Back projection soft shadows

float visibility = 1.0f;
float searchTexelSpread = 1.0f;
float oneTexelSize = lightsData[lightInput.id].shadowScale;
float halfTexelSize = oneTexelSize * 0.5f;
float2 ndcCurrentFragmentLightmapTexel = lNDC.xy;
float lightSizeScale = 1.0f / lightsData[lightInput.id].data3.x;
float near_plane = spotLight.shadowNear;
float far_plane = spotLight.shadowFar;
float lightmapFarPlaneSize = tan(spotLight.spotAngle * 0.5f) * 2.0f * far_plane;
float oneFarPlaneTexelSize = lightmapFarPlaneSize * lightsData[lightInput.id].shadowScale;

float z = lNDC.z;

for (int i = -KERNEL_SEARCH_SIZE; i < KERNEL_SEARCH_SIZE; i++)
{
    for (int j = -KERNEL_SEARCH_SIZE; j < KERNEL_SEARCH_SIZE; j++)
    {
        float sampleTexX = lNDC.x + oneTexelSize * searchTexelSpread * float(i);
        float sampleTexY = lNDC.y + oneTexelSize * searchTexelSpread * float(j);

        float2 sampleCoords = float2(sampleTexX, sampleTexY);

        float zs = shadowMap.SampleLevel(shadowSampler, sampleCoords.xy, 0.0f).x;

        if (zs > z)
        {
            continue;
        }

        float thisSampleZPlaneSize = lightmapFarPlaneSize * zs;

        float2 ndcNextFragmentLightmapTexel = sampleCoords;
        float scaleFactor = z / (z - zs);

        float left = (ndcNextFragmentLightmapTexel.x - ndcCurrentFragmentLightmapTexel.x - halfTexelSize) * thisSampleZPlaneSize * scaleFactor * lightSizeScale;
        float right = (ndcNextFragmentLightmapTexel.x - ndcCurrentFragmentLightmapTexel.x + halfTexelSize) * thisSampleZPlaneSize * scaleFactor * lightSizeScale;
        float top = (ndcNextFragmentLightmapTexel.y - ndcCurrentFragmentLightmapTexel.y + halfTexelSize) * thisSampleZPlaneSize * scaleFactor * lightSizeScale;
        float bottom = (ndcNextFragmentLightmapTexel.y - ndcCurrentFragmentLightmapTexel.y - halfTexelSize) * thisSampleZPlaneSize * scaleFactor * lightSizeScale;

        left = clamp(left, -0.5f, 0.5f);
        right = clamp(right, -0.5f, 0.5f);
        top = clamp(top, -0.5f, 0.5f);
        bottom = clamp(bottom, -0.5f, 0.5f);

        float width = right - left;
        float height = top - bottom;

        float area = width * height;

        visibility -= area * 1.0f;
    }
}

if (visibility < 0.0f)
{
    visibility = 0.0f;
}

shadowMask *= visibility;