CCPR: Process quadratic flat edges without soft msaa

The artifacts previously thought to require msaa can be handled by
(1) converting near-linear quadratics into lines, and (2) ensuring all
quadratic segments are monotonic with respect to the vector of their
closing edge [P2 -> P0].

No. 1 was already in effect.

No. 2 is implemented by this change.

Now we only fall back on soft msaa for the two corner pixels.

This change also does some generic housekeeping in the quadratic
processor.

Bug: skia:
Change-Id: Ib3309c2ed86d3d8bec5f451125a69326e82eeb1c
Reviewed-on: https://skia-review.googlesource.com/29721
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>

1 files changed, 60 insertions, 0 deletions

diff --git a/src/gpu/GrPathUtils.cpp b/src/gpu/GrPathUtils.cpp
index b6711a0df4..9a79f2061e 100644
--- a/src/gpu/GrPathUtils.cpp
+++ b/src/gpu/GrPathUtils.cpp
@@ -567,6 +567,66 @@ void GrPathUtils::convertCubicToQuadsConstrainToTangents(const SkPoint p[4],
     }
 }
 
+static inline Sk2f normalize(const Sk2f& n) {
+    Sk2f nn = n*n;
+    return n * (nn + SkNx_shuffle<1,0>(nn)).rsqrt();
+}
+
+bool GrPathUtils::chopMonotonicQuads(const SkPoint p[3], SkPoint dst[5]) {
+    GR_STATIC_ASSERT(SK_SCALAR_IS_FLOAT);
+    GR_STATIC_ASSERT(2 * sizeof(float) == sizeof(SkPoint));
+    GR_STATIC_ASSERT(0 == offsetof(SkPoint, fX));
+
+    Sk2f p0 = Sk2f::Load(&p[0]);
+    Sk2f p1 = Sk2f::Load(&p[1]);
+    Sk2f p2 = Sk2f::Load(&p[2]);
+
+    Sk2f tan0 = p1 - p0;
+    Sk2f tan1 = p2 - p1;
+    Sk2f v = p2 - p0;
+
+    // Check if the curve is already monotonic (i.e. (tan0 dot v) >= 0 and (tan1 dot v) >= 0).
+    // This should almost always be this case for well-behaved curves in the real world.
+    float dot0[2], dot1[2];
+    (tan0 * v).store(dot0);
+    (tan1 * v).store(dot1);
+    if (dot0[0] + dot0[1] >= 0 && dot1[0] + dot1[1] >= 0) {
+        return false;
+    }
+
+    // Chop the curve into two segments with equal curvature. To do this we find the T value whose
+    // tangent is perpendicular to the vector that bisects tan0 and -tan1.
+    Sk2f n = normalize(tan0) - normalize(tan1);
+
+    // This tangent can be found where (dQ(t) dot n) = 0:
+    //
+    //   0 = (dQ(t) dot n) = | 2*t  1 | * | p0 - 2*p1 + p2 | * | n |
+    //                                    | -2*p0 + 2*p1   |   | . |
+    //
+    //                     = | 2*t  1 | * | tan1 - tan0 | * | n |
+    //                                    | 2*tan0      |   | . |
+    //
+    //                     = 2*t * ((tan1 - tan0) dot n) + (2*tan0 dot n)
+    //
+    //   t = (tan0 dot n) / ((tan0 - tan1) dot n)
+    Sk2f dQ1n = (tan0 - tan1) * n;
+    Sk2f dQ0n = tan0 * n;
+    Sk2f t = (dQ0n + SkNx_shuffle<1,0>(dQ0n)) / (dQ1n + SkNx_shuffle<1,0>(dQ1n));
+    t = Sk2f::Min(Sk2f::Max(t, 0), 1); // Clamp for FP error.
+
+    Sk2f p01 = SkNx_fma(t, tan0, p0);
+    Sk2f p12 = SkNx_fma(t, tan1, p1);
+    Sk2f p012 = SkNx_fma(t, p12 - p01, p01);
+
+    p0.store(&dst[0]);
+    p01.store(&dst[1]);
+    p012.store(&dst[2]);
+    p12.store(&dst[3]);
+    p2.store(&dst[4]);
+
+    return true;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 
 /**