4 files changed, 211 insertions, 0 deletions
diff --git a/src/mbgl/shaders/hillshade.cpp b/src/mbgl/shaders/hillshade.cpp
new file mode 100644
index 0000000000..4083faa4b4
--- /dev/null
+++ b/src/mbgl/shaders/hillshade.cpp
@@ -0,0 +1,80 @@
+// NOTE: DO NOT CHANGE THIS FILE. IT IS AUTOMATICALLY GENERATED.
+
+#include <mbgl/shaders/hillshade.hpp>
+
+namespace mbgl {
+namespace shaders {
+
+const char* hillshade::name = "hillshade";
+const char* hillshade::vertexSource = R"MBGL_SHADER(
+uniform mat4 u_matrix;
+
+attribute vec2 a_pos;
+attribute vec2 a_texture_pos;
+
+varying vec2 v_pos;
+
+void main() {
+    gl_Position = u_matrix * vec4(a_pos, 0, 1);
+    v_pos = a_texture_pos / 8192.0;
+}
+
+)MBGL_SHADER";
+const char* hillshade::fragmentSource = R"MBGL_SHADER(
+uniform sampler2D u_image;
+varying vec2 v_pos;
+
+uniform vec2 u_latrange;
+uniform vec2 u_light;
+uniform vec4 u_shadow;
+uniform vec4 u_highlight;
+uniform vec4 u_accent;
+
+#define PI 3.141592653589793
+
+void main() {
+    vec4 pixel = texture2D(u_image, v_pos);
+
+    vec2 deriv = ((pixel.rg * 2.0) - 1.0);
+
+    // We divide the slope by a scale factor based on the cosin of the pixel's approximate latitude
+    // to account for mercator projection distortion. see #4807 for details
+    float scaleFactor = cos(radians((u_latrange[0] - u_latrange[1]) * (1.0 - v_pos.y) + u_latrange[1]));
+    // We also multiply the slope by an arbitrary z-factor of 1.25
+    float slope = atan(1.25 * length(deriv) / scaleFactor);
+    float aspect = deriv.x != 0.0 ? atan(deriv.y, -deriv.x) : PI / 2.0 * (deriv.y > 0.0 ? 1.0 : -1.0);
+
+    float intensity = u_light.x;
+    // We add PI to make this property match the global light object, which adds PI/2 to the light's azimuthal
+    // position property to account for 0deg corresponding to north/the top of the viewport in the style spec
+    // and the original shader was written to accept (-illuminationDirection - 90) as the azimuthal.
+    float azimuth = u_light.y + PI;
+
+    // We scale the slope exponentially based on intensity, using a calculation similar to
+    // the exponential interpolation function in the style spec:
+    // https://github.com/mapbox/mapbox-gl-js/blob/master/src/style-spec/expression/definitions/interpolate.js#L217-L228
+    // so that higher intensity values create more opaque hillshading.
+    float base = 1.875 - intensity * 1.75;
+    float maxValue = 0.5 * PI;
+    float scaledSlope = intensity != 0.5 ? ((pow(base, slope) - 1.0) / (pow(base, maxValue) - 1.0)) * maxValue : slope;
+
+    // The accent color is calculated with the cosine of the slope while the shade color is calculated with the sine
+    // so that the accent color's rate of change eases in while the shade color's eases out.
+    float accent = cos(scaledSlope);
+    // We multiply both the accent and shade color by a clamped intensity value
+    // so that intensities >= 0.5 do not additionally affect the color values
+    // while intensity values < 0.5 make the overall color more transparent.
+    vec4 accent_color = (1.0 - accent) * u_accent * clamp(intensity * 2.0, 0.0, 1.0);
+    float shade = abs(mod((aspect + azimuth) / PI + 0.5, 2.0) - 1.0);
+    vec4 shade_color = mix(u_shadow, u_highlight, shade) * sin(scaledSlope) * clamp(intensity * 2.0, 0.0, 1.0);
+    gl_FragColor = accent_color * (1.0 - shade_color.a) + shade_color;
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
+}
+
+)MBGL_SHADER";
+
+} // namespace shaders
+} // namespace mbgl
diff --git a/src/mbgl/shaders/hillshade.hpp b/src/mbgl/shaders/hillshade.hpp
new file mode 100644
index 0000000000..a4a27cb595
--- /dev/null
+++ b/src/mbgl/shaders/hillshade.hpp
@@ -0,0 +1,16 @@
+// NOTE: DO NOT CHANGE THIS FILE. IT IS AUTOMATICALLY GENERATED.
+
+#pragma once
+
+namespace mbgl {
+namespace shaders {
+
+class hillshade {
+public:
+    static const char* name;
+    static const char* vertexSource;
+    static const char* fragmentSource;
+};
+
+} // namespace shaders
+} // namespace mbgl
diff --git a/src/mbgl/shaders/hillshade_prepare.cpp b/src/mbgl/shaders/hillshade_prepare.cpp
new file mode 100644
index 0000000000..733658435e
--- /dev/null
+++ b/src/mbgl/shaders/hillshade_prepare.cpp
@@ -0,0 +1,99 @@
+// NOTE: DO NOT CHANGE THIS FILE. IT IS AUTOMATICALLY GENERATED.
