Add procedural skybox scene coverage

engine/assets/builtin/shaders/skybox/skybox.frag.glsl

@@ -0,0 +1,35 @@
+// XC_BUILTIN_SKYBOX_OPENGL_PS
+#version 430
+
+layout(std140, binding = 0) uniform EnvironmentConstants {
+    vec4 gSkyboxTopColor;
+    vec4 gSkyboxHorizonColor;
+    vec4 gSkyboxBottomColor;
+    vec4 gCameraRightAndTanHalfFov;
+    vec4 gCameraUpAndAspect;
+    vec4 gCameraForwardAndUnused;
+};
+
+in vec2 vNdc;
+
+layout(location = 0) out vec4 fragColor;
+
+void main() {
+    float tanHalfFov = gCameraRightAndTanHalfFov.w;
+    float aspect = gCameraUpAndAspect.w;
+
+    vec3 viewRay = normalize(
+        gCameraForwardAndUnused.xyz +
+        vNdc.x * aspect * tanHalfFov * gCameraRightAndTanHalfFov.xyz +
+        vNdc.y * tanHalfFov * gCameraUpAndAspect.xyz);
+
+    float vertical = clamp(viewRay.y, -1.0, 1.0);
+    vec3 color = gSkyboxHorizonColor.rgb;
+    if (vertical >= 0.0) {
+        color = mix(gSkyboxHorizonColor.rgb, gSkyboxTopColor.rgb, pow(clamp(vertical, 0.0, 1.0), 0.65));
+    } else {
+        color = mix(gSkyboxHorizonColor.rgb, gSkyboxBottomColor.rgb, pow(clamp(-vertical, 0.0, 1.0), 0.55));
+    }
+
+    fragColor = vec4(color, 1.0);
+}