phase(N.5b) Task 5: terrain_modern.frag

Fragment shader for the modern terrain dispatcher. Bit-identical math
to today's terrain.frag (per-cell maskBlend3 + Phase G fog + lightning
flash). Same #version 460 + GL_ARB_bindless_texture preamble change
as terrain_modern.vert. Sampling syntax unchanged — the bindless-ness
is invisible at the GLSL level.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/Shaders/terrain_modern.frag

@@ -0,0 +1,140 @@
+#version 460 core
+#extension GL_ARB_bindless_texture : require
+
+// Phase N.5b: terrain fragment shader on the modern bindless dispatcher.
+// Math identical to terrain.frag (Phase 3c per-cell maskBlend3 +
+// Phase G fog + lightning flash). uTerrain and uAlpha are bound via
+// glProgramUniformHandleARB on the C# side; GLSL sampling is unchanged.
+
+in  vec2  vBaseUV;
+in  vec3  vWorldNormal;
+in  vec3  vWorldPos;
+in  vec3  vLightingRGB;
+in  vec4  vOverlay0;
+in  vec4  vOverlay1;
+in  vec4  vOverlay2;
+in  vec4  vRoad0;
+in  vec4  vRoad1;
+flat in float vBaseTexIdx;
+
+out vec4 fragColor;
+
+uniform sampler2DArray uTerrain;
+uniform sampler2DArray uAlpha;
+
+struct Light {
+    vec4 posAndKind;
+    vec4 dirAndRange;
+    vec4 colorAndIntensity;
+    vec4 coneAngleEtc;
+};
+layout(std140, binding = 1) uniform SceneLighting {
+    Light uLights[8];
+    vec4  uCellAmbient;
+    vec4  uFogParams;
+    vec4  uFogColor;
+    vec4  uCameraAndTime;
+};
+
+const float TILE = 1.0;
+
+vec4 maskBlend3(vec4 t0, vec4 t1, vec4 t2, float h0, float h1, float h2) {
+    float a0 = h0 == 0.0 ? 1.0 : t0.a;
+    float a1 = h1 == 0.0 ? 1.0 : t1.a;
+    float a2 = h2 == 0.0 ? 1.0 : t2.a;
+    float aR = 1.0 - (a0 * a1 * a2);
+    float aRsafe = max(aR, 1e-6);
+    a0 = 1.0 - a0;
+    a1 = 1.0 - a1;
+    a2 = 1.0 - a2;
+    vec3 r0 = (a0 * t0.rgb + (1.0 - a0) * a1 * t1.rgb + (1.0 - a1) * a2 * t2.rgb);
+    return vec4(r0 / aRsafe, aR);
+}
+
+vec4 combineOverlays(vec2 baseUV, vec4 pOverlay0, vec4 pOverlay1, vec4 pOverlay2) {
+    float h0 = pOverlay0.z < 0.0 ? 0.0 : 1.0;
+    float h1 = pOverlay1.z < 0.0 ? 0.0 : 1.0;
+    float h2 = pOverlay2.z < 0.0 ? 0.0 : 1.0;
+    vec4 t0 = vec4(0.0), t1 = vec4(0.0), t2 = vec4(0.0);
+
+    if (h0 > 0.0) {
+        t0 = texture(uTerrain, vec3(baseUV * TILE, pOverlay0.z));
+        if (pOverlay0.w >= 0.0) {
+            vec4 a = texture(uAlpha, vec3(pOverlay0.xy, pOverlay0.w));
+            t0.a = a.a;
+        }
+    }
+    if (h1 > 0.0) {
+        t1 = texture(uTerrain, vec3(baseUV * TILE, pOverlay1.z));
+        if (pOverlay1.w >= 0.0) {
+            vec4 a = texture(uAlpha, vec3(pOverlay1.xy, pOverlay1.w));
+            t1.a = a.a;
+        }
+    }
+    if (h2 > 0.0) {
+        t2 = texture(uTerrain, vec3(baseUV * TILE, pOverlay2.z));
+        if (pOverlay2.w >= 0.0) {
+            vec4 a = texture(uAlpha, vec3(pOverlay2.xy, pOverlay2.w));
+            t2.a = a.a;
+        }
+    }
+    return maskBlend3(t0, t1, t2, h0, h1, h2);
+}
+
+vec4 combineRoad(vec2 baseUV, vec4 pRoad0, vec4 pRoad1) {
+    float h0 = pRoad0.z < 0.0 ? 0.0 : 1.0;
+    float h1 = pRoad1.z < 0.0 ? 0.0 : 1.0;
+    vec4 result = vec4(0.0);
+    if (h0 > 0.0) {
+        result = texture(uTerrain, vec3(baseUV * TILE, pRoad0.z));
+        if (pRoad0.w >= 0.0) {
+            vec4 a0 = texture(uAlpha, vec3(pRoad0.xy, pRoad0.w));
+            result.a = 1.0 - a0.a;
+            if (h1 > 0.0 && pRoad1.w >= 0.0) {
+                vec4 a1 = texture(uAlpha, vec3(pRoad1.xy, pRoad1.w));
+                result.a = 1.0 - (a0.a * a1.a);
+            }
+        }
+    }
+    return result;
+}
+
+vec3 applyFog(vec3 lit, vec3 worldPos) {
+    int mode = int(uFogParams.w);
+    if (mode == 0) return lit;
+    float d = length(worldPos - uCameraAndTime.xyz);
+    float fogStart = uFogParams.x;
+    float fogEnd   = uFogParams.y;
+    float span = max(1e-3, fogEnd - fogStart);
+    float fog = clamp((d - fogStart) / span, 0.0, 1.0);
+    return mix(lit, uFogColor.xyz, fog);
+}
+
+void main() {
+    vec4 baseColor = vec4(0.0);
+    if (vBaseTexIdx >= 0.0) {
+        baseColor = texture(uTerrain, vec3(vBaseUV * TILE, vBaseTexIdx));
+    }
+
+    vec4 overlays = vec4(0.0);
+    if (vOverlay0.z >= 0.0)
+        overlays = combineOverlays(vBaseUV, vOverlay0, vOverlay1, vOverlay2);
+
+    vec4 roads = vec4(0.0);
+    if (vRoad0.z >= 0.0)
+        roads = combineRoad(vBaseUV, vRoad0, vRoad1);
+
+    vec3 baseMasked = baseColor.rgb * ((1.0 - overlays.a) * (1.0 - roads.a));
+    vec3 ovlMasked  = overlays.rgb  * (overlays.a        * (1.0 - roads.a));
+    vec3 roadMasked = roads.rgb     *  roads.a;
+    vec3 rgb = clamp(baseMasked + ovlMasked + roadMasked, 0.0, 1.0);
+
+    vec3 lit = rgb * min(vLightingRGB, vec3(1.0));
+
+    float flash = uFogParams.z;
+    lit += flash * vec3(0.6, 0.6, 0.75);
+
+    lit = applyFog(lit, vWorldPos);
+
+    fragColor = vec4(lit, 1.0);
+}