consolidate functions to only use stops

refs mapbox/mapbox-gl-style-spec#96

5 files changed, 72 insertions, 195 deletions

diff --git a/include/mbgl/style/function_properties.hpp b/include/mbgl/style/function_properties.hpp
index 74ac80f83c..8cd7ce6e28 100644
--- a/include/mbgl/style/function_properties.hpp
+++ b/include/mbgl/style/function_properties.hpp
@@ -17,43 +17,19 @@ private:
 };
 
 template <typename T>
-struct LinearFunction {
-    inline LinearFunction(const T &value, float z_base, float slope, const T &min, const T &max)
-        : value(value), min(min), max(max), z_base(z_base), slope(slope) {}
-    T evaluate(float z) const;
-
-private:
-    const T value, min, max;
-    const float z_base, slope;
-};
-
-template <typename T>
-struct ExponentialFunction {
-    inline ExponentialFunction(const T &value, float z_base, float exp_base, float slope, const T &min,
-                               const T &max)
-        : value(value), min(min), max(max), z_base(z_base), exp_base(exp_base), slope(slope) {}
-    T evaluate(float z) const;
-
-private:
-    const T value, min, max;
-    const float z_base, exp_base, slope;
-};
-
-template <typename T>
 struct StopsFunction {
-    inline StopsFunction(const std::vector<std::pair<float, T>> &values) : values(values) {}
+    inline StopsFunction(const std::vector<std::pair<float, T>> &values, float base) : values(values), base(base) {}
     T evaluate(float z) const;
 
 private:
     const std::vector<std::pair<float, T>> values;
+    const float base;
 };
 
 template <typename T>
 using Function = util::variant<
     std::false_type,
     ConstantFunction<T>,
-    LinearFunction<T>,
-    ExponentialFunction<T>,
     StopsFunction<T>
 >;
 
diff --git a/src/style/function_properties.cpp b/src/style/function_properties.cpp
index 2b378fb6b9..879de84f85 100644
--- a/src/style/function_properties.cpp
+++ b/src/style/function_properties.cpp
@@ -6,82 +6,36 @@
 namespace mbgl {
 
 
-template <>
-bool LinearFunction<bool>::evaluate(float z) const {
-    return z < z_base ? slope >= 0 : z > z_base ? slope >= 0 : value;
-}
-
-template <>
-float LinearFunction<float>::evaluate(float z) const {
-    return std::fmin(std::fmax(min, value + (z - z_base) * slope), max);
-}
-
-template <>
-Color LinearFunction<Color>::evaluate(float z) const {
-    return {{
-        std::fmin(std::fmax(min[0], value[0] + (z - z_base) * slope), max[0]),
-        std::fmin(std::fmax(min[1], value[1] + (z - z_base) * slope), max[1]),
-        std::fmin(std::fmax(min[2], value[2] + (z - z_base) * slope), max[2]),
-        std::fmin(std::fmax(min[3], value[3] + (z - z_base) * slope), max[3])
-    }};
-}
-
-
-template <>
-bool ExponentialFunction<bool>::evaluate(float z) const {
-    return z < z_base ? slope >= 0 : z > z_base ? slope >= 0 : value;
-}
-
-template <>
-float ExponentialFunction<float>::evaluate(float z) const {
-    return std::fmin(std::fmax(min, value + std::pow(exp_base, (z - z_base)) * slope), max);
-}
-
-template <>
-Color ExponentialFunction<Color>::evaluate(float z) const {
-    return {{
-        std::fmin(std::fmax(min[0], value[0] + float(std::pow(exp_base, (z - z_base))) * slope), max[0]),
-        std::fmin(std::fmax(min[1], value[1] + float(std::pow(exp_base, (z - z_base))) * slope), max[1]),
-        std::fmin(std::fmax(min[2], value[2] + float(std::pow(exp_base, (z - z_base))) * slope), max[2]),
-        std::fmin(std::fmax(min[3], value[3] + float(std::pow(exp_base, (z - z_base))) * slope), max[3])
-    }};
-}
-
 template <typename T>
-inline T exponentialInterpolate(T smaller, T larger, const float factor);
+inline T interpolate(T smaller, T larger, const float factor);
 
 template <>
-inline float exponentialInterpolate(const float smaller, const float larger, const float factor) {
-    // Linear interpolation if base is 0
-    if (smaller == 0.0f) {
-        return factor * larger;
-    }
-    // Exponential interpolation between the values
-    return smaller * std::pow(larger / smaller, factor);
+inline float interpolate(const float smaller, const float larger, const float factor) {
+    return (smaller * (1 - factor)) + (larger * factor);
 }
 
 template <>
-inline bool exponentialInterpolate(const bool smaller, const bool larger, const float factor) {
-    return exponentialInterpolate(float(smaller), float(larger), factor);
+inline bool interpolate(const bool smaller, const bool larger, const float factor) {
+    return interpolate(float(smaller), float(larger), factor);
 }
 
