Add atan_ primitive to SkRasterPipeline.
The atan approximation logic was lifted from SkVM.
Change-Id: Id2dcc369a84666d9dafed6d7365988f906e89b18
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/635500
Reviewed-by: Herb Derby <herb@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
diff --git a/src/opts/SkRasterPipeline_opts.h b/src/opts/SkRasterPipeline_opts.h
index cf6a085..0ec9146 100644
--- a/src/opts/SkRasterPipeline_opts.h
+++ b/src/opts/SkRasterPipeline_opts.h
@@ -1502,6 +1502,34 @@
return x;
}
+/* Use 4th order polynomial approximation from https://arachnoid.com/polysolve/
+ with 129 values of x,atan(x) for x:[0...1]
+ This only works for 0 <= x <= 1
+ */
+SI F approx_atan_unit(F x) {
+ // y = 0.14130025741326729 x⁴
+ // - 0.34312835980675116 x³
+ // - 0.016172900528248768 x²
+ // + 1.00376969762003850 x
+ // - 0.00014758242182738969
+ return x * (x * (x * (x * 0.14130025741326729f - 0.34312835980675116f)
+ - 0.016172900528248768f)
+ + 1.0037696976200385f)
+ - 0.00014758242182738969f;
+}
+
+// Use identity atan(x) = pi/2 - atan(1/x) for x > 1
+SI F atan_(F x) {
+ I32 neg = (x < 0.0f);
+ x = if_then_else(neg, -x, x);
+ I32 flip = (x > 1.0f);
+ x = if_then_else(flip, 1/x, x);
+ x = approx_atan_unit(x);
+ x = if_then_else(flip, SK_ScalarPI/2 - x, x);
+ x = if_then_else(neg, -x, x);
+ return x;
+}
+
// Used by gather_ stages to calculate the base pointer and a vector of indices to load.
template <typename T>
SI U32 ix_and_ptr(T** ptr, const SkRasterPipeline_GatherCtx* ctx, F x, F y) {
diff --git a/tests/SkRasterPipelineOptsTest.cpp b/tests/SkRasterPipelineOptsTest.cpp
index 926a825..438ee6b 100644
--- a/tests/SkRasterPipelineOptsTest.cpp
+++ b/tests/SkRasterPipelineOptsTest.cpp
@@ -121,3 +121,16 @@
}
}
}
+
+DEF_TEST(SkRasterPipelineOpts_Atan, r) {
+ using F = SK_OPTS_NS::F;
+
+ constexpr float kTolerance = 0.00175f;
+ for (float x = -10.0f; x <= 10.0f; x += 0.1f) {
+ F result = SK_OPTS_NS::atan_(x);
+ F expected = atanf(x);
+ F delta = SK_OPTS_NS::abs_(expected - result);
+
+ REPORTER_ASSERT(r, SK_OPTS_NS::all(delta < kTolerance));
+ }
+}
