How to make smoother borders using Fragment shader in OpenGL?

I have been trying to draw border of an Image with transparent background using OpenGL in Android. I am using Fragment Shader & Vertex Shader. (From the GPUImage Library)

Below I have added Fig. A & Fig B.

Fig A.

Fig B.

I have achieved Fig A. With the customised Fragment Shader. But Unable to make the border smoother as in Fig B. I am attaching the Shader code that I have used (to achieve rough border). Can someone here help me on how to make the border smoother?

Here is my Vertex Shader :

    attribute vec4 position;
    attribute vec4 inputTextureCoordinate;        
    varying vec2 textureCoordinate;
    
    void main()
    {
        gl_Position = position;
        textureCoordinate = inputTextureCoordinate.xy;
    }

Here is my Fragment Shader :

I have calculated 8 pixels around the current pixel. If any one pixel of those 8 is opaque(having alpha greater than 0.4), It it drawn as a border color.

                precision mediump float;
                uniform sampler2D inputImageTexture;
                varying vec2 textureCoordinate;
                uniform lowp float thickness;
                uniform lowp vec4 color;

                void main() {
                    float x = textureCoordinate.x;
                    float y = textureCoordinate.y;
                    vec4 current = texture2D(inputImageTexture, vec2(x,y));

                    if ( current.a != 1.0 ) {
                        float offset = thickness * 0.5;
                        
                        vec4 top = texture2D(inputImageTexture, vec2(x, y - offset));
                        vec4 topRight = texture2D(inputImageTexture, vec2(x + offset,y - offset));
                        vec4 topLeft = texture2D(inputImageTexture, vec2(x - offset, y - offset));
                        vec4 right = texture2D(inputImageTexture, vec2(x + offset, y ));
                        vec4 bottom = texture2D(inputImageTexture, vec2(x , y + offset));
                        vec4 bottomLeft  = texture2D(inputImageTexture, vec2(x - offset, y + offset));
                        vec4 bottomRight = texture2D(inputImageTexture, vec2(x + offset, y + offset));
                        vec4 left = texture2D(inputImageTexture, vec2(x - offset, y ));
                        
                        if ( top.a > 0.4 || bottom.a > 0.4 || left.a > 0.4 || right.a > 0.4 || topLeft.a > 0.4 || topRight.a > 0.4 || bottomLeft.a > 0.4 || bottomRight.a > 0.4 ) {
                             if (current.a != 0.0) {
                                 current = mix(color , current , current.a);
                             } else {
                                 current = color;
                             }
                        }
                    }
                    
                    gl_FragColor = current;
                }

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