 template <>
-inline Color exponentialInterpolate(const Color smaller, const Color larger, const float factor) {
+inline Color interpolate(const Color smaller, const Color larger, const float factor) {
     return {{
-        exponentialInterpolate(smaller[0], larger[0], factor),
-        exponentialInterpolate(smaller[1], larger[1], factor),
-        exponentialInterpolate(smaller[2], larger[2], factor),
-        exponentialInterpolate(smaller[3], larger[3], factor)
+        interpolate(smaller[0], larger[0], factor),
+        interpolate(smaller[1], larger[1], factor),
+        interpolate(smaller[2], larger[2], factor),
+        interpolate(smaller[3], larger[3], factor)
     }};
 }
 
 
 template <typename T>
-T exponentialDefault();
+inline T defaultStopsValue();
 
-template <> bool exponentialDefault() { return true; }
-template <> float exponentialDefault() { return 1.0f; }
-template <> Color exponentialDefault() { return {{ 0, 0, 0, 1 }}; }
+template <> inline bool defaultStopsValue() { return true; }
+template <> inline float defaultStopsValue() { return 1.0f; }
+template <> inline Color defaultStopsValue() { return {{ 0, 0, 0, 1 }}; }
 
 
 template <typename T>
@@ -112,15 +66,22 @@ T StopsFunction<T>::evaluate(float z) const {
         if (larger_z == smaller_z || larger_val == smaller_val) {
             return smaller_val;
         }
-        float factor = (z - smaller_z) / (larger_z - smaller_z);
-        return exponentialInterpolate<T>(smaller_val, larger_val, factor);
+        const float zoomDiff = larger_z - smaller_z;
+        const float zoomProgress = z - smaller_z;
+        if (base == 1.0f) {
+            const float t = zoomProgress / zoomDiff;
+            return interpolate<T>(smaller_val, larger_val, t);
+        } else {
+            const float t = (std::pow(base, zoomProgress) - 1) / (std::pow(base, zoomDiff) - 1);
+            return interpolate<T>(smaller_val, larger_val, t);
+        }
     } else if (larger) {
         return larger_val;
     } else if (smaller) {
         return smaller_val;
     } else {
         // No stop defined.
-        return exponentialDefault<T>();
+        return defaultStopsValue<T>();
     }
 }
 
diff --git a/src/style/style_parser.cpp b/src/style/style_parser.cpp
index faa8caeb98..3c79ee3c92 100644
--- a/src/style/style_parser.cpp
+++ b/src/style/style_parser.cpp
@@ -230,95 +230,56 @@ Color StyleParser::parseFunctionArgument(JSVal value) {
     return parseColor(rvalue);
 }
 
+template <typename T> inline float defaultBaseValue() { return 1.75; }
+template <> inline float defaultBaseValue<Color>() { return 1.0; }
+
 template <typename T>
 std::tuple<bool, Function<T>> StyleParser::parseFunction(JSVal value) {
-    if (!value.HasMember("fn")) {
-        fprintf(stderr, "[WARNING] function must specify a function name\n");
-        return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-    }
-
-    JSVal value_fn = value["fn"];
-    if (!value_fn.IsString()) {
-        fprintf(stderr, "[WARNING] function must specify a function type\n");
+    if (!value.HasMember("stops")) {
+        fprintf(stderr, "[WARNING] stops function must specify a stops array\n");
         return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
     }
 
-    const std::string type { value_fn.GetString(), value_fn.GetStringLength() };
+    float base = defaultBaseValue<T>();
 
-    if (type == "linear") {
-        if (!value.HasMember("z")) {
-            fprintf(stderr, "[WARNING] linear function must specify a base zoom level\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
-        if (!value.HasMember("val")) {
-            fprintf(stderr, "[WARNING] linear function must specify a base value\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+    if (value.HasMember("base")) {
+        JSVal value_base = value["base"];
+        if (value_base.IsNumber()) {
+            base = value_base.GetDouble();
+        } else {
+            fprintf(stderr, "[WARNING] base must be numeric\n");
         }
-        const float z_base = parseFunctionArgument<float>(value["z"]);
-        const T val = parseFunctionArgument<T>(value["val"]);
-        const float slope = value.HasMember("slope") ? parseFunctionArgument<float>(value["slope"]) : 1;
-        const T min = value.HasMember("min") ? parseFunctionArgument<T>(value["min"]) : T();
-        const T max = value.HasMember("max") ? parseFunctionArgument<T>(value["max"]) : T();
-        return std::tuple<bool, Function<T>> { true, LinearFunction<T>(val, z_base, slope, min, max) };
     }
-    else if (type == "exponential") {
-        if (!value.HasMember("z")) {
-            fprintf(stderr, "[WARNING] exponential function must specify a base zoom level\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
-        if (!value.HasMember("val")) {
-            fprintf(stderr, "[WARNING] exponential function must specify a base value\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
-        const float z_base = parseFunctionArgument<float>(value["z"]);
-        const float exp_base =  value.HasMember("base") ? parseFunctionArgument<float>(value["base"]) : 1.75;
-        const T val = parseFunctionArgument<T>(value["val"]);
-        const float slope = value.HasMember("slope") ? parseFunctionArgument<float>(value["slope"]) : 1;
-        const T min = value.HasMember("min") ? parseFunctionArgument<T>(value["min"]) : T();
-        const T max = value.HasMember("max") ? parseFunctionArgument<T>(value["max"]) : T();
-        return std::tuple<bool, Function<T>> { true, ExponentialFunction<T>(val, z_base, exp_base, slope, min, max) };
-    }
-    else if (type == "stops") {
-        if (!value.HasMember("stops")) {
-            fprintf(stderr, "[WARNING] stops function must specify a stops array\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
 
