Exponential shadow maps sampling with PCF for sampler2DShadow instead of sampler2D

Question

I am trying to make use of free hardware PCF sampling with sampler2DShadow and extend it to more samples.

So far I am sampling shadow map (sampler2D) in ESM way and extending this to PCF and it works. So I have no acne (thanks to ESM) and soft result, but I need a lot of PCF samples:

float shadowMapSize = 512.0;
float c = 5000.0;
float receiver = lightCoords.z;
float shadow = 0.0;
for(int i = 0; i < SAMPLE_NUM; ++i)
{
    vec2 off = (POISSON_SAMPLES[i])/shadowMapSize;
    float occluder = texture(shadowmap, lightCoords.xy+off).r;
    shadow += exp(c*(occluder-receiver));
}
shadow /= SAMPLE_NUM;

shadow = clamp(shadow, 0.0, 1.0);

But I wanted to use sampler2DShadow and get 4 PCF tabs for free and extend this to 16 taps for almost free with only 4 poisson samples like that:

float shadowMapSize = 512.0;
float c = 5000.0;
float receiver = lightCoords.z;
float shadow = 0.0;
for(int i = 0; i < SAMPLE_NUM; ++i)
{
    vec2 off = (POISSON_SAMPLES[i])/shadowMapSize;
    float occluder = texture(shadowmap, vec3(lightCoords.xy+off, receiver)).r;
    shadow += exp(c*(occluder-receiver));
}
shadow /= SAMPLE_NUM;
shadow = clamp(shadow, 0.0, 1.0);

But unfortunately it doesnt work as expected:

Is there a way to make it work (in ESM way, not traditional due to acne) or due to sampler2DShadow specification it is rather impossible? In traditional way it is possible.

Answer 1

PCF does N shadow tests, where each test returns 1 (in light) or 0 (in shadow), and returns the average of the results. sampler2DShadow is a special sampler where PCF is applied to the nearest texels to a sample, based on the depth coordinate provided. So you can't use it with ESM.