Spot Light with Soft Shadow Shader


This post shows some shaders used to create a Shadow effect using the classic technic Shadow Mapping. The objective of this post is not to explaint how shadows are calculated, we are only going to show the shaders, the creation of the render targets for example will not be shown.

The first step in shadow mapping is to create the depth map of each light, the following shader shows the process. (draw each object of your scene as if the camera is in the light position )

VertexToPixel MyVertexShader(half4 inPos: POSITION0)
{
	VertexToPixel Output = (VertexToPixel)0;
	Output.Position = mul(inPos, WVP);
	Output.ScreenPos = Output.Position .zw;
	return Output;
}

PixelToFrame MyPixelShader(VertexToPixel PSIn) : COLOR0
{
	PixelToFrame Output = (PixelToFrame)0;
	Output.Color = PSIn.ScreenPos.x/PSIn.ScreenPos.y;
	return Output;
}

The idea of this shader is just to output the Z/W of the objects seen from the light perspective.

With the depth map in hands, the following shader shows how to calculate the spot light effect (draw all the objects using this shader =P).

float4 PixelShaderFunctionPCFAttenuation(VertexShaderOutput input) : COLOR0
{
    //read depth calculated before
    float depthVal = tex2D(depthSampler,input.TexCoord).r; ///storead as Z/W

    //compute screen-space position
    float4 position;
    position.x = input.TexCoord1.x * 2.0f - 1.0f;
    position.y = -(input.TexCoord1.y * 2.0f - 1.0f);
    position.z = depthVal;
    position.w = 1.0f;
    //transform to world space
    position = mul(position, InvertViewProjection);
    position /= position.w;    

    //surface-to-light vector
    float3 lightVector = lightPosition - position;  

    //compute attenuation based on distance - linear attenuation
    float attenuation = saturate(1.0f - length(lightVector)/lightRadius);   

    //normalize light vector
    lightVector = normalize(lightVector);       

    //find screen position as seen by the light
    float4 lightScreenPos = mul(position, xLightViewProjection);
    lightScreenPos /= lightScreenPos.w;

    //find sample position in shadow map
    float2 lightSamplePos;
    lightSamplePos.x = lightScreenPos.x/2.0f+0.5f;
    lightSamplePos.y = (-lightScreenPos.y/2.0f+0.5f);

    // Offset the coordinate by half a texel so we sample it correctly
    lightSamplePos += (0.5f / shadowBufferSize);

    float SdL = dot(lightDirection, -lightVector);   	

    if(SdL > lightAngleCosine)
    {
		float shadowOcclusion  = 0;
		if(shadown)
		{
			shadowOcclusion = CalcShadowTermSoftPCF(lightScreenPos.z, lightSamplePos, 7); ///soft shadow effect
		}
		else
		{
			shadowOcclusion = 1;
		}

        float spotIntensity = pow(SdL, lightDecayExponent);   

        //compute diffuse light
        float NdL = max(0,dot(normal,lightVector));
        float3 diffuseLight = NdL * Color.rgb;  

        //reflection vector
        float3 reflectionVector = normalize(reflect(-lightVector, normal));
        //camera-to-surface vector
        float3 directionToCamera = normalize(cameraPosition - position);
        //compute specular light
        float specularLight = specularIntensity * pow( saturate(dot(reflectionVector, directionToCamera)), specularPower);  

        attenuation *= spotIntensity;  

        //take into account attenuation and lightIntensity.
        return shadowOcclusion * attenuation * lightIntensity * float4(diffuseLight.rgb,specularLight);
    }
}

The idea is simple, it is pretty much the same of the commom spot lights shaders, the only difference is the Soft shadow term. (Check our sample about light types without shadow )

The following part shows how the soft term is calculated:

// Calculates the shadow term using PCF soft-shadowing
float CalcShadowTermSoftPCF(float fLightDepth, float2 vShadowTexCoord, int iSqrtSamples)
{
	float fShadowTerm = 0.0f;  

	float fRadius = (iSqrtSamples - 1.0f) / 2;
	float fWeightAccum = 0.0f;

	for (float y = -fRadius; y<= fRadius; y++)
	{
		for (float x = -fRadius; x <= fRadius; x++)
		{
			float2 vOffset = 0;
			vOffset = float2(x, y);				
			vOffset /= shadowBufferSize;
			float2 vSamplePoint = vShadowTexCoord + vOffset;			
			float fDepth = tex2D(ShadowMapSampler, vSamplePoint).x;			
			float fSample = (fLightDepth <= fDepth + BIAS);
			
			// Edge tap smoothing
			float xWeight = 1;
			float yWeight = 1;
			
			if (x == -fRadius)
				xWeight = 1 - frac(vShadowTexCoord.x * shadowBufferSize);
			else if (x == fRadius)
				xWeight = frac(vShadowTexCoord.x * shadowBufferSize);
				
			if (y == -fRadius)
				yWeight = 1 - frac(vShadowTexCoord.y * shadowBufferSize);
			else if (y == fRadius)
				yWeight = frac(vShadowTexCoord.y * shadowBufferSize);
				
			fShadowTerm += fSample * xWeight * yWeight;
			fWeightAccum = xWeight * yWeight;
		}											
	}		
	
	fShadowTerm /= (iSqrtSamples * iSqrtSamples);
	fShadowTerm *= 1.55f;	
	
	return fShadowTerm;
}

You can use this soft term will all light types. Experiment changing the parameter iSqrtSamples.

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