#version 300 es
precision highp float;

in vec2 v_texcoord;
uniform sampler2D tex;
out vec4 fragColor;

void main() {
    float o = 0.002;

    vec4 c = texture(tex, v_texcoord) * 5.0;
    c -= texture(tex, v_texcoord + vec2( 0, -o));
    c -= texture(tex, v_texcoord + vec2(-o,  0));
    c -= texture(tex, v_texcoord + vec2( o,  0));
    c -= texture(tex, v_texcoord + vec2( 0,  o));

    fragColor = clamp(c, 0.0, 1.0);
}