Frame: guard sliver-producing refinement splits

README.md

@@ -148,8 +148,8 @@ Adaptive refinement is disabled by default (`--refine-max-level 0`), so the
 existing coarse-mesh renders remain unchanged.  When enabled, its defaults are
 an absolute direction error of `1e-3` degrees, relative error `0.1`, minimum
 long edge `0.5` pixels, minimum area `0.25` pixel-squared, and a provisional
-minimum discrete-Jacobian magnitude of `1e-3`.  Each value can be overridden
-independently:
+minimum discrete-Jacobian magnitude of `1e-3`, and minimum triangle quality
+`0.2`.  Each value can be overridden independently:
 
 ```text
 --refine-max-level N
@@ -158,6 +158,7 @@ independently:
 --refine-jacobian-min J
 --refine-min-edge-pixels P
 --refine-min-area-pixels2 A
+--refine-min-triangle-quality Q
 ```
 
 `N` caps the triangle refinement level.  Let `e` be the angle between the
@@ -170,7 +171,12 @@ source-sky/lens-map length.  A locally escaped triangle is split only when
 **both** `e > D_rad` (the converted `--refine-angle-abs-deg D`) and
 `e / max(s, 1e-15) > --refine-angle-rel`.  `P` and `A`
 prevent selecting a leaf already at or below the requested image-plane
-long-edge and area scales.
+long-edge and area scales.  `Q` is the normalized image-plane triangle quality
+`4 sqrt(3) A / (l0^2 + l1^2 + l2^2)`: it is one for an equilateral triangle
+and approaches zero for a sliver.  Its default is `0.2`; `0` disables this
+quality promotion.  If a requested
+single-edge split would create a child below `Q`, that leaf is promoted to red
+refinement before the conforming shared-edge closure is applied.
 Triangles whose three vertices disagree between capture and escape are split
 independently of the direction-error thresholds, allowing the mesh to follow a
 shadow boundary.