source: data/branches/Shader_HS18/programs/GLSL/DepthShadowmapReceiverFp.glsl

#version 120

uniform float inverseShadowmapSize;
uniform float fixedDepthBias;
uniform float gradientClamp;
uniform float gradientScaleBias;

uniform sampler2D shadowMap;

varying vec4 oUv;
varying vec4 outColor;

void main()
{
        vec4 shadowUV = oUv;
        // point on shadowmap
        shadowUV = shadowUV / shadowUV.w;
        float centerdepth = texture2D(shadowMap, shadowUV.xy).x;
    
    // gradient calculation
        float pixeloffset = inverseShadowmapSize;
    vec4 depths = vec4(
        texture2D(shadowMap, shadowUV.xy + vec2(-pixeloffset, 0)).x,
        texture2D(shadowMap, shadowUV.xy + vec2(+pixeloffset, 0)).x,
        texture2D(shadowMap, shadowUV.xy + vec2(0, -pixeloffset)).x,
        texture2D(shadowMap, shadowUV.xy + vec2(0, +pixeloffset)).x);

        vec2 differences = abs( depths.yw - depths.xz );
        float gradient = min(gradientClamp, max(differences.x, differences.y));
        float gradientFactor = gradient * gradientScaleBias;

        // visibility function
        float depthAdjust = gradientFactor + (fixedDepthBias * centerdepth);
        float finalCenterDepth = centerdepth + depthAdjust;

        // shadowUV.z contains lightspace position of current object
#if PCF
        // use depths from prev, calculate diff
        depths += depthAdjust;
        float final = (finalCenterDepth > shadowUV.z) ? 1.0 : 0.0;
        final += (depths.x > shadowUV.z) ? 1.0 : 0.0;
        final += (depths.y > shadowUV.z) ? 1.0 : 0.0;
        final += (depths.z > shadowUV.z) ? 1.0 : 0.0;
        final += (depths.w > shadowUV.z) ? 1.0 : 0.0;
        
        final *= 0.2;

        gl_FragColor = vec4(outColor.xyz * final, 1);
        
#else
        gl_FragColor = (centerdepth > shadowUV.z) ? vec4(outColor.xyz,1) : vec4(0,0,0,1);
#endif
}