[[vertex]]

uniform mat4 mvp;

in vec4 pos;
in vec2 uvs;
out vec2 uv;

void main()
{
    gl_Position = mvp * vec4(pos.xy, 0.0, 1.0);
    uv = uvs;
}

[[fragment]]

uniform sampler2D tex_bg;

in highp vec2 uv;
out highp vec4 frag;

void main()
{
    highp vec4 bg = texture(tex_bg, uv);
    if (bg.a == 0.0)
    {
        highp vec4 sum = vec4(0.0);
        for (int y = -1; y <= 1; y++)
        {
            for (int x = -1; x <= 1; x++)
            {
                highp vec4 c = textureOffset(tex_bg, uv, ivec2(x, y));
                sum += vec4(c.rgb * c.a, c.a);
            }
        }
        frag = sum.a > 0.0 ? vec4(sum.rgb / sum.a, 0.0) : bg;
    }
    else
    {
        frag = bg;
    }
}