Fix one pixel mis-alignment of rounded corners when either the cell dimensions or the thickness of the line is an odd number of pixels

The control points of the Bezier curve and the spread due to thickness
must match up with the values used to draw straight lines. In order to
ensure that, one calculates them before upscaling by the super sampling
factor, rather than after. Fixes #2907

Author: kovidgoyal

docs/changelog.rst

@@ -17,6 +17,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - Fix image leaving behind a black rectangle when switch away and back to
   alternate screen (:iss:`2901`)
 
+- Fix one pixel mis-alignment of rounded corners when either the cell
+  dimensions or the thickness of the line is an odd number of pixels
+  (:iss:`2907`)
 
 0.18.2 [2020-07-28]
 --------------------

kitty/fonts/box_drawing.py

@@ -364,20 +364,19 @@ def D(buf: BufType, width: int, height: int, left: bool = True) -> None:
                 buf[offset + dest_x] = mbuf[offset + src_x]
 
 
-def draw_parametrized_curve(buf: BufType, width: int, height: int, thickness_in_pixels: int, xfunc: BezierFunc, yfunc: BezierFunc) -> None:
+def draw_parametrized_curve(buf: BufType, width: int, height: int, delta: int, extra: int, xfunc: BezierFunc, yfunc: BezierFunc) -> None:
     num_samples = height*4
     seen = set()
-    delta, extra = divmod(thickness_in_pixels, 2)
     for i in range(num_samples + 1):
         t = (i / num_samples)
         p = x_p, y_p = int(xfunc(t)), int(yfunc(t))
         if p in seen:
             continue
         seen.add(p)
-        for y in range(int(y_p) - delta, int(y_p) + delta + extra):
+        for y in range(y_p - delta, y_p + delta + extra):
             if 0 <= y < height:
                 offset = y * width
-                for x in range(int(x_p) - delta, int(x_p) + delta + extra):
+                for x in range(x_p - delta, x_p + delta + extra):
                     if 0 <= x < width:
                         pos = offset + x
                         buf[pos] = min(255, buf[pos] + 255)
@@ -386,29 +385,31 @@ def draw_parametrized_curve(buf: BufType, width: int, height: int, thickness_in_
 @supersampled()
 def rounded_corner(buf: BufType, width: int, height: int, level: int = 1, which: str = '╭') -> None:
     supersample_factor = getattr(buf, 'supersample_factor')
-    thickness_in_pixels = thickness(level) * supersample_factor
+    delta, extra = divmod(thickness(level), 2)
+    hw = ((width / supersample_factor) // 2) * supersample_factor
+    hh = ((height / supersample_factor) // 2) * supersample_factor
     if which == '╭':
-        start = width // 2, height - 1
-        end = width - 1, height // 2
-        c1 = width // 2, int(0.75 * height)
-        c2 = width // 2, height // 2 + 1
+        start = hw, height - 1
+        end = width - 1, hh
+        c1 = hw, int(0.75 * height)
+        c2 = hw, hh + 1
     elif which == '╮':
-        start = 0, height // 2
-        end = width // 2, height - 1
-        c1 = width // 2, height // 2 + 1
-        c2 = width // 2, int(0.75 * height)
+        start = 0, hh
+        end = hw, height - 1
+        c1 = hw, hh + 1
+        c2 = hw, int(0.75 * height)
     elif which == '╰':
         start = width // 2, 0
-        end = width - 1, height // 2
-        c1 = width // 2, int(0.25 * height)
-        c2 = width // 2 - 1, height // 2 - 1
+        end = width - 1, hh
+        c1 = hw, int(0.25 * height)
+        c2 = hw, hh - 1
     elif which == '╯':
-        start = 0, height // 2
-        end = width // 2, 0
-        c1 = width // 2 - 1, height // 2 - 1
-        c2 = width // 2, int(0.25 * height)
+        start = 0, hh
+        end = hw, 0
+        c1 = hw, hh - 1
+        c2 = hw, int(0.25 * height)
     xfunc, yfunc = cubic_bezier(start, end, c1, c2)
-    draw_parametrized_curve(buf, width, height, thickness_in_pixels, xfunc, yfunc)
+    draw_parametrized_curve(buf, width, height, delta * supersample_factor, extra * supersample_factor, xfunc, yfunc)
 
 
 def half_dhline(buf: BufType, width: int, height: int, level: int = 1, which: str = 'left', only: Optional[str] = None) -> Tuple[int, int]:
@@ -686,7 +687,7 @@ def quad(buf: BufType, width: int, height: int, x: int = 0, y: int = 0) -> None:
 for start in '┌┐└┘':
     for i, (hlevel, vlevel) in enumerate(((t, t), (f, t), (t, f), (f, f))):
         box_chars[chr(ord(start) + i)] = [p(corner, which=start, hlevel=hlevel, vlevel=vlevel)]
-for ch in '╭╮╯╰':
+for ch in '╭╮╰╯':
     box_chars[ch] = [p(rounded_corner, which=ch)]
 
 for i, (a_, b_, c_, d_) in enumerate((