To add the PCF technique to the shadow mapping algorithm, use the following steps:

1. When setting up the FBO for the shadow map, make sure to use linear filtering on the depth texture. Replace the corresponding lines with the following code:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER,  
                GL_LINEAR); 
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,      
                GL_LINEAR); 

2. Use the following code for the shadeWithShadow function within the fragment shader:

subroutine (RenderPassType) 
void shadeWithShadow() {
  vec3 ambient = vec3(0.2); 
  vec3 diffSpec = diffAndSpec(); 
 
  // The sum of the comparisons with nearby texels 
  float sum = 0; 
 
  // Sum contributions from texels around ShadowCoord 
  sum += textureProjOffset(ShadowMap, ShadowCoord,  
                           ivec2(-1,-1)); 
  sum += textureProjOffset(ShadowMap, ShadowCoord,  
                           ivec2(-1,1)); 
  sum += textureProjOffset(ShadowMap, ShadowCoord,  
                           ivec2(1,1)); 
  sum += textureProjOffset(ShadowMap, ShadowCoord,  
                           ivec2(1,-1)); 
  float shadow = sum * 0.25; 
 
  FragColor = vec4(ambient + diffSpec * shadow,1.0); 
}