Add procedural skybox scene coverage

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

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+// 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);
+}

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

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+// XC_BUILTIN_SKYBOX_VULKAN_PS
+#version 450
+
+layout(set = 0, binding = 0, std140) uniform EnvironmentConstants {
+    vec4 gSkyboxTopColor;
+    vec4 gSkyboxHorizonColor;
+    vec4 gSkyboxBottomColor;
+    vec4 gCameraRightAndTanHalfFov;
+    vec4 gCameraUpAndAspect;
+    vec4 gCameraForwardAndUnused;
+};
+
+layout(location = 0) 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);
+}

engine/assets/builtin/shaders/skybox/skybox.ps.hlsl

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+// XC_BUILTIN_SKYBOX_D3D12_PS
+cbuffer EnvironmentConstants : register(b0) {
+    float4 gSkyboxTopColor;
+    float4 gSkyboxHorizonColor;
+    float4 gSkyboxBottomColor;
+    float4 gCameraRightAndTanHalfFov;
+    float4 gCameraUpAndAspect;
+    float4 gCameraForwardAndUnused;
+}
+
+struct PSInput {
+    float4 position : SV_POSITION;
+    float2 ndc : TEXCOORD0;
+};
+
+float4 MainPS(PSInput input) : SV_Target {
+    const float tanHalfFov = gCameraRightAndTanHalfFov.w;
+    const float aspect = gCameraUpAndAspect.w;
+
+    float3 viewRay = normalize(
+        gCameraForwardAndUnused.xyz +
+        input.ndc.x * aspect * tanHalfFov * gCameraRightAndTanHalfFov.xyz +
+        input.ndc.y * tanHalfFov * gCameraUpAndAspect.xyz);
+
+    const float vertical = clamp(viewRay.y, -1.0f, 1.0f);
+    float3 color = gSkyboxHorizonColor.rgb;
+    if (vertical >= 0.0f) {
+        color = lerp(gSkyboxHorizonColor.rgb, gSkyboxTopColor.rgb, pow(saturate(vertical), 0.65f));
+    } else {
+        color = lerp(gSkyboxHorizonColor.rgb, gSkyboxBottomColor.rgb, pow(saturate(-vertical), 0.55f));
+    }
+
+    return float4(color, 1.0f);
+}

engine/assets/builtin/shaders/skybox/skybox.shader

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+Shader "Builtin Skybox"
+{
+    SubShader
+    {
+        Pass
+        {
+            Name "Skybox"
+            Tags { "LightMode" = "Skybox" }
+            Resources
+            {
+                EnvironmentConstants (ConstantBuffer, 0, 0) [Semantic(Environment)]
+            }
+            HLSLPROGRAM
+            #pragma vertex MainVS
+            #pragma fragment MainPS
+            #pragma backend D3D12 HLSL "skybox.vs.hlsl" "skybox.ps.hlsl" vs_5_0 ps_5_0
+            #pragma backend OpenGL GLSL "skybox.vert.glsl" "skybox.frag.glsl"
+            #pragma backend Vulkan GLSL "skybox.vert.vk.glsl" "skybox.frag.vk.glsl"
+            ENDHLSL
+        }
+    }
+}

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

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+// XC_BUILTIN_SKYBOX_OPENGL_VS
+#version 430
+
+out vec2 vNdc;
+
+void main() {
+    const vec2 positions[3] = vec2[3](
+        vec2(-1.0, -1.0),
+        vec2(-1.0,  3.0),
+        vec2( 3.0, -1.0)
+    );
+
+    vec2 position = positions[gl_VertexID];
+    gl_Position = vec4(position, 1.0, 1.0);
+    vNdc = position;
+}

engine/assets/builtin/shaders/skybox/skybox.vert.vk.glsl

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+// XC_BUILTIN_SKYBOX_VULKAN_VS
+#version 450
+
+layout(location = 0) out vec2 vNdc;
+
+void main() {
+    const vec2 positions[3] = vec2[3](
+        vec2(-1.0, -1.0),
+        vec2(-1.0,  3.0),
+        vec2( 3.0, -1.0)
+    );
+
+    vec2 position = positions[gl_VertexIndex];
+    gl_Position = vec4(position, 1.0, 1.0);
+    vNdc = position;
+}

engine/assets/builtin/shaders/skybox/skybox.vs.hlsl

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+// XC_BUILTIN_SKYBOX_D3D12_VS
+struct VSOutput {
+    float4 position : SV_POSITION;
+    float2 ndc : TEXCOORD0;
+};
+
+VSOutput MainVS(uint vertexId : SV_VertexID) {
+    const float2 positions[3] = {
+        float2(-1.0f, -1.0f),
+        float2(-1.0f,  3.0f),
+        float2( 3.0f, -1.0f)
+    };
+
+    VSOutput output;
+    output.position = float4(positions[vertexId], 1.0f, 1.0f);
+    output.ndc = positions[vertexId];
+    return output;
+}