float4 ToLightVS(float3 pos, float w = 1)
...

float AvailabilityFromVS(in float4 pos)
...

void RayMarch(
	inout float4 PosLightVS,
	in    float  fStepSize,
	in    float4 InvDirLightVS,
	inout float3 accum,
	in float l
	)
{
	// Increment
	PosLightVS.xyz += fStepSize * InvDirLightVS.xyz;

	// Check for availability
	float3 occlusion = AvailabilityFromVS(PosLightVS).xxx;

	// Get Distance
	float d = length(PosLightVS.xyz);
	float dRcp = rcp(d);

	// Calculate Lighting Energy
	float3 energy =
		TAU * (occlusion * (PHI * 0.25 * PI_RCP) * dRcp * dRcp) * exp(-d * TAU) * exp(-l * TAU)
		* fStepSize;

	accum += energy;
}

float3 VolumetricLighting(VS_Output inp)
{
#define STEPS 50.0f
#define STEPS_RCP (1.0f / STEPS)

	// Max resolving distance
	float fDistMax = 99.0;

	// Get WS Position
	float depth = t_dX.Sample(ss, inp.Tex);
	float3 viewRay = normalize(inp.ViewRay);
	float3 positionWS = cameraPosition + viewRay * depth;

	// View space positions
	float4 CamLightVS = ToLightVS(cameraPosition, 1);
	float4 PosLightVS = ToLightVS(positionWS, 1);

	float3 DirLight = positionWS - cameraPosition;
	float4 DirLightVS = ToLightVS(DirLight, 0);

	// Noise reducing
	float fDist = trunc(clamp(length(
		distance(CamLightVS.xyz, PosLightVS.xyz)
		), 0.0, fDistMax));

	// Size of each step
	float fStepSize = fDist * STEPS_RCP;
	float4 RayLightVS = PosLightVS;

	// Total Light Accum
	float3 accum = 0;

	// Ray march
	[loop] for (float l = fDist; l > fStepSize; l -= fStepSize)
	{
		RayMarch(RayLightVS, fStepSize, DirLightVS, accum, l);
	}

	return accum;
}