; each pixel it reaches takes what it has room for and passes the
 rest on, until it runs out or leaves the frame. Into the neighbors, each of
 the 8 takes an eighth, and what overfills them drains.
 
+How far a streak runs is the excess over the full well, times the
+efficiency, divided among pixels that each hold one full well. For a bright
+star only a pixel or two saturate, by a little, and the streak is short and
+a pixel or two wide. The Sun is different. Its disk is about 85 pixels
+across in the example camera, so the streak is 85 pixels wide, and at 0.4 ms
+each pixel of it holds about 100,000 full wells: a column through the middle
+of the disk has some 8 million full wells of excess. With no drain
+(efficiency 1), half of that runs each way, enough to fill thousands of
+frames' worth of column, so every pixel in those columns saturates and the
+rest runs off the edges: a white band the width of the Sun, top to bottom
+(`examples/scenes/sun-glare.toml`). That is what a CCD without an
+anti-blooming drain records. A drain rated for 100 to 1,000 times
+overexposure, as is typical, still leaves the Sun at about 100,000 times;
+only an efficiency of about 5e-5 would shorten this streak to about 200
+pixels each way.
+
 Blooming happens to the charge, so it comes after the shot noise (and the
 fixed pattern) and before the read noise and the ADC. With blooming on, the
 sensor pass works in three steps rather than one: it writes each value's