matrix: Enable rounding for matrix multiplication (#531)

Round multiplied cells in `mul-mat`.

CHANGES.md

@@ -17,6 +17,7 @@
 - Fixed `decorations.flat-brace` vertical positioning
 - Fixed drawing of mirrored plot axis ticks.
 - Fixed plots with only annotions.
+- Added matrix rounding to fix rounding errors when using lots of transforms
 
 # 0.2.1
 

src/matrix.typ

@@ -1,11 +1,13 @@
 #import "vector.typ"
 
+// Global rounding precision
+#let precision = 8
+
 #let cos(angle) = {
-  return calc.round(calc.cos(angle), digits: 10)
+  return calc.round(calc.cos(angle), digits: precision)
 }
 
 #let sin = calc.sin
-
 #let pi = calc.pi
 
 /// Create identity matrix with dimensions $m times n$
@@ -152,9 +154,8 @@
    (0,0,0,1))
 }
 
-// Multiply matrices on top of each other.
+/// Multiply matrices on top of each other.
 #let mul-mat(..matrices) = {
-  // assert(ms.named() == (:), message: "Unexpected named arguments: " + repr(ms.named()))
   matrices = matrices.pos()
   let out = matrices.remove(0)
   for matrix in matrices {
@@ -166,7 +167,7 @@
       for i in range(m) {
         (
           for j in range(p) {
-            (range(n).map(k => out.at(i).at(k) * matrix.at(k).at(j)).sum(),)
+            (calc.round(range(n).map(k => out.at(i).at(k) * matrix.at(k).at(j)).sum(), digits: precision),)
           }
         ,)
       }

tests/transform/precission/test.typ

@@ -0,0 +1,20 @@
+#set page(width: auto, height: auto)
+#import "/src/lib.typ": *
+#import "/tests/helper.typ": *
+
+#test-case({
+  import cetz.draw: *
+  for i in range(0, 90 + 1) {
+    rotate(22deg)
+    translate((0,1))
+    rotate(-25deg)
+    translate((0,-1))
+    scale(y: -1)
+  }
+
+  // With rounding errors, the line and decoration
+  // won't be at the same location.
+  line((-1,0), (1,0), stroke: red)
+
+  cetz.decorations.wave(line((-1,0), (1,0), stroke: green))
+})