2pt conical stage for concentric case

If the circles are concentric, we can use the radial stage and
post-massage t with a 2x3 stage.

Change-Id: I744bc889d509d5f9ff162c179e8ec7ca35410e3c
Reviewed-on: https://skia-review.googlesource.com/20972
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>

2 files changed, 32 insertions, 0 deletions

diff --git a/src/shaders/gradients/SkTwoPointConicalGradient.cpp b/src/shaders/gradients/SkTwoPointConicalGradient.cpp
index 399bce6675..846b5f41d6 100644
--- a/src/shaders/gradients/SkTwoPointConicalGradient.cpp
+++ b/src/shaders/gradients/SkTwoPointConicalGradient.cpp
@@ -422,3 +422,31 @@ void SkTwoPointConicalGradient::toString(SkString* str) const {
     str->append(")");
 }
 #endif
+
+bool SkTwoPointConicalGradient::adjustMatrixAndAppendStages(SkArenaAlloc* alloc,
+                                                            SkMatrix* matrix,
+                                                            SkRasterPipeline* p) const {
+    // When the two circles are concentric, we can pretend we're radial (with a tiny *twist).
+    if (SkScalarNearlyZero((fCenter1 - fCenter2).length())) {
+        matrix->postTranslate(-fCenter1.fX, -fCenter1.fY);
+        matrix->postScale(1 / fRadius2, 1 / fRadius2);
+        p->append(SkRasterPipeline::xy_to_radius);
+
+        // Tiny twist: radial computes a t for [0, r2], but we want a t for [r1, r2].
+        auto dR = fRadius2 - fRadius1;
+        SkASSERT(dR > 0);
+        auto scale =  fRadius2 / dR;
+        auto bias  = -fRadius1 / dR;
+
+        auto mad_matrix = SkMatrix::Concat(SkMatrix::MakeTrans(bias, 0),
+                                           SkMatrix::MakeScale(scale, 1));
+        auto m = alloc->makeArrayDefault<float>(6);
+        SkAssertResult(mad_matrix.asAffine(m));
+        p->append(SkRasterPipeline::matrix_2x3, m);
+
+        return true;
+    }
+
+    // TODO
+    return false;
+}
diff --git a/src/shaders/gradients/SkTwoPointConicalGradient.h b/src/shaders/gradients/SkTwoPointConicalGradient.h
index b32f52c1e0..8b61b8e9d2 100644
--- a/src/shaders/gradients/SkTwoPointConicalGradient.h
+++ b/src/shaders/gradients/SkTwoPointConicalGradient.h
@@ -79,6 +79,10 @@ protected:
     Context* onMakeContext(const ContextRec&, SkArenaAlloc*) const override;
     sk_sp<SkShader> onMakeColorSpace(SkColorSpaceXformer* xformer) const override;
 
+    bool adjustMatrixAndAppendStages(SkArenaAlloc* alloc,
+                                     SkMatrix* matrix,
+                                     SkRasterPipeline* p) const final;
+
 private:
     SkPoint fCenter1;
     SkPoint fCenter2;