Babylon.js/src/Shaders/glowBlurPostProcess.fragment.fx

// Samplers
varying vec2 vUV;
uniform sampler2D textureSampler;

// Parameters
uniform vec2 screenSize;
uniform vec2 direction;
uniform float blurWidth;

// Transform color to luminance.
float getLuminance(vec3 color)
{
    return dot(color, vec3(0.2126, 0.7152, 0.0722));
}

void main(void)
{
	float weights[7];
	weights[0] = 0.05;
	weights[1] = 0.1;
	weights[2] = 0.2;
	weights[3] = 0.3;
	weights[4] = 0.2;
	weights[5] = 0.1;
	weights[6] = 0.05;

	vec2 texelSize = vec2(1.0 / screenSize.x, 1.0 / screenSize.y);
	vec2 texelStep = texelSize * direction * blurWidth;
	vec2 start = vUV - 3.0 * texelStep;

	vec4 baseColor = vec4(0., 0., 0., 0.);
	vec2 texelOffset = vec2(0., 0.);

	for (int i = 0; i < 7; i++)
	{
		// alpha blur.
		vec4 texel = texture2D(textureSampler, start + texelOffset);
		baseColor.a += texel.a * weights[i];

		// Highest Luma for outline.
		float luminance = getLuminance(baseColor.rgb);
		float luminanceTexel = getLuminance(texel.rgb);
		float choice = step(luminanceTexel, luminance);
		baseColor.rgb = choice * baseColor.rgb + (1.0 - choice) * texel.rgb;

		texelOffset += texelStep;
	}

	gl_FragColor = baseColor;
}