-        JSVal value_stops = value["stops"];
-        if (!value_stops.IsArray()) {
-            fprintf(stderr, "[WARNING] stops function must specify a stops array\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
-
-        std::vector<std::pair<float, T>> stops;
-        for (rapidjson::SizeType i = 0; i < value_stops.Size(); ++i) {
-            JSVal stop = value_stops[i];
-            if (stop.IsArray()) {
-                if (stop.Size() != 2) {
-                    fprintf(stderr, "[WARNING] stop must have zoom level and value specification\n");
-                    return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-                }
+    JSVal value_stops = value["stops"];
+    if (!value_stops.IsArray()) {
+        fprintf(stderr, "[WARNING] stops function must specify a stops array\n");
+        return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+    }
 
-                JSVal z = stop[rapidjson::SizeType(0)];
-                if (!z.IsNumber()) {
-                    fprintf(stderr, "[WARNING] zoom level in stop must be a number\n");
-                    return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-                }
+    std::vector<std::pair<float, T>> stops;
+    for (rapidjson::SizeType i = 0; i < value_stops.Size(); ++i) {
+        JSVal stop = value_stops[i];
+        if (stop.IsArray()) {
+            if (stop.Size() != 2) {
+                fprintf(stderr, "[WARNING] stop must have zoom level and value specification\n");
+                return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+            }
 
-                stops.emplace_back(z.GetDouble(), parseFunctionArgument<T>(stop[rapidjson::SizeType(1)]));
-            } else {
-                fprintf(stderr, "[WARNING] function argument must be a numeric value\n");
+            JSVal z = stop[rapidjson::SizeType(0)];
+            if (!z.IsNumber()) {
+                fprintf(stderr, "[WARNING] zoom level in stop must be a number\n");
                 return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
             }
-        }
 
-        return std::tuple<bool, Function<T>> { true, StopsFunction<T>(stops) };
-    }
-    else {
-        fprintf(stderr, "[WARNING] function type '%s' is unknown\n", type.c_str());
+            stops.emplace_back(z.GetDouble(), parseFunctionArgument<T>(stop[rapidjson::SizeType(1)]));
+        } else {
+            fprintf(stderr, "[WARNING] function argument must be a numeric value\n");
+            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+        }
     }
 
-    return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+    return std::tuple<bool, Function<T>> { true, StopsFunction<T>(stops, base) };
 }
 
 
diff --git a/test/fixtures/styles/road-width.style.json b/test/fixtures/styles/road-width.style.json
index 5738546b33..76d7b5258c 100644
--- a/test/fixtures/styles/road-width.style.json
+++ b/test/fixtures/styles/road-width.style.json
@@ -23,7 +23,6 @@
       "type": "line",
       "style": {
         "line-width": {
-          "fn": "stops",
           "stops": [[13, 0], [13.999, 0], [14, 4], [14.1, 10], [14.2, 20]]
         }
       }
diff --git a/test/functions.cpp b/test/functions.cpp
index a2ad93c4e1..56b2a31706 100644
--- a/test/functions.cpp
+++ b/test/functions.cpp
@@ -19,11 +19,11 @@ TEST(Function, Constant) {
 
 TEST(Function, Stops) {
     // Explicit constant slope in fringe regions.
-    mbgl::StopsFunction<float> slope_1({ { 0, 1.5 }, { 6, 1.5 }, { 8, 3 }, { 22, 3 } });
+    mbgl::StopsFunction<float> slope_1({ { 0, 1.5 }, { 6, 1.5 }, { 8, 3 }, { 22, 3 } }, 1.75);
     EXPECT_EQ(1.5, slope_1.evaluate(0));
     EXPECT_EQ(1.5, slope_1.evaluate(4));
     EXPECT_EQ(1.5, slope_1.evaluate(6));
-    ASSERT_FLOAT_EQ(2.12132, slope_1.evaluate(7));
+    ASSERT_FLOAT_EQ(2.0454545454545454, slope_1.evaluate(7));
     EXPECT_EQ(3.0, slope_1.evaluate(8));
     EXPECT_EQ(3.0, slope_1.evaluate(9));
     EXPECT_EQ(3.0, slope_1.evaluate(15));
@@ -31,48 +31,28 @@ TEST(Function, Stops) {
 
