axmol/tests/cpp-tests/Resources/Shaders/example_Outline.fsh

/*
  Created by guanghui on 4/8/14.
http://www.idevgames.com/forums/thread-3010.html
*/

varying vec2 v_texCoord;
varying vec4 v_fragmentColor;

uniform vec3 u_outlineColor;
uniform float u_threshold;
uniform float u_radius;

uniform sampler2D u_texture;

void main()
{
    float radius = u_radius;
    vec4 accum = vec4(0.0);
    vec4 normal = vec4(0.0);
    
    normal = texture2D(u_texture, vec2(v_texCoord.x, v_texCoord.y));
    
    accum += texture2D(u_texture, vec2(v_texCoord.x - radius, v_texCoord.y - radius));
    accum += texture2D(u_texture, vec2(v_texCoord.x + radius, v_texCoord.y - radius));
    accum += texture2D(u_texture, vec2(v_texCoord.x + radius, v_texCoord.y + radius));
    accum += texture2D(u_texture, vec2(v_texCoord.x - radius, v_texCoord.y + radius));
    
    accum *= u_threshold;
    accum.rgb =  u_outlineColor * accum.a;
    accum.a = 1.0;
    
    normal = ( accum * (1.0 - normal.a)) + (normal * normal.a);
    
    gl_FragColor = v_fragmentColor * normal;
}