// 与粒子系统 Renderer › Custom Vertex Streams 的顺序一一对应：
//   Position(POSITION) · Color(COLOR) · UV(TEXCOORD0.xy) · UV2(TEXCOORD0.zw) · AnimBlend(TEXCOORD1.x)
struct Attributes
{
    float4 positionOS : POSITION;
    half4  color      : COLOR;
#if defined(_FLIPBOOKBLENDING_ON)
    float4 texcoords  : TEXCOORD0;   // xy = 当前帧 UV，zw = 下一帧 UV
    float  animBlend  : TEXCOORD1;   // 两帧之间的进度 0..1
#else
    float2 texcoords  : TEXCOORD0;
#endif
    UNITY_VERTEX_INPUT_INSTANCE_ID
};

half4 frag(Varyings i) : SV_Target
{
    half4 tex = SAMPLE_TEXTURE2D(_BaseMap, sampler_BaseMap, i.uv.xy);
#if defined(_FLIPBOOKBLENDING_ON)
    half4 tex2 = SAMPLE_TEXTURE2D(_BaseMap, sampler_BaseMap, i.uv.zw);
    tex = lerp(tex, tex2, i.extra.x);
#endif

    // 顶点色 = 粒子系统的 Start Color × Color over Lifetime：整个粒子系统的颜色变化都从这里进来
    half4 c = tex * i.color * _BaseColor;

#if defined(_RAMP_ON)
    // 渐变映射：用“贴图浓度 × 粒子剩余寿命（顶点色 alpha）”去查火焰渐变，越老越暗红
    half k = saturate(tex.a * i.color.a * _RampScale);
    c.rgb = SAMPLE_TEXTURE2D(_RampTex, sampler_RampTex, float2(k, 0.5)).rgb * _BaseColor.rgb;
    c.a = saturate(k);
#endif

    float thisZ = i.screenData.z;
#if defined(_SOFTPARTICLES_ON)
    // 软粒子：比较场景深度与粒子自身深度，越接近（快要插进物体）越透明
    float2 suv = i.screenData.xy / i.screenData.w;
    float sceneZ = LinearEyeDepth(SampleSceneDepth(suv), _ZBufferParams);
    c.a *= saturate((sceneZ - thisZ) / _SoftDistance);
#endif
#if defined(_CAMERAFADE_ON)
    // 近相机淡出：粒子贴脸时会糊满屏幕、产生巨大 Overdraw，提前淡掉
    c.a *= saturate((thisZ - _CameraFadeNear) / max(_CameraFadeFar - _CameraFadeNear, 1e-3));
#endif

    c.rgb *= lerp(1.0, c.a, _Premultiply);   // 预乘：Blend One OneMinusSrcAlpha 时需要
    // 正片叠底（Blend DstColor Zero）没有 alpha 参与：把 alpha 折算进颜色，透明处趋向白色（=不改变背景）
    c.rgb = lerp(c.rgb, lerp(1.0, c.rgb, c.a), _Modulate);
    // 雾：Alpha 混合直接 MixFog；叠加模式应往黑色雾化 MixFogColor(c.rgb, 0, fog)，否则雾里越叠越亮
    c.rgb = MixFog(c.rgb, i.extra.y);
    return c;
}

/// 用一组朝向相机的 Quad 摆出“一团粒子”，每个 Quad 带不同的顶点色（模拟粒子的 Start Color）
static void FakeParticles(Shot s, Material m, Vector3 center, int count, Vector3 spread, float size, Color c0, Color c1, int seed)
{
    var rnd = new System.Random(seed);
    for (int i = 0; i < count; i++)
    {
        var p = center + Vector3.Scale(new Vector3((float)rnd.NextDouble() - 0.5f, (float)rnd.NextDouble() - 0.5f, (float)rnd.NextDouble() - 0.5f), spread);
        var q = s.Quad(m, p, size * Mathf.Lerp(0.7f, 1.2f, (float)rnd.NextDouble()));
        q.transform.rotation = Quaternion.LookRotation(p - s.Cam.transform.position) * Quaternion.Euler(0, 0, (float)rnd.NextDouble() * 360);
        var mesh = Object.Instantiate(q.GetComponent<MeshFilter>().sharedMesh);
        var col = Color.Lerp(c0, c1, (float)rnd.NextDouble());
        mesh.colors = new[] { col, col, col, col };
        q.GetComponent<MeshFilter>().sharedMesh = mesh;
        s.Temp.Add(mesh);
    }
}