[core] C++ port of TinySDF

3 files changed, 127 insertions, 0 deletions

diff --git a/cmake/core-files.cmake b/cmake/core-files.cmake
index d6c9493742..138fea1bcb 100644
--- a/cmake/core-files.cmake
+++ b/cmake/core-files.cmake
@@ -689,6 +689,8 @@ set(MBGL_CORE_FILES
     src/mbgl/util/tile_coordinate.hpp
     src/mbgl/util/tile_cover.cpp
     src/mbgl/util/tile_cover.hpp
+    src/mbgl/util/tiny_sdf.cpp
+    src/mbgl/util/tiny_sdf.hpp
     src/mbgl/util/token.hpp
     src/mbgl/util/url.cpp
     src/mbgl/util/url.hpp
diff --git a/src/mbgl/util/tiny_sdf.cpp b/src/mbgl/util/tiny_sdf.cpp
new file mode 100644
index 0000000000..60839357d5
--- /dev/null
+++ b/src/mbgl/util/tiny_sdf.cpp
@@ -0,0 +1,105 @@
+#include <mbgl/util/tiny_sdf.hpp>
+
+#include <mbgl/util/math.hpp>
+
+#include <algorithm>
+
+namespace mbgl {
+namespace util {
+
+namespace tinysdf {
+
+static const double INF = 1e20;
+
+// 1D squared distance transform
+void edt1d(std::vector<double>& f,
+           std::vector<double>& d,
+           std::vector<int16_t>& v,
+           std::vector<double>& z,
+           uint32_t n) {
+    v[0] = 0;
+    z[0] = -INF;
+    z[1] = +INF;
+
+    for (uint32_t q = 1, k = 0; q < n; q++) {
+        double s = ((f[q] + q * q) - (f[v[k]] + v[k] * v[k])) / (2 * q - 2 * v[k]);
+        while (s <= z[k]) {
+            k--;
+            s = ((f[q] + q * q) - (f[v[k]] + v[k] * v[k])) / (2 * q - 2 * v[k]);
+        }
+        k++;
+        v[k] = q;
+        z[k] = s;
+        z[k + 1] = +INF;
+    }
+
+    for (uint32_t q = 0, k = 0; q < n; q++) {
+        while (z[k + 1] < q) k++;
+        d[q] = (q - v[k]) * (q - v[k]) + f[v[k]];
+    }
+}
+
+
+// 2D Euclidean distance transform by Felzenszwalb & Huttenlocher https://cs.brown.edu/~pff/dt/
+void edt(std::vector<double>& data,
+         uint32_t width,
+         uint32_t height,
+         std::vector<double>& f,
+         std::vector<double>& d,
+         std::vector<int16_t>& v,
+         std::vector<double>& z) {
+    for (uint32_t x = 0; x < width; x++) {
+        for (uint32_t y = 0; y < height; y++) {
+            f[y] = data[y * width + x];
+        }
+        edt1d(f, d, v, z, height);
+        for (uint32_t y = 0; y < height; y++) {
+            data[y * width + x] = d[y];
+        }
+    }
+    for (uint32_t y = 0; y < height; y++) {
+        for (uint32_t x = 0; x < width; x++) {
+            f[x] = data[y * width + x];
+        }
+        edt1d(f, d, v, z, width);
+        for (uint32_t x = 0; x < width; x++) {
+            data[y * width + x] = std::sqrt(d[x]);
+        }
+    }
+}
+
+} // namespace tinysdf
+
+AlphaImage transformRasterToSDF(const AlphaImage& rasterInput, double radius, double cutoff) {
+    uint32_t size = rasterInput.size.width * rasterInput.size.height;
+    uint32_t maxDimension = std::max(rasterInput.size.width, rasterInput.size.height);
+    
+    AlphaImage sdf(rasterInput.size);
+    
+    // temporary arrays for the distance transform
+    std::vector<double> gridOuter(size);
+    std::vector<double> gridInner(size);
+    std::vector<double> f(maxDimension);
+    std::vector<double> d(maxDimension);
+    std::vector<double> z(maxDimension + 1);
+    std::vector<int16_t> v(maxDimension);
+    
+    for (uint32_t i = 0; i < size; i++) {
+        double a = double(rasterInput.data[i]) / 255; // alpha value
+        gridOuter[i] = a == 1.0 ? 0.0 : a == 0.0 ? tinysdf::INF : std::pow(std::max(0.0, 0.5 - a), 2.0);
+        gridInner[i] = a == 1.0 ? tinysdf::INF : a == 0.0 ? 0.0 : std::pow(std::max(0.0, a - 0.5), 2.0);
+    }
+
+    tinysdf::edt(gridOuter, rasterInput.size.width, rasterInput.size.height, f, d, v, z);
+    tinysdf::edt(gridInner, rasterInput.size.width, rasterInput.size.height, f, d, v, z);
+
+    for (uint32_t i = 0; i < size; i++) {
+        double distance = gridOuter[i] - gridInner[i];
+        sdf.data[i] = std::max(0l, std::min(255l, std::lround(255.0 - 255.0 * (distance / radius + cutoff))));
+    }
+
+    return sdf;
+}
+
+} // namespace util
+} // namespace mbgl
diff --git a/src/mbgl/util/tiny_sdf.hpp b/src/mbgl/util/tiny_sdf.hpp
new file mode 100644
index 0000000000..33c9280cbd
--- /dev/null
+++ b/src/mbgl/util/tiny_sdf.hpp
@@ -0,0 +1,20 @@
+#pragma once
+
+#include <mbgl/util/image.hpp>
+
+namespace mbgl {
+namespace util {
+
+/*
+    C++ port of https://github.com/mapbox/tiny-sdf, which is in turn based on the
+    Felzenszwalb/Huttenlocher distance transform paper (https://cs.brown.edu/~pff/papers/dt-final.pdf).
+    Note there exists an alternative C++ implementation from the paper’s authors at
+    https://cs.brown.edu/~pff/dt/, which this implementation is not based on.
+ 
+    Takes an alpha channel raster input and transforms it into an alpha channel
+    Signed Distance Field (SDF) output of the same dimensions.
+*/
+AlphaImage transformRasterToSDF(const AlphaImage& rasterInput, double radius, double cutoff);
+
+} // namespace util
+} // namespace mbgl