optimised contour generation

OTFbuild/bitmap_tracer.py

@@ -1,14 +1,15 @@
 """
-Convert 1-bit bitmap arrays to TrueType quadratic outlines.
+Convert 1-bit bitmap arrays to CFF outlines by tracing connected pixel blobs.
 
-Each set pixel becomes part of a rectangle contour drawn clockwise.
+Each connected component of filled pixels becomes a single closed contour
-Adjacent identical horizontal runs are merged vertically into rectangles.
+(plus additional contours for any holes). Adjacent collinear edges are
+merged, minimising vertex count.
 
-Scale: x_left = col * SCALE, y_top = (BASELINE_ROW - row) * SCALE
+Scale: x = col * SCALE, y = (BASELINE_ROW - row) * SCALE
 where BASELINE_ROW = 16 (ascent in pixels).
 """
 
-from typing import Dict, List, Tuple
+from typing import List, Tuple
 
 import sheet_config as SC
 
@@ -16,15 +17,56 @@ SCALE = SC.SCALE
 BASELINE_ROW = 16  # pixels from top to baseline
 
 
+def _turn_priority(in_dx, in_dy, out_dx, out_dy):
+    """
+    Return priority for outgoing direction relative to incoming.
+    Lower = preferred (rightmost turn in y-down grid coordinates).
+
+    This produces outer contours that are CW in font coordinates (y-up)
+    and hole contours that are CCW, matching the non-zero winding rule.
+    """
+    # Normalise to unit directions
+    nidx = (1 if in_dx > 0 else -1) if in_dx else 0
+    nidy = (1 if in_dy > 0 else -1) if in_dy else 0
+    ndx = (1 if out_dx > 0 else -1) if out_dx else 0
+    ndy = (1 if out_dy > 0 else -1) if out_dy else 0
+
+    # Right turn in y-down coords: (-in_dy, in_dx)
+    if (ndx, ndy) == (-nidy, nidx):
+        return 0
+    # Straight
+    if (ndx, ndy) == (nidx, nidy):
+        return 1
+    # Left turn: (in_dy, -in_dx)
+    if (ndx, ndy) == (nidy, -nidx):
+        return 2
+    # U-turn
+    return 3
+
+
+def _simplify(contour):
+    """Remove collinear intermediate vertices from a rectilinear contour."""
+    n = len(contour)
+    if n < 3:
+        return contour
+    result = []
+    for i in range(n):
+        p = contour[(i - 1) % n]
+        c = contour[i]
+        q = contour[(i + 1) % n]
+        # Cross product of consecutive edge vectors
+        if (c[0] - p[0]) * (q[1] - c[1]) - (c[1] - p[1]) * (q[0] - c[0]) != 0:
+            result.append(c)
+    return result if len(result) >= 3 else contour
+
+
 def trace_bitmap(bitmap, glyph_width_px):
     """
-    Convert a bitmap to a list of rectangle contours.
+    Convert a bitmap to polygon contours by tracing connected pixel blobs.
 
-    Each rectangle is ((x0, y0), (x1, y1)) in font units, where:
+    Returns a list of contours, where each contour is a list of (x, y)
-    - (x0, y0) is bottom-left
+    tuples in font units.  Outer contours are clockwise, hole contours
-    - (x1, y1) is top-right
+    counter-clockwise (non-zero winding rule).
-
-    Returns list of (x0, y0, x1, y1) tuples representing rectangles.
     """
     if not bitmap or not bitmap[0]:
         return []
@@ -32,66 +74,79 @@ def trace_bitmap(bitmap, glyph_width_px):
     h = len(bitmap)
     w = len(bitmap[0])
 
-    # Step 1: Find horizontal runs per row
+    def filled(r, c):
-    runs = []  # list of (row, col_start, col_end)
+        return 0 <= r < h and 0 <= c < w and bitmap[r][c]
-    for row in range(h):
-        col = 0
-        while col < w:
-            if bitmap[row][col]:
-                start = col
-                while col < w and bitmap[row][col]:
-                    col += 1
-                runs.append((row, start, col))
-            else:
-                col += 1
 
-    # Step 2: Merge vertically adjacent identical runs into rectangles
+    # -- Step 1: collect directed boundary edges --
-    rects = []  # (row_start, row_end, col_start, col_end)
+    # Pixel (r, c) occupies grid square (c, r)-(c+1, r+1).
-    used = [False] * len(runs)
+    # Edge direction keeps the filled region to the left (in y-down coords).
+    edge_map = {}  # start_vertex -> [end_vertex, ...]
 
