More general version of image filtering; reworked to be robust and easier to SSE

src/core/SkBitmapFilter.h

Index: src/core/SkBitmapFilter.h
diff --git a/src/core/SkBitmapFilter.h b/src/core/SkBitmapFilter.h
new file mode 100644
index 0000000000000000000000000000000000000000..4e4778876c6e8a1491d83186afda157e49f167f7
--- /dev/null
+++ b/src/core/SkBitmapFilter.h
@@ -0,0 +1,150 @@
+
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+
+#ifndef SkBitmapFilter_DEFINED
+#define SkBitmapFilter_DEFINED
+
+#include "SkMath.h"
+
+// size of the precomputed bitmap filter tables for high quality filtering.
+// Used to precompute the shape of the filter kernel.
+// Table size chosen from experiments to see where I could start to see a difference.
+
+#define SKBITMAP_FILTER_TABLE_SIZE 32
+
+class SkBitmapFilter {
+  public:
+      SkBitmapFilter(float width)
+      : fWidth(width), fInvWidth(1.f/width) {
+          precomputed = false;
+      }
+      
+      SkFixed lookup( float x ) const {
+          if (!precomputed) {
+              precomputeTable();
+          }
+          int filter_idx = int(fabsf(x * invWidth() * SKBITMAP_FILTER_TABLE_SIZE));
+          return fFilterTable[ SkTMin(filter_idx, SKBITMAP_FILTER_TABLE_SIZE-1) ];
+      }      
+      
+      SkFixed lookupFloat( float x ) const {
+          if (!precomputed) {
+              precomputeTable();
+          }
+          int filter_idx = int(fabsf(x * invWidth() * SKBITMAP_FILTER_TABLE_SIZE));
+          return fFilterTableFloat[ SkTMin(filter_idx, SKBITMAP_FILTER_TABLE_SIZE-1) ];
+      }      
+      
+      float width() const { return fWidth; }
+      float invWidth() const { return fInvWidth; }
+      virtual float evaluate(float x) const = 0;
+  protected:
+      float fWidth;
+      float fInvWidth;
+      
+      mutable bool precomputed;
+      mutable SkFixed fFilterTable[SKBITMAP_FILTER_TABLE_SIZE];
+      mutable float fFilterTableFloat[SKBITMAP_FILTER_TABLE_SIZE];
+  private:
+      void precomputeTable() const {
+          precomputed = true;
+          SkFixed *ftp = fFilterTable;
+          float *ftp_float = fFilterTableFloat;
+          for (int x = 0; x < SKBITMAP_FILTER_TABLE_SIZE; ++x) {
+              float fx = ((float)x + .5f) * this->width() / SKBITMAP_FILTER_TABLE_SIZE;
+              float filter_value = evaluate(fx);
+              *ftp_float++ = filter_value;
+              *ftp++ = SkFloatToFixed(filter_value);
+          }
+      }
+};
+
+class SkMitchellFilter: public SkBitmapFilter {
+  public:
+      SkMitchellFilter(float b, float c, float width=2.0f)
+      : SkBitmapFilter(width), B(b), C(c) { 
+      }
+      
+      virtual float evaluate(float x) const SK_OVERRIDE {
+          x = fabsf(x);
+          float ret;
+          if (x > 2.f) {
+              return 0;
+          } else if (x > 1.f) {
+              return ((-B - 6*C) * x*x*x + (6*B + 30*C) * x*x +
+                      (-12*B - 48*C) * x + (8*B + 24*C)) * (1.f/6.f);
+          } else {
+              return ((12 - 9*B - 6*C) * x*x*x +
+                      (-18 + 12*B + 6*C) * x*x +
+                      (6 - 2*B)) * (1.f/6.f);
+          }
+      }
+  protected:
+      float B, C;
+};
+
+class SkGaussianFilter: public SkBitmapFilter {
+  public:
+      SkGaussianFilter(float a, float width=2.0f)
+      : SkBitmapFilter(width), alpha(a), expWidth(expf(-alpha * width * width)) { 
+      }
+      
+      virtual float evaluate(float x) const SK_OVERRIDE {
+          return SkTMax(0.f, float(expf(-alpha*x*x) - expWidth));
+      }
+  protected:
+      float alpha, expWidth;
+};
+
+class SkTriangleFilter: public SkBitmapFilter {
+  public:
+      SkTriangleFilter(float width=1)
+      : SkBitmapFilter(width) { 
+      }
+      
+      virtual float evaluate(float x) const SK_OVERRIDE {
+          return SkTMax(0.f, fWidth - fabsf(x));
+      }
+  protected:
+};
+
+class SkBoxFilter: public SkBitmapFilter {
+  public:
+      SkBoxFilter(float width=0.5f)
+      : SkBitmapFilter(width) { 
+      }
+      
+      virtual float evaluate(float x) const SK_OVERRIDE {
+          return 1;
+      }
+  protected:
+};
+
+
+class SkSincFilter: public SkBitmapFilter {
+  public:
+      SkSincFilter(float t, float width=3.f)
+      : SkBitmapFilter(width), tau(t) { 
+      }
+      
+      virtual float evaluate(float x) const SK_OVERRIDE {
+          x = sk_float_abs(x * fInvWidth);
+          if (x < 1e-5f) return 1.f;
+          if (x > 1.f)   return 0.f;
+          x *= M_PI;
+          float sinc = sk_float_sin(x) / x;
+          float lanczos = sk_float_sin(x * tau) / (x * tau);
+          return sinc * lanczos;
+      }
+  protected:
+      float tau;
+};
+
+
+#endif