outline.frag

precision mediump float;

varying vec2 vTextureCoord;
uniform sampler2D uSampler;
uniform vec2 dimensions;

void main(void) {
    vec2 pixelSize  = vec2(1.0) / dimensions;
    vec4 pixel      = texture2D(uSampler, vTextureCoord);
    vec4 pixelUp    = texture2D(uSampler, vTextureCoord - vec2(0.0, pixelSize.y));
    vec4 pixelDown  = texture2D(uSampler, vTextureCoord + vec2(0.0, pixelSize.y));
    vec4 pixelLeft  = texture2D(uSampler, vTextureCoord - vec2(pixelSize.x, 0.0));
    vec4 pixelRight = texture2D(uSampler, vTextureCoord + vec2(pixelSize.x, 0.0));
    if (pixel.a == 0.0 && (pixelUp.a    > 0.0 ||
                           pixelDown.a  > 0.0 ||
                           pixelLeft.a  > 0.0 ||
                           pixelRight.a > 0.0)) {
        pixel = vec4(1.0, 0.0, 0.0, 1.0);
    }
    gl_FragColor = pixel;
}