+
+#include <mbgl/shaders/hillshade_prepare.hpp>
+
+namespace mbgl {
+namespace shaders {
+
+const char* hillshade_prepare::name = "hillshade_prepare";
+const char* hillshade_prepare::vertexSource = R"MBGL_SHADER(
+uniform mat4 u_matrix;
+
+attribute vec2 a_pos;
+attribute vec2 a_texture_pos;
+
+varying vec2 v_pos;
+
+void main() {
+    gl_Position = u_matrix * vec4(a_pos, 0, 1);
+    v_pos = (a_texture_pos / 8192.0) / 2.0 + 0.25;
+}
+
+)MBGL_SHADER";
+const char* hillshade_prepare::fragmentSource = R"MBGL_SHADER(
+#ifdef GL_ES
+precision highp float;
+#endif
+
+uniform sampler2D u_image;
+varying vec2 v_pos;
+uniform vec2 u_dimension;
+uniform float u_zoom;
+
+float getElevation(vec2 coord, float bias) {
+    // Convert encoded elevation value to meters
+    vec4 data = texture2D(u_image, coord) * 255.0;
+    return (data.r + data.g * 256.0 + data.b * 256.0 * 256.0) / 4.0;
+}
+
+void main() {
+    vec2 epsilon = 1.0 / u_dimension;
+
+    // queried pixels:
+    // +-----------+
+    // |   |   |   |
+    // | a | b | c |
+    // |   |   |   |
+    // +-----------+
+    // |   |   |   |
+    // | d | e | f |
+    // |   |   |   |
+    // +-----------+
+    // |   |   |   |
+    // | g | h | i |
+    // |   |   |   |
+    // +-----------+
+
+    float a = getElevation(v_pos + vec2(-epsilon.x, -epsilon.y), 0.0);
+    float b = getElevation(v_pos + vec2(0, -epsilon.y), 0.0);
+    float c = getElevation(v_pos + vec2(epsilon.x, -epsilon.y), 0.0);
+    float d = getElevation(v_pos + vec2(-epsilon.x, 0), 0.0);
+    float e = getElevation(v_pos, 0.0);
+    float f = getElevation(v_pos + vec2(epsilon.x, 0), 0.0);
+    float g = getElevation(v_pos + vec2(-epsilon.x, epsilon.y), 0.0);
+    float h = getElevation(v_pos + vec2(0, epsilon.y), 0.0);
+    float i = getElevation(v_pos + vec2(epsilon.x, epsilon.y), 0.0);
+
+    // here we divide the x and y slopes by 8 * pixel size
+    // where pixel size (aka meters/pixel) is:
+    // circumference of the world / (pixels per tile * number of tiles)
+    // which is equivalent to: 8 * 40075016.6855785 / (512 * pow(2, u_zoom))
+    // which can be reduced to: pow(2, 19.25619978527 - u_zoom)
+    // we want to vertically exaggerate the hillshading though, because otherwise
+    // it is barely noticeable at low zooms. to do this, we multiply this by some
+    // scale factor pow(2, (u_zoom - 14) * a) where a is an arbitrary value and 14 is the
+    // maxzoom of the tile source. here we use a=0.3 which works out to the
+    // expression below. see nickidlugash's awesome breakdown for more info
+    // https://github.com/mapbox/mapbox-gl-js/pull/5286#discussion_r148419556
+    float exaggeration = u_zoom < 2.0 ? 0.4 : u_zoom < 4.5 ? 0.35 : 0.3;
+
+    vec2 deriv = vec2(
+        (c + f + f + i) - (a + d + d + g),
+        (g + h + h + i) - (a + b + b + c)
+    ) /  pow(2.0, (u_zoom - 14.0) * exaggeration + 19.2562 - u_zoom);
+
+    gl_FragColor = clamp(vec4(
+        deriv.x / 2.0 + 0.5,
+        deriv.y / 2.0 + 0.5,
+        1.0,
+        1.0), 0.0, 1.0);
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
+}
+
+)MBGL_SHADER";
+
+} // namespace shaders
+} // namespace mbgl
diff --git a/src/mbgl/shaders/hillshade_prepare.hpp b/src/mbgl/shaders/hillshade_prepare.hpp
new file mode 100644
index 0000000000..c38b4a0d19
--- /dev/null
+++ b/src/mbgl/shaders/hillshade_prepare.hpp
@@ -0,0 +1,16 @@
+// NOTE: DO NOT CHANGE THIS FILE. IT IS AUTOMATICALLY GENERATED.
+
+#pragma once
+
+namespace mbgl {
+namespace shaders {
+
+class hillshade_prepare {
+public:
+    static const char* name;
+    static const char* vertexSource;
+    static const char* fragmentSource;
+};
+
+} // namespace shaders
+} // namespace mbgl