Under the beam, a tilted crystal answers with a pattern: bands of brightness, each one cast by a whole family of atomic planes. Find the bands, let them vote, and one orientation wins. That is a single pixel's interview.
One pixel's interview has four steps: catch the pattern, find the bands, let them vote in Hough space, and crown the orientation that explains them all.
Then do it again, four million times. Pixel by pixel, a gray bar becomes a country nobody had ever seen: provinces of shared orientation, with borders drawn wherever neighbors disagree.
A map is that interview at industrial speed, and not every pixel passes. The ones that fail stay dark, and what you do with them afterwards quietly decides your grain count.
But be careful with the colors. They are not properties. They are answers to a question you chose: which way is up. Change the question, and every province repaints itself while nothing in the metal moves.
The key to the rainbow is a triangle. A grain's color is just where its crystal direction lands inside it, and the landing spot depends on which sample direction you asked about.
Two bars, one melt, two maps. Bar A is a country of small provinces stitched together with twins. Bar B is a country of long, unbroken borders. Cracks read maps too, and they prefer the open roads.
Boundaries have statistics. Bar A's histogram spikes at sixty degrees, the signature of twins: tight, low-energy, crack-resistant borders. Bar B's boundaries are random strangers, and strangers make poor neighbors.
A grain has no color. The rainbow is a coordinate system, and you choose the question it answers. Ask it well, and a gray bar becomes a country: provinces, borders, and a border policy that decides where cracks may travel.
Two maps, one rainbow,
different countries.
The crack lived on B's borders.