[docs] Include uncompressed shader source in .cpp comments

This is meant to
(1) Make it easier for new developers to find the source
(2) Make it easier to look at shader diffs when the GL JS pin changes

1 files changed, 73 insertions, 0 deletions

diff --git a/src/mbgl/shaders/hillshade.cpp b/src/mbgl/shaders/hillshade.cpp
index 4ff8416d5e..4a2babe278 100644
--- a/src/mbgl/shaders/hillshade.cpp
+++ b/src/mbgl/shaders/hillshade.cpp
@@ -10,5 +10,78 @@ const char* hillshade::name = "hillshade";
 const char* hillshade::vertexSource = source() + 38765;
 const char* hillshade::fragmentSource = source() + 38964;
 
+// Uncompressed source of hillshade.vertex.glsl:
+/*
+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;
+}
+
+*/
+
+// Uncompressed source of hillshade.fragment.glsl:
+/*
+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
+}
+
+*/
+
 } // namespace shaders
 } // namespace mbgl