better shortcut detection, improves polygon test

src/pixie/internal.nim

@@ -1,4 +1,4 @@
-import chroma, common, system/memory, vmath
+import bumpy, chroma, common, system/memory, vmath
 
 const allowSimd* = not defined(pixieNoSimd) and not defined(tcc)
 
@@ -51,6 +51,20 @@ proc `*`*(color: ColorRGBX, opacity: float32): ColorRGBX {.raises: [].} =
       a = ((color.a * x) div 255).uint8
     rgbx(r, g, b, a)
 
+proc intersectsInside*(a, b: Segment, at: var Vec2): bool {.inline.} =
+  ## Checks if the a segment intersects b segment (excluding endpoints).
+  ## If it returns true, at will have point of intersection
+  let
+    s1 = a.to - a.at
+    s2 = b.to - b.at
+    denominator = (-s2.x * s1.y + s1.x * s2.y)
+    s = (-s1.y * (a.at.x - b.at.x) + s1.x * (a.at.y - b.at.y)) / denominator
+    t = (s2.x * (a.at.y - b.at.y) - s2.y * (a.at.x - b.at.x)) / denominator
+
+  if s > 0 and s < 1 and t > 0 and t < 1:
+    at = a.at + (t * s1)
+    return true
+
 proc fillUnsafe*(
   data: var seq[uint8], value: uint8, start, len: int
 ) {.inline, raises: [].} =

src/pixie/paths.nim

@@ -1205,7 +1205,7 @@ proc partitionSegments(
         entry0 = partition.entries[0].segment
         entry1 = partition.entries[1].segment
       var at: Vec2
-      if not intersects(entry0, entry1, at):
+      if not intersectsInside(entry0, entry1, at):
         # These two segments do not intersect, enable shortcut
         partition.twoNonintersectingSpanningSegments = true
         # Ensure entry[0] is on the left
@@ -1916,7 +1916,7 @@ proc fillShapes(
 
     if allEntriesInScanlineSpanIt and tmp == 2:
       var at: Vec2
-      if not intersects(
+      if not intersectsInside(
         partitions[partitionIndex].entries[entryIndices[0]].segment,
         partitions[partitionIndex].entries[entryIndices[1]].segment,
         at
@@ -1925,9 +1925,13 @@ proc fillShapes(
         var
           left = partitions[partitionIndex].entries[entryIndices[0]]
           right = partitions[partitionIndex].entries[entryIndices[1]]
-        # Ensure left is on the left
-        if left.segment.at.x > right.segment.at.x:
-          swap left, right
+        block:
+          # Ensure left is actually on the left
+          let
+            maybeLeftMaxX = max(left.segment.at.x, left.segment.to.x)
+            maybeRightMaxX = max(right.segment.at.x, right.segment.to.x)
+          if maybeLeftMaxX > maybeRightMaxX:
+            swap left, right
 
         let requiresAntiAliasing =
           left.segment.requiresAntiAliasing or