High Performance Outdoor Light Scattering Using Epipolar Sampling - GPU Pro: Advanced Rendering Techniques - page 117

// Compute ray termination point , full ray length and view
// direction ; truncate the ray against the top of the atmosphere
float3 f3RlghIn =0;
float3 f3MieIn =0;
float2 f2NetDensFromCam =0;

for ( uint Cascade = g_StartCscd ; Cascade < g_NumCascades ;++ Cascade )
{ // Truncate view ray against min/max z range for the current
// cascade , project onto the shadow map

// Load slice origin and direction , compute integration
// step f3UVAndDepthStep in shadow map space and in world
// space , set up current position f3CurrUVAndDepth

// Compute initial sample location in 1D height/map
uint uiSamplePos =
length ( f2StartUV . xy − f2SliceOriginUV . xy )/ fUVStepLen +0.5;

uint uiLevel =0;
int iDataOffset = − g_iSMDim ; // Level 0 is not stored
float fMarchedDist =0;
uint uiMinMaxTexYInd =
uiSliceInd +( Cascade − g_StartCscd ) ￿ g_NumSlices ;

float fStep =1. f ;
while ( fMarchedDist < fRayLength )
{ IsInLight =0;

// If the sample is located at the appropriate position ,
// advance to the next coarser level
if (( uiSamplePos & ((2 << uiLevel ) − 1)) == 0 )
{ iDataOffset += g_iSMDim >> uiLevel ;
uiLevel ++;
fStep ￿ =2. f ;

}

while ( uiLevel > 0)
{ // Compute depths at the ends of the current ray section
float2 f2StartEndDepth ;
f2StartEndDepth . x = f3CurrUVAndDepth . z ;
f2StartEndDepth . y = f3CurrUVAndDepth . z +
f3UVAndDepthStep . z ￿ ( fStep −

1) ;

// Load min/max depths for the node
float2 f2MinMaxDepth = g_tex2DMinMaxDepth . Load (
uint3 (( uiSamplePos >> uiLevel )+ iDataOffset ,
uiMinMaxTexYInd ,
0) ) ;

IsInShadow = all ( f2StartEndDepth > = f2MinMaxDepth . yy );
IsInLight = all ( f2StartEndDepth <

f2MinMaxDepth . xx );

if ( IsInLight || IsInShadow )
break ; // If the ray section is fully li t or shadowed ,
// we can break the loop
// If the ray section is neither fully lit , nor
// shadowed , we have to go to the finer level
uiLevel −− ;
iDataOffset − = g_iSMDim >> uiLevel ;
fStep /= 2 . f ;

} ;