float ao = smoothstep(0.0, 0.7, vGrassUv.y);
ao = mix(0.05, 1.0, pow(ao, 1.6));

vec3 rootColor = diffuseColor.rgb * 0.15;
vec3 midColor = diffuseColor.rgb;
vec3 tipColor = diffuseColor.rgb * 1.3 + vec3(0.06, 0.08, 0.0);

float heightParam = vGrassUv.y;
vec3 grassColor;
if (heightParam < 0.4) {
    float t = smoothstep(0.0, 0.4, heightParam);
    grassColor = mix(rootColor, midColor, t);
} else {
    float t = smoothstep(0.4, 1.0, heightParam);
    grassColor = mix(midColor, tipColor, t);
}

vec3 viewDir = normalize(cameraPosition - vWorldPos);
vec3 lightDir = normalize(vec3(15.0, 25.0, 15.0));

float VdotL = max(0.0, dot(viewDir, -lightDir));
float sss = pow(VdotL, 3.0) * smoothstep(0.2, 0.9, vGrassUv.y);

vec3 sssColor = diffuseColor.rgb * vec3(0.6, 1.0, 0.15) * 1.8;
grassColor += sssColor * sss * 2.0;

float NdotV = 1.0 - max(0.0, dot(normalize(vNormal), viewDir));
float rim = pow(NdotV, 3.0) * smoothstep(0.3, 1.0, vGrassUv.y) * 0.15;
grassColor += vec3(0.3, 0.5, 0.1) * rim;

diffuseColor.rgb = grassColor * ao;