vector: measure the fixed point rasterizer's accuracy.
When commit 8874bef1 changed ϕ from 10 to 9, this total diff grew from:
Total diff = 3051, n = 208, avg = 14.66827 out of 65535, or 0.02238%.
to:
Total diff = 6412, n = 208, avg = 30.82692 out of 65535, or 0.04704%.
Also fix a comment typo.
Change-Id: I91cc861a3d3ab282b5431cfb141fb010fd1f8d14
Reviewed-on: https://go-review.googlesource.com/32771
Reviewed-by: David Crawshaw <crawshaw@golang.org>
diff --git a/vector/acc_test.go b/vector/acc_test.go
index 0e2ae58..d80f765 100644
--- a/vector/acc_test.go
+++ b/vector/acc_test.go
@@ -612,3 +612,40 @@
0x00d4, 0xa6a1, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xd54d, 0x8399, 0xffff, 0xffff, 0x764b,
0x0000, 0x001b, 0x4ffc, 0xbb4a, 0xe3f5, 0xeee3, 0xbd4c, 0x7e42, 0x0900, 0x1b0c, 0xb6fc, 0xb6fc, 0x7e04,
}
+
+// TestFixedFloatingCloseness compares the closeness of the fixed point and
+// floating point rasterizer.
+func TestFixedFloatingCloseness(t *testing.T) {
+ if len(fxMask16) != len(flMask16) {
+ t.Fatalf("len(fxMask16) != len(flMask16)")
+ }
+
+ total := uint32(0)
+ for i := range fxMask16 {
+ a := fxMask16[i]
+ b := flMask16[i]
+ if a > b {
+ total += a - b
+ } else {
+ total += b - a
+ }
+ }
+ n := len(fxMask16)
+
+ // This log message is useful when changing the fixed point rasterizer
+ // implementation, such as by changing ϕ. Assuming that the floating point
+ // rasterizer is accurate, the average difference is a measure of how
+ // inaccurate the (faster) fixed point rasterizer is.
+ //
+ // Smaller is better.
+ percent := float64(total*100) / float64(n*65535)
+ t.Logf("Comparing closeness of the fixed point and floating point rasterizer.\n"+
+ "Specifically, the elements of fxMask16 and flMask16.\n"+
+ "Total diff = %d, n = %d, avg = %.5f out of 65535, or %.5f%%.\n",
+ total, n, float64(total)/float64(n), percent)
+
+ const thresholdPercent = 1.0
+ if percent > thresholdPercent {
+ t.Errorf("average difference: got %.5f%%, want <= %.5f%%", percent, thresholdPercent)
+ }
+}
diff --git a/vector/raster_fixed.go b/vector/raster_fixed.go
index ccfa342..5b0fe7a 100644
--- a/vector/raster_fixed.go
+++ b/vector/raster_fixed.go
@@ -127,8 +127,8 @@
if i := clamp(x0i, width); i < uint(len(buf)) {
// In ideal math: buf[i] += uint32(d * a0)
- D := oneMinusX0fSquared // D ranges up to ±1<<(1*ϕ).
- D *= d // D ranges up to ±1<<(2*ϕ).
+ D := oneMinusX0fSquared // D ranges up to ±1<<(2*ϕ).
+ D *= d // D ranges up to ±1<<(3*ϕ).
D /= twoOverS
buf[i] += uint32(D)
}