-    for i, (row, cs, ce) in enumerate(runs):
+    for r in range(h):
-        if used[i]:
+        for c in range(w):
-            continue
+            if not bitmap[r][c]:
-        # Try to extend this run downward
+                continue
-        row_end = row + 1
+            if not filled(r - 1, c):          # top boundary
-        j = i + 1
+                edge_map.setdefault((c, r), []).append((c + 1, r))
-        while j < len(runs):
+            if not filled(r + 1, c):          # bottom boundary
-            r2, cs2, ce2 = runs[j]
+                edge_map.setdefault((c + 1, r + 1), []).append((c, r + 1))
-            if r2 > row_end:
+            if not filled(r, c - 1):          # left boundary
-                break
+                edge_map.setdefault((c, r + 1), []).append((c, r))
-            if r2 == row_end and cs2 == cs and ce2 == ce and not used[j]:
+            if not filled(r, c + 1):          # right boundary
-                used[j] = True
+                edge_map.setdefault((c + 1, r), []).append((c + 1, r + 1))
-                row_end = r2 + 1
-            j += 1
-        rects.append((row, row_end, cs, ce))
 
-    # Step 3: Convert to font coordinates
+    if not edge_map:
+        return []
+
+    # -- Step 2: trace contours using rightmost-turn rule --
+    used = set()
     contours = []
-    for row_start, row_end, col_start, col_end in rects:
+
-        x0 = col_start * SCALE
+    for sv in sorted(edge_map):
-        x1 = col_end * SCALE
+        for ev in edge_map[sv]:
-        y_top = (BASELINE_ROW - row_start) * SCALE
+            if (sv, ev) in used:
-        y_bottom = (BASELINE_ROW - row_end) * SCALE
+                continue
-        contours.append((x0, y_bottom, x1, y_top))
+
+            path = [sv]
+            prev, curr = sv, ev
+            used.add((sv, ev))
+
+            while curr != sv:
+                path.append(curr)
+                idx, idy = curr[0] - prev[0], curr[1] - prev[1]
+
+                candidates = [e for e in edge_map.get(curr, [])
+                              if (curr, e) not in used]
+                if not candidates:
+                    break
+
+                best = min(candidates,
+                           key=lambda e: _turn_priority(
+                               idx, idy, e[0] - curr[0], e[1] - curr[1]))
+
+                used.add((curr, best))
+                prev, curr = curr, best
+
+            path = _simplify(path)
+            if len(path) >= 3:
+                contours.append([
+                    (x * SCALE, (BASELINE_ROW - y) * SCALE)
+                    for x, y in path
+                ])
 
     return contours
 
 
 def draw_glyph_to_pen(contours, pen, x_offset=0, y_offset=0):
     """
-    Draw rectangle contours to a TTGlyphPen or similar pen.
+    Draw polygon contours to a T2CharStringPen (or compatible pen).
-    Each rectangle is drawn as a clockwise closed contour (4 on-curve points).
 
+    Each contour is a list of (x, y) vertices forming a closed polygon.
     x_offset/y_offset shift all contours (used for alignment positioning).
     """
-    for x0, y0, x1, y1 in contours:
+    for contour in contours:
-        ax0 = x0 + x_offset
+        x, y = contour[0]
-        ax1 = x1 + x_offset
+        pen.moveTo((x + x_offset, y + y_offset))
-        ay0 = y0 + y_offset
+        for x, y in contour[1:]:
-        ay1 = y1 + y_offset
+            pen.lineTo((x + x_offset, y + y_offset))
-        # Clockwise: bottom-left -> top-left -> top-right -> bottom-right
-        pen.moveTo((ax0, ay0))
-        pen.lineTo((ax0, ay1))
-        pen.lineTo((ax1, ay1))
-        pen.lineTo((ax1, ay0))
         pen.closePath()