snow_sm4.hlsl.h

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#define SRC(...) #__VA_ARGS__
SRC(
struct UBO
{
 float4x4 modelViewProj;
 float2 OutputSize;
 float time;
};
uniform UBO global;

struct PSInput
{
 float4 position : SV_POSITION;
 float2 texcoord : TEXCOORD0;
};

PSInput VSMain(float4 position : POSITION, float2 texcoord : TEXCOORD0)
{
 PSInput result;
 result.position = mul(global.modelViewProj, position);
 result.texcoord = texcoord;
 return result;
}

static const float baseScale = 3.5; // [1.0 .. 10.0]
static const float density = 0.7; // [0.01 .. 1.0]
static const float speed = 0.25; // [0.1 .. 1.0]

float rand(float2 co)
{
 return frac(sin(dot(co.xy, float2(12.9898, 78.233))) * 43758.5453);
}

float dist_func(float2 distv)
{
 float dist = sqrt((distv.x * distv.x) + (distv.y * distv.y)) * (40.0 / baseScale);
 dist = clamp(dist, 0.0, 1.0);
 return cos(dist * (3.14159265358 * 0.5)) * 0.5;
}

float random_dots(float2 co)
{
 float part = 1.0 / 20.0;
 float2 cd = floor(co / part);
 float p = rand(cd);

 if (p > 0.005 * (density * 40.0))
 return 0.0;

 float2 dpos = (float2(frac(p * 2.0) , p) + float2(2.0, 2.0)) * 0.25;

 float2 cellpos = frac(co / part);
 float2 distv = (cellpos - dpos);

 return dist_func(distv);
}

float snow(float2 pos, float time, float scale)
{
 // add wobble
 pos.x += cos(pos.y * 1.2 + time * 3.14159 * 2.0 + 1.0 / scale) / (8.0 / scale) * 4.0;
 // add gravity
 pos += time * scale * float2(-0.5, 1.0) * 4.0;
 return random_dots(pos / scale) * (scale * 0.5 + 0.5);
}


float4 PSMain(PSInput input) : SV_TARGET
{
 float tim = global.time * 0.4 * speed;
 float2 pos = input.position.xy / global.OutputSize.xx;
 pos.y = 1.0 - pos.y; // Flip Y
 float a = 0.0;
 // Each of these is a layer of snow
 // Remove some for better performance
 // Changing the scale (3rd value) will mess with the looping
 a += snow(pos, tim, 1.0);
 a += snow(pos, tim, 0.7);
 a += snow(pos, tim, 0.6);
 a += snow(pos, tim, 0.5);
 a += snow(pos, tim, 0.4);
 a += snow(pos, tim, 0.3);
 a += snow(pos, tim, 0.25);
 a += snow(pos, tim, 0.125);
 a = a * min(pos.y * 4.0, 1.0);
 return float4(1.0, 1.0, 1.0, a);
};
)