fixed disconnected shadows

lib/ShadowMap.lua

@@ -362,6 +362,34 @@ local function fit(cx, cy, vw, vh)
   ShadowMap.bias = ShadowMap.slack / math.max(1, far - near)
 end
 
+-- A CASTER sunk slightly down the sun ray before it is drawn into the map.
+--
+-- The depth compare forgives `slack` world pixels (BIAS + the SLOPE term)
+-- so lit surfaces do not acne against their own texels -- but that same
+-- forgiveness is what lets the ground right next to a standing figure read
+-- as lit: the first `slack` of its shadow falls inside the compare's
+-- tolerance, so on screen the shadow starts that far from the feet, and the
+-- gap grows as the sun drops (the same bias reaches further horizontally
+-- the more oblique the ray). Peter-panning, in the literature; it went
+-- unseen while the sun hung at a fixed 45 degrees and shows plainly at a
+-- day/night golden hour.
+--
+-- Storing the card that much DEEPER along the ray cancels the bias for the
+-- shadow it throws -- the root lands back under the feet -- while touching
+-- nothing else: the card's own lookup now reads a deeper stored value and
+-- is more confidently lit, no terrain moved, and the acne margin the slack
+-- exists for is intact. For sprite cards and other thin stand-ins only;
+-- sinking the TERRAIN would cancel the bias everywhere, which is the acne.
+--
+-- Valid between begin() and finish(), which is when `slack` matches the
+-- frustum the map is being drawn with.
+function ShadowMap.sink(model)
+  local f = sunDir()
+  local s = ShadowMap.slack
+  return Mat4.mul(Mat4.translate(f[1] * s, f[2] * s, f[3] * s),
+                  model or IDENTITY)
+end
+
 -- Whether the map has to be redrawn for `sig` -- a caller-built stamp of
 -- everything the pass depends on (camera, terrain meshes, every pose). A
 -- frame that changes none of it reuses the map it already has, which is