2 files changed, 32 insertions, 7 deletions
diff --git a/libs/hwui/utils/Blur.cpp b/libs/hwui/utils/Blur.cpp
index 85d90d0..c020b40 100644
--- a/libs/hwui/utils/Blur.cpp
+++ b/libs/hwui/utils/Blur.cpp
@@ -23,6 +23,31 @@
 namespace android {
 namespace uirenderer {
 
+// This constant approximates the scaling done in the software path's
+// "high quality" mode, in SkBlurMask::Blur() (1 / sqrt(3)).
+static const float BLUR_SIGMA_SCALE = 0.57735f;
+
+float Blur::convertRadiusToSigma(float radius) {
+    return radius > 0 ? BLUR_SIGMA_SCALE * radius + 0.5f : 0.0f;
+}
+
+float Blur::convertSigmaToRadius(float sigma) {
+    return sigma > 0.5f ? (sigma - 0.5f) / BLUR_SIGMA_SCALE : 0.0f;
+}
+
+/**
+ * HWUI has used a slightly different equation than Skia to generate the value
+ * for sigma and to preserve compatibility we have kept that logic.
+ *
+ * Based on some experimental radius and sigma values we approximate the
+ * equation sigma = f(radius) as sigma = radius * 0.3  + 0.6.  The larger the
+ * radius gets, the more our gaussian blur will resemble a box blur since with
+ * large sigma the gaussian curve begins to lose its shape.
+ */
+static float legacyConvertRadiusToSigma(float radius) {
+    return radius > 0 ? 0.3f * radius + 0.6f : 0.0f;
+}
+
 void Blur::generateGaussianWeights(float* weights, int32_t radius) {
     // Compute gaussian weights for the blur
     // e is the euler's number
@@ -31,13 +56,7 @@ void Blur::generateGaussianWeights(float* weights, int32_t radius) {
     // g(x) = ( 1 / sqrt( 2 * pi ) * sigma) * e ^ ( -x^2 / 2 * sigma^2 )
     // x is of the form [-radius .. 0 .. radius]
     // and sigma varies with radius.
-    // Based on some experimental radius values and sigma's
-    // we approximately fit sigma = f(radius) as
-    // sigma = radius * 0.3  + 0.6
-    // The larger the radius gets, the more our gaussian blur
-    // will resemble a box blur since with large sigma
-    // the gaussian curve begins to lose its shape
-    float sigma = 0.3f * (float) radius + 0.6f;
+    float sigma = legacyConvertRadiusToSigma((float) radius);
 
     // Now compute the coefficints
     // We will store some redundant values to save some math during
diff --git a/libs/hwui/utils/Blur.h b/libs/hwui/utils/Blur.h
index 6c176e9..79aff65 100644
--- a/libs/hwui/utils/Blur.h
+++ b/libs/hwui/utils/Blur.h
@@ -18,12 +18,18 @@
 #define ANDROID_HWUI_BLUR_H
 
 #include <stdint.h>
+#include <cutils/compiler.h>
 
 namespace android {
 namespace uirenderer {
 
 class Blur {
 public:
+    // If radius > 0, return the corresponding sigma, else return 0
+    ANDROID_API static float convertRadiusToSigma(float radius);
+    // If sigma > 0.6, return the corresponding radius, else return 0
+    ANDROID_API static float convertSigmaToRadius(float sigma);
+
     static void generateGaussianWeights(float* weights, int32_t radius);
     static void horizontal(float* weights, int32_t radius, const uint8_t* source,
         uint8_t* dest, int32_t width, int32_t height);