 
     // Test constant values in fringe regions.
-    mbgl::StopsFunction<float> slope_2({ { 6, 1.5 }, { 8, 3 } });
+    mbgl::StopsFunction<float> slope_2({ { 6, 1.5 }, { 8, 3 } }, 1.75);
     EXPECT_EQ(1.5, slope_2.evaluate(0));
     EXPECT_EQ(1.5, slope_2.evaluate(4));
     EXPECT_EQ(1.5, slope_2.evaluate(6));
-    ASSERT_FLOAT_EQ(2.12132, slope_2.evaluate(7));
+    ASSERT_FLOAT_EQ(2.0454545454545454, slope_2.evaluate(7));
     EXPECT_EQ(3.0, slope_2.evaluate(8));
     EXPECT_EQ(3.0, slope_2.evaluate(9));
     EXPECT_EQ(3.0, slope_2.evaluate(15));
     EXPECT_EQ(3.0, slope_2.evaluate(22));
 
     // Test no values.
-    mbgl::StopsFunction<float> slope_3({});
+    mbgl::StopsFunction<float> slope_3({}, 1.75);
     EXPECT_EQ(1, slope_3.evaluate(2));
     EXPECT_EQ(1, slope_3.evaluate(6));
     EXPECT_EQ(1, slope_3.evaluate(12));
-}
-
-
-TEST(Function, Linear) {
-    mbgl::LinearFunction<float> slope_1(/* val */ 7.5, /* z_base */ 4, /* slope */ 2, /* min */ 7.5, /* max */ 20);
-    ASSERT_FLOAT_EQ(7.5, slope_1.evaluate(3));
-    ASSERT_FLOAT_EQ(7.5, slope_1.evaluate(4));
-    ASSERT_FLOAT_EQ(8.5, slope_1.evaluate(4.5));
-    ASSERT_FLOAT_EQ(9.5, slope_1.evaluate(5));
-    ASSERT_FLOAT_EQ(11.5, slope_1.evaluate(6));
-    ASSERT_FLOAT_EQ(19.5, slope_1.evaluate(10));
-    ASSERT_FLOAT_EQ(20, slope_1.evaluate(11));
-    ASSERT_FLOAT_EQ(20, slope_1.evaluate(20));
-}
 
 
-TEST(Function, Exponential) {
-    mbgl::ExponentialFunction<float> slope_1(/* val */ 7.5, /* z_base */ 4, /* exp_base */ 1.75, /* slope */ 2, /* min */ 7.5, /* max */ 20);
-    ASSERT_FLOAT_EQ(8.6428576, slope_1.evaluate(3)); // 7.5 + 1.75^(3 - 4) * 2
-    ASSERT_FLOAT_EQ(9.5, slope_1.evaluate(4)); // 7.5 + 1.75^(4 - 4) * 2
-    ASSERT_FLOAT_EQ(10.145751, slope_1.evaluate(4.5)); // 7.5 + 1.75^(4.5 - 4) * 2
-    ASSERT_FLOAT_EQ(11, slope_1.evaluate(5)); // 7.5 + 1.75^(5 - 4) * 2
-    ASSERT_FLOAT_EQ(13.625, slope_1.evaluate(6)); // 7.5 + 1.75^(6 - 4) * 2
-    ASSERT_FLOAT_EQ(18.21875, slope_1.evaluate(7)); // 7.5 + 1.75^(7 - 4) * 2
-    ASSERT_FLOAT_EQ(20, slope_1.evaluate(8)); // 7.5 + 1.75^(8 - 4) * 2  ==> clamped to 20
-    ASSERT_FLOAT_EQ(20, slope_1.evaluate(20)); // 7.5 + 1.75^(20 - 4) * 2 ==> clamped to 20
+    // Explicit constant slope in fringe regions.
+    mbgl::StopsFunction<float> slope_4({ { 0, 2 }, { 8, 10 } }, 1);
+    EXPECT_EQ(2, slope_4.evaluate(0));
+    EXPECT_EQ(3, slope_4.evaluate(1));
+    EXPECT_EQ(4, slope_4.evaluate(2));
+    EXPECT_EQ(4.75, slope_4.evaluate(2.75));
+    EXPECT_EQ(10, slope_4.evaluate(8));
 }
-
-
-