ccpr: Fix flatness and triangle-ness detection for conics
We should detect these cases by examining the curve at max height; not
midtangent.
Bug: skia:7821
Change-Id: I3d9e3a10798f0d825916840cb99d054b2a6284c3
Reviewed-on: https://skia-review.googlesource.com/122620
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
diff --git a/src/gpu/ccpr/GrCCGeometry.cpp b/src/gpu/ccpr/GrCCGeometry.cpp
index 2593273..59d040d 100644
--- a/src/gpu/ccpr/GrCCGeometry.cpp
+++ b/src/gpu/ccpr/GrCCGeometry.cpp
@@ -18,6 +18,8 @@
GR_STATIC_ASSERT(2 * sizeof(float) == sizeof(SkPoint));
GR_STATIC_ASSERT(0 == offsetof(SkPoint, fX));
+static constexpr float kFlatnessThreshold = 1/16.f; // 1/16 of a pixel.
+
void GrCCGeometry::beginPath() {
SkASSERT(!fBuildingContour);
fVerbs.push_back(Verb::kBeginPath);
@@ -59,7 +61,7 @@
}
static inline bool are_collinear(const Sk2f& p0, const Sk2f& p1, const Sk2f& p2,
- float tolerance = 1/16.f) { // 1/16 of a pixel.
+ float tolerance = kFlatnessThreshold) {
Sk2f l = p2 - p0; // Line from p0 -> p2.
// lwidth = Manhattan width of l.
@@ -87,7 +89,7 @@
return std::abs(d) <= lwidth * tolerance;
}
-static inline bool are_collinear(const SkPoint P[4], float tolerance = 1/16.f) { // 1/16 of a pixel.
+static inline bool are_collinear(const SkPoint P[4], float tolerance = kFlatnessThreshold) {
Sk4f Px, Py; // |Px Py| |p0 - p3|
Sk4f::Load2(P, &Px, &Py); // |. . | = |p1 - p3|
Px -= Px[3]; // |. . | |p2 - p3|
@@ -586,56 +588,40 @@
Sk2f p1 = Sk2f::Load(P+1);
Sk2f p2 = Sk2f::Load(P+2);
- // Don't crunch on the curve if it is nearly flat (or just very small). Collinear control points
- // can break the midtangent-finding math below.
- if (are_collinear(p0, p1, p2)) {
- this->appendLine(p2);
- return;
- }
-
Sk2f tan0 = p1 - p0;
Sk2f tan1 = p2 - p1;
- // The derivative of a conic has a cumbersome order-4 denominator. However, this isn't necessary
- // if we are only interested in a vector in the same *direction* as a given tangent line. Since
- // the denominator scales dx and dy uniformly, we can throw it out completely after evaluating
- // the derivative with the standard quotient rule. This leaves us with a simpler quadratic
- // function that we use to find the midtangent.
- float midT = find_midtangent(tan0, tan1, 1, (w - 1) * (p2 - p0),
- 1, (p2 - p0) - 2*w*(p1 - p0),
- 1, w*(p1 - p0));
- // Use positive logic since NaN fails comparisons. (However midT should not be NaN since we cull
- // near-linear conics above. And while w=0 is flat, it's not a line and has valid midtangents.)
- if (!(midT > 0 && midT < 1)) {
- // The conic is flat. Otherwise there would be a real midtangent inside T=0..1.
- this->appendLine(p2);
- return;
- }
-
- // Evaluate the conic at midT.
- Sk4f p3d0 = Sk4f(p0[0], p0[1], 1, 0);
- Sk4f p3d1 = Sk4f(p1[0], p1[1], 1, 0) * w;
- Sk4f p3d2 = Sk4f(p2[0], p2[1], 1, 0);
- Sk4f midT4 = midT;
-
- Sk4f p3d01 = lerp(p3d0, p3d1, midT4);
- Sk4f p3d12 = lerp(p3d1, p3d2, midT4);
- Sk4f p3d012 = lerp(p3d01, p3d12, midT4);
-
- Sk2f midpoint = Sk2f(p3d012[0], p3d012[1]) / p3d012[2];
-
- if (are_collinear(p0, midpoint, p2, 1) || // Check if the curve is within one pixel of flat.
- ((midpoint - p1).abs() < 1).allTrue()) { // Check if the curve is almost a triangle.
- // Draw the conic as a triangle instead. Our AA approximation won't do well if the curve
- // gets wrapped too tightly, and if we get too close to p1 we will pick up artifacts from
- // the implicit function's reflection.
- this->appendLine(midpoint);
- this->appendLine(p2);
- return;
- }
if (!is_convex_curve_monotonic(p0, tan0, p2, tan1)) {
+ // The derivative of a conic has a cumbersome order-4 denominator. However, this isn't
+ // necessary if we are only interested in a vector in the same *direction* as a given
+ // tangent line. Since the denominator scales dx and dy uniformly, we can throw it out
+ // completely after evaluating the derivative with the standard quotient rule. This leaves
+ // us with a simpler quadratic function that we use to find the midtangent.
+ float midT = find_midtangent(tan0, tan1, 1, (w - 1) * (p2 - p0),
+ 1, (p2 - p0) - 2*w*(p1 - p0),
+ 1, w*(p1 - p0));
+ // Use positive logic since NaN fails comparisons. (However midT should not be NaN since we
+ // cull near-linear conics above. And while w=0 is flat, it's not a line and has valid
+ // midtangents.)
+ if (!(midT > 0 && midT < 1)) {
+ // The conic is flat. Otherwise there would be a real midtangent inside T=0..1.
+ this->appendLine(p2);
+ return;
+ }
+
// Chop the conic at midtangent to produce two monotonic segments.
+ Sk4f p3d0 = Sk4f(p0[0], p0[1], 1, 0);
+ Sk4f p3d1 = Sk4f(p1[0], p1[1], 1, 0) * w;
+ Sk4f p3d2 = Sk4f(p2[0], p2[1], 1, 0);
+ Sk4f midT4 = midT;
+
+ Sk4f p3d01 = lerp(p3d0, p3d1, midT4);
+ Sk4f p3d12 = lerp(p3d1, p3d2, midT4);
+ Sk4f p3d012 = lerp(p3d01, p3d12, midT4);
+
+ Sk2f midpoint = Sk2f(p3d012[0], p3d012[1]) / p3d012[2];
Sk2f ww = Sk2f(p3d01[2], p3d12[2]) * Sk2f(p3d012[2]).rsqrt();
+
this->appendMonotonicConic(p0, Sk2f(p3d01[0], p3d01[1]) / p3d01[2], midpoint, ww[0]);
this->appendMonotonicConic(midpoint, Sk2f(p3d12[0], p3d12[1]) / p3d12[2], p2, ww[1]);
return;
@@ -645,10 +631,32 @@
}
void GrCCGeometry::appendMonotonicConic(const Sk2f& p0, const Sk2f& p1, const Sk2f& p2, float w) {
+ SkASSERT(w >= 0);
SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
- // Don't send curves to the GPU if we know they are nearly flat (or just very small).
- if (are_collinear(p0, p1, p2)) {
+ Sk2f base = p2 - p0;
+ Sk2f baseAbs = base.abs();
+ float baseWidth = baseAbs[0] + baseAbs[1];
+
+ // Find the height of the curve. Max height always occurs at T=.5 for conics.
+ Sk2f d = (p1 - p0) * SkNx_shuffle<1,0>(base);
+ float h1 = std::abs(d[1] - d[0]); // Height of p1 above the base.
+ float ht = h1*w, hs = 1 + w; // Height of the conic = ht/hs.
+
+ if (ht < (baseWidth*hs) * kFlatnessThreshold) { // i.e. ht/hs < baseWidth * kFlatnessThreshold
+ // We are flat. (See rationale in are_collinear.)
+ this->appendLine(p2);
+ return;
+ }
+
+ if (w > 1 && h1*hs - ht < baseWidth*hs) { // i.e. w > 1 && h1 - ht/hs < baseWidth
+ // If we get within 1px of p1 when w > 1, we will pick up artifacts from the implicit
+ // function's reflection. Chop at max height (T=.5) and draw a triangle instead.
+ Sk2f p1w = p1*w;
+ Sk2f ab = p0 + p1w;
+ Sk2f bc = p1w + p2;
+ Sk2f highpoint = (ab + bc) / (2*(1 + w));
+ this->appendLine(highpoint);
this->appendLine(p2);
return;
}