Origin: https://github.com/nleseul/obs-shaderfilter/blob/master/data/examples/drop_shadow.shader

fake_blur.shader

uniform int shadow_blur_unit_dist;
uniform float shadow_blur_gain;
uniform float4 shadow_color;

float4 mainImage(VertData v_in) : TARGET {
	float invsqrt2 = 0.70710678118654;

	float2x2 rotateClockwise45 = {
		invsqrt2, -invsqrt2,
		invsqrt2, invsqrt2,
	};

	float sampled_shadow_alpha = 0;

	float2 offset = float2(0, shadow_blur_unit_dist);
	sampled_shadow_alpha += image.Sample(textureSampler, v_in.uv + offset * uv_pixel_interval).a;

	offset = mul(rotateClockwise45, offset);
	sampled_shadow_alpha += image.Sample(textureSampler, v_in.uv + offset * uv_pixel_interval).a;

	offset = mul(rotateClockwise45, offset);
	sampled_shadow_alpha += image.Sample(textureSampler, v_in.uv + offset * uv_pixel_interval).a;

	offset = mul(rotateClockwise45, offset);
	sampled_shadow_alpha += image.Sample(textureSampler, v_in.uv + offset * uv_pixel_interval).a;

	offset = mul(rotateClockwise45, offset);
	sampled_shadow_alpha += image.Sample(textureSampler, v_in.uv + offset * uv_pixel_interval).a;

	offset = mul(rotateClockwise45, offset);
	sampled_shadow_alpha += image.Sample(textureSampler, v_in.uv + offset * uv_pixel_interval).a;

	offset = mul(rotateClockwise45, offset);
	sampled_shadow_alpha += image.Sample(textureSampler, v_in.uv + offset * uv_pixel_interval).a;

	offset = mul(rotateClockwise45, offset);
	sampled_shadow_alpha += image.Sample(textureSampler, v_in.uv + offset * uv_pixel_interval).a;

	sampled_shadow_alpha /= 8;
	sampled_shadow_alpha *= shadow_blur_gain;

	float4 final_shadow_color = float4(shadow_color.rgb, sampled_shadow_alpha);
	float4 color = image.Sample(textureSampler, v_in.uv);

	return final_shadow_color * (1-color.a) + color;
}

	GPU Util
Disable	6%
drop_shadow.shader (shadow_blur_size: 50)	69% (+63%)
fake_blur.shader	6% (+<1%)