kernel vec4 sketch(sampler image, float strength)
{
    vec2 d = destCoord();

    vec2 bottomLeftTextureCoordinate = samplerTransform(image, d + vec2(-1.0, -1.0));
    vec2 topRightTextureCoordinate = samