Apply fog in fragment shader in terrain_raster program (#12423)

* Add render test for fog with terrain and zero exaggeration

* Return early in elevation function when zero exaggeration is used

* Calculate fog opacity in terrain raster fragment shader when zero exaggeration is used

src/render/painter.js

@@ -859,11 +859,13 @@ class Painter {
      */
     currentGlobalDefines(): string[] {
         const rtt = this.terrain && this.terrain.renderingToTexture;
+        const zeroExaggeration = this.terrain && this.terrain.exaggeration() === 0.0;
         const fog = this.style && this.style.fog;
         const defines = [];
 
         if (this.terrainRenderModeElevated()) defines.push('TERRAIN');
         if (this.transform.projection.name === 'globe') defines.push('GLOBE');
+        if (zeroExaggeration) defines.push('ZERO_EXAGGERATION');
         // When terrain is active, fog is rendered as part of draping, not as part of tile
         // rendering. Removing the fog flag during tile rendering avoids additional defines.
         if (fog && !rtt && fog.getOpacity(this.transform.pitch) !== 0.0) {

src/shaders/_prelude_terrain.vertex.glsl

@@ -100,12 +100,18 @@ float prevElevation(vec2 apos) {
 
 #ifdef TERRAIN_VERTEX_MORPHING
 float elevation(vec2 apos) {
+    #ifdef ZERO_EXAGGERATION
+        return 0.0;
+    #endif
     float nextElevation = currentElevation(apos);
     float prevElevation = prevElevation(apos);
     return mix(prevElevation, nextElevation, u_dem_lerp);
 }
 #else
 float elevation(vec2 apos) {
+    #ifdef ZERO_EXAGGERATION
+        return 0.0;
+    #endif
     return currentElevation(apos);
 }
 #endif

src/shaders/terrain_raster.fragment.glsl

@@ -19,7 +19,11 @@ void main() {
 #endif
 
 #ifdef FOG
+#ifdef ZERO_EXAGGERATION
+    color = fog_dither(fog_apply_premultiplied(color, v_fog_pos));
+#else
     color = fog_dither(fog_apply_from_vert(color, v_fog_opacity));
+#endif
 #endif
     gl_FragColor = color;
 #ifdef TERRAIN_WIREFRAME

src/shaders/terrain_raster.vertex.glsl

@@ -32,8 +32,12 @@ void main() {
     gl_Position = u_matrix * vec4(decodedPos, elevation, 1.0);
 
 #ifdef FOG
+#ifdef ZERO_EXAGGERATION
+    v_fog_pos = fog_position(decodedPos);
+#else
     v_fog_opacity = fog(fog_position(vec3(decodedPos, elevation)));
 #endif
+#endif
 
 #ifdef RENDER_SHADOWS
     vec3 pos = vec3(decodedPos, elevation);

test/integration/render-tests/fog/terrain/zero-exaggeration/style.json

@@ -0,0 +1,59 @@
+{
+  "version": 8,
+  "metadata": {
+    "test": {
+      "height": 256,
+      "width": 256
+    }
+  },
+  "center": [-113.32296, 35.94662],
+  "zoom": 5.5,
+  "pitch": 80.0,
+  "bearing": 64.5,
+  "terrain": {
+    "source": "rgbterrain",
+    "exaggeration": 0.0
+  },
+  "fog": {
+    "range": [-0.2, 0.5],
+    "color": "white",
+    "horizon-blend": 0.15
+  },
+  "sources": {
+    "rgbterrain": {
+      "type": "raster-dem",
+      "tiles": [
+        "local://tiles/{z}-{x}-{y}.terrain.png"
+      ],
+      "maxzoom": 12,
+      "tileSize": 256
+    },
+    "satellite": {
+      "type": "raster",
+      "tiles": [
+        "local://tiles/{z}-{x}-{y}.satellite.png"
+      ],
+      "maxzoom": 17,
+      "tileSize": 256
+    }
+  },
+  "layers": [
+    {
+      "id": "sky",
+      "type": "sky",
+      "paint": {
+        "sky-type": "atmosphere",
+        "sky-atmosphere-sun": [0, 0],
+        "sky-atmosphere-sun-intensity": 15
+      }
+    },
+    {
+      "id": "raster",
+      "type": "raster",
+      "source": "satellite",
+      "paint": {
+        "raster-fade-duration": 0
+      }
+    }
+  ]
+}