Terrain: shader support for sampling DEM elevation from float textures

Feature is implemented in gl-native first. Since shader code is shared, code here is
not used by gl-js (`TERRAIN_DEM_FLOAT_FORMAT` is not defined).

src/shaders/_prelude_terrain.vertex.glsl

@@ -4,8 +4,13 @@
 
 #ifdef TERRAIN
 
+#ifdef TERRAIN_DEM_FLOAT_FORMAT
+uniform highp sampler2D u_dem;
+uniform highp sampler2D u_dem_prev;
+#else
 uniform sampler2D u_dem;
 uniform sampler2D u_dem_prev;
+#endif
 uniform vec4 u_dem_unpack;
 uniform vec2 u_dem_tl;
 uniform vec2 u_dem_tl_prev;
@@ -31,6 +36,10 @@ float decodeElevation(vec4 v) {
 }
 
 float currentElevation(vec2 apos) {
+#ifdef TERRAIN_DEM_FLOAT_FORMAT
+    vec2 pos = (u_dem_size * (apos / 8192.0 * u_dem_scale + u_dem_tl) + 1.0) / (u_dem_size + 2.0);
+    return u_exaggeration * texture2D(u_dem, pos).a;
+#else
     float dd = 1.0 / (u_dem_size + 2.0);
     vec4 r = tileUvToDemSample(apos / 8192.0, u_dem_size, u_dem_scale, u_dem_tl);
     vec2 pos = r.xy;
@@ -45,9 +54,14 @@ float currentElevation(vec2 apos) {
     float br = decodeElevation(texture2D(u_dem, pos + vec2(dd, dd)));
 
     return u_exaggeration * mix(mix(tl, tr, f.x), mix(bl, br, f.x), f.y);
+#endif
 }
 
 float prevElevation(vec2 apos) {
+#ifdef TERRAIN_DEM_FLOAT_FORMAT
+    vec2 pos = (u_dem_size * (apos / 8192.0 * u_dem_scale_prev + u_dem_tl_prev) + 1.0) / (u_dem_size + 2.0);
+    return u_exaggeration * texture2D(u_dem_prev, pos).a;
+#else
     float dd = 1.0 / (u_dem_size + 2.0);
     vec4 r = tileUvToDemSample(apos / 8192.0, u_dem_size, u_dem_scale_prev, u_dem_tl_prev);
     vec2 pos = r.xy;
@@ -59,6 +73,7 @@ float prevElevation(vec2 apos) {
     float br = decodeElevation(texture2D(u_dem_prev, pos + vec2(dd, dd)));
 
     return u_exaggeration * mix(mix(tl, tr, f.x), mix(bl, br, f.x), f.y);
+#endif
 }
 
 #ifdef TERRAIN_VERTEX_MORPHING
@@ -106,6 +121,12 @@ float occlusionFade(vec4 frag) {
  // This is so that rendering changes are reflected on CPU side for feature querying.
 
 vec4 fourSample(vec2 pos, vec2 off) {
+#ifdef TERRAIN_DEM_FLOAT_FORMAT
+    float tl = texture2D(u_dem, pos).a;
+    float tr = texture2D(u_dem, pos + vec2(off.x, 0.0)).a;
+    float bl = texture2D(u_dem, pos + vec2(0.0, off.y)).a;
+    float br = texture2D(u_dem, pos + off).a;
+#else
     vec4 demtl = vec4(texture2D(u_dem, pos).xyz * 255.0, -1.0);
     float tl = dot(demtl, u_dem_unpack);
     vec4 demtr = vec4(texture2D(u_dem, pos + vec2(off.x, 0.0)).xyz * 255.0, -1.0);
@@ -114,6 +135,7 @@ vec4 fourSample(vec2 pos, vec2 off) {
     float bl = dot(dembl, u_dem_unpack);
     vec4 dembr = vec4(texture2D(u_dem, pos + off).xyz * 255.0, -1.0);
     float br = dot(dembr, u_dem_unpack);
+#endif
     return vec4(tl, tr, bl, br);
 }
 

src/shaders/hillshade_prepare.fragment.glsl

@@ -8,11 +8,15 @@ uniform vec2 u_dimension;
 uniform float u_zoom;
 uniform vec4 u_unpack;
 
-float getElevation(vec2 coord, float bias) {
+float getElevation(vec2 coord) {
+#ifdef TERRAIN_DEM_FLOAT_FORMAT
+    return texture2D(u_image, coord).a / 4.0;
+#else
     // Convert encoded elevation value to meters
     vec4 data = texture2D(u_image, coord) * 255.0;
     data.a = -1.0;
     return dot(data, u_unpack) / 4.0;
+#endif
 }
 
 void main() {
@@ -33,15 +37,15 @@ void main() {
     // |   |   |   |
     // +-----------+
 
-    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);
+    float a = getElevation(v_pos + vec2(-epsilon.x, -epsilon.y));
+    float b = getElevation(v_pos + vec2(0, -epsilon.y));
+    float c = getElevation(v_pos + vec2(epsilon.x, -epsilon.y));
+    float d = getElevation(v_pos + vec2(-epsilon.x, 0));
+    float e = getElevation(v_pos);
+    float f = getElevation(v_pos + vec2(epsilon.x, 0));
+    float g = getElevation(v_pos + vec2(-epsilon.x, epsilon.y));
+    float h = getElevation(v_pos + vec2(0, epsilon.y));
+    float i = getElevation(v_pos + vec2(epsilon.x, epsilon.y));
 
     // Here we divide the x and y slopes by 8 * pixel size
     // where pixel size (aka meters/pixel) is: