Ahmed Elhamy

3D Shadow Shimmering When Moving Objects

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Dears,

I am having a shadow shimmering in my scene. I am using shadow mapping technique. I know that there is a lot of posts over the internet dealing with this subject but My issue is slightly different.

I know that the problem comes from sub-texel issue when moving the camera BUT My scene is using a different technique as the camera and the light sources are stable and the objects inside the scene is moving Relative To Eye concept (RTE). The issue is when implementing cascaded shadow mapping and variance shadow mapping as stated in Directx examples, Every thing goes well except that the shadows are shimmering (Flickering). The shimmering is coming from that the objects are moving not the camera. So when I try to solve the problem with adjusting the sub-texel problem with the camera movement it didn't solve the problem as the camera is stable but the objects are not. 

Any help will be appreciated.

Thanks in advance.

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This is expected. The typical "stabilization"  techniques for cascade shadow maps only fix flickering for the case of completely static geometry. If the geometry moves or otherwise animates, you'll get flickering due to changes in how the geometry rasterizes into the depth map. The only way to avoid this this is to apply filtering when sampling the shadow map (or potentially pre-filter, if you're using a technique like VSM that supports pre-filtering).

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8 minutes ago, Aressera said:

Maybe you could quantize the positions of objects to texels when rendering the shadow map (but only the shadow map)?

I think you could only do that for ortho shadow maps, not for perspective or omnidirectional. And it would introduce wrong self shadowing anyway if you only do it for the shadow maps.

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What works pretty well is temporal filtering, this causes temporal lag (the shadow lags behind a moving object), but simmering is totally eliminated. In fact moving objects have a nicer shadow edge than static objects, because static objects can reveal the individual texels even with VSM, but moving objects blur this away nicely.

I get this as a side effect of realtime GI i'm working on, and i assume it is difficult to achieve it with traditional shadow maps.

The easiest way would be to do it in screen space with temporal aliasing (which is the same idea), but i guess you can't get the filter duration long enough to be effective?

The proper way would be to render the shadows to texture maps and apply the filter there, which boils down to texture (or object) space lighting and is probably too much work for just this purpose alone.

Edit:

Hey, you could apply the temporal filter just to the shadow map itself! E.g. shadowMapTexel = shadowMapTexel * 0.95 + newValue * 0.05.

I assume this works well with VSM and would be easy to test. Let me know how it works if you try it... unpractical self shadows?

 

 

Edited by JoeJ

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18 hours ago, JoeJ said:

Hey, you could apply the temporal filter just to the shadow map itself! E.g. shadowMapTexel = shadowMapTexel * 0.95 + newValue * 0.05.

 

Should I add this code to the pixel shader in the first pass, render to shadow map?

This is the code I use in the shader for rendering the shadow map texture.

VS_OUTPUT VSMain( VS_INPUT Input )
{
    VS_OUTPUT Output;
    
    
    Output.vPosition = mul( Input.vPosition, g_mWorldViewProjection );

    return Output;
}


float2 PSMain (VS_OUTPUT Input) : SV_TARGET 
{
    float2 rt;
    rt.x = Input.vPosition.z;
    rt.y = rt.x * rt.x;
    return rt;
}

This is also the code I use to calculate the percent lit in the scene rendering in the variance shadow mapping.

void CalculateVarianceShadow ( in float4 vShadowTexCoord, in float4 vShadowMapTextureCoordViewSpace, int iCascade, out float fPercentLit , in float depth) 
{
    fPercentLit = 0.0f;
    // This loop could be unrolled, and texture immediate offsets could be used if the kernel size were fixed.
    // This would be a performance improvment.
            
    float2 mapDepth = 0;


    // In orderto pull the derivative out of divergent flow control we calculate the 
    // derivative off of the view space coordinates an then scale the deriviative.
    
    float3 vShadowTexCoordDDX = 
        ddx(vShadowMapTextureCoordViewSpace );
    //float3 vShadowTexCoordDDX = vShadowMapTextureCoordViewSpace;
    vShadowTexCoordDDX *= m_vCascadeScale[iCascade].xyz; 
    float3 vShadowTexCoordDDY = 
        ddy(vShadowMapTextureCoordViewSpace );
    //float3 vShadowTexCoordDDY = vShadowMapTextureCoordViewSpace;
    vShadowTexCoordDDY *= m_vCascadeScale[iCascade].xyz; 
    
    mapDepth += g_txShadow.SampleGrad( g_samShadow, vShadowTexCoord.xyz, 
                                       vShadowTexCoordDDX,
                                       vShadowTexCoordDDY);
    //mapDepth += g_txShadow.Sample(g_samShadow, vShadowTexCoord.xyz);
    // The sample instruction uses gradients for some filters.
                
    float  fAvgZ  = mapDepth.x; // Filtered z
    float  fAvgZ2 = mapDepth.y; // Filtered z-squared
    
    if ( vShadowTexCoord.w <= fAvgZ ) // We put the z value in w so that we can index the texture array with Z.
    {
        fPercentLit = 1;
    }
    else 
    {
        float variance = ( fAvgZ2 ) - ( fAvgZ * fAvgZ );
        //variance       = min( 1.0f, max( 0.0f, variance + 0.00001f ) );
        variance = min(1.0f, max(0.0f, variance + 0.00001f));
    
        float mean     = fAvgZ;
        float d        = vShadowTexCoord.w - mean; // We put the z value in w so that we can index the texture array with Z.
        float p_max    = variance / ( variance + d*d );

        // To combat light-bleeding, experiment with raising p_max to some power
        // (Try values from 0.1 to 100.0, if you like.)
        fPercentLit = pow(p_max, 2);
    }

 

Edited by Ahmed Elhamy

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This implies you do not blur the shadow map? You should, because blurring gives soft shadows and soft shadows are usually enough to make the problem acceptable (although popping of individual shadow texels is still visible).

So i'd do the temporal filter on the blurred results which requires to keep a copy of the old blurred shadow map.

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I already do blurring for the shadow map texture, below is the shader code for the blurring.

#ifndef SEPERABLE_BLUR_KERNEL_SIZE
#define SEPERABLE_BLUR_KERNEL_SIZE 3
#endif

static const int BLUR_KERNEL_BEGIN = SEPERABLE_BLUR_KERNEL_SIZE / -2; 
static const int BLUR_KERNEL_END = SEPERABLE_BLUR_KERNEL_SIZE / 2 + 1;
static const float FLOAT_BLUR_KERNEL_SIZE = (float)SEPERABLE_BLUR_KERNEL_SIZE;

cbuffer cbblurVS : register( b2)
{
    int2        g_iWidthHeight            : packoffset( c0 );
    int            g_iKernelStart          : packoffset( c0.z );
    int            g_iKernelEnd            : packoffset( c0.w );
};

//--------------------------------------------------------------------------------------
// defines
//--------------------------------------------------------------------------------------

Texture2DArray g_txShadow : register( t5 );
SamplerState g_samShadow : register( s5 );

//--------------------------------------------------------------------------------------
// Input/Output structures
//--------------------------------------------------------------------------------------

struct PSIn
{
    float4      Pos        : SV_Position;        //Position
    float2      Tex        : TEXCOORD;            //Texture coordinate
    float2      ITex    : TEXCOORD2;
};

struct VSIn
{
    uint Pos    : SV_VertexID ;
};


PSIn VSMain(VSIn inn)
{
    PSIn output;

    output.Pos.y  = -1.0f + (inn.Pos%2) * 2.0f ;
    output.Pos.x  = -1.0f + (inn.Pos/2) * 2.0f;
    output.Pos.z = .5;
    output.Pos.w = 1;
    output.Tex.x = inn.Pos/2;
    output.Tex.y = 1.0f - inn.Pos%2;
    output.ITex.x = (float)(g_iWidthHeight.x * output.Tex.x);
    output.ITex.y = (float)(g_iWidthHeight.y * output.Tex.y);
    return output;
}

//float PSDepth

//------------------------------------------------------------------------------
// Logarithmic filtering
//------------------------------------------------------------------------------

float log_conv ( float x0, float X, float y0, float Y )
{
    return (X + log(x0 + (y0 * exp(Y - X))));
}


//--------------------------------------------------------------------------------------
// Pixel shader that performs bump mapping on the final vertex
//--------------------------------------------------------------------------------------
float2 PSBlurX(PSIn input) : SV_Target
{    
  
  
    float2 dep=0;
    [unroll]for ( int x = BLUR_KERNEL_BEGIN; x < BLUR_KERNEL_END; ++x ) {
        dep += g_txShadow.Sample( g_samShadow,  float3( input.Tex.x, input.Tex.y, 0 ), int2( x,0 ) ).rg;
    }
    dep /= FLOAT_BLUR_KERNEL_SIZE;
    return dep;  
    
//    return g_txShadow.Sample(g_samShadow,  float3(input.Tex.x, input.Tex.y, 0) ).rg;
    
}

//--------------------------------------------------------------------------------------
// Pixel shader that performs bump mapping on the final vertex
//--------------------------------------------------------------------------------------
float2 PSBlurY(PSIn input) : SV_Target
{    

    
    
    float2 dep=0;
    [unroll]for ( int y = BLUR_KERNEL_BEGIN; y < BLUR_KERNEL_END; ++y ) {
        dep += g_txShadow.Sample( g_samShadow,  float3( input.Tex.x, input.Tex.y, 0 ), int2( 0,y ) ).rg;
    }
    dep /= FLOAT_BLUR_KERNEL_SIZE;
    return dep;  
    
    //return g_txShadow.Sample(g_samShadow,  float3(input.Tex.x, input.Tex.y, 0) ).rg;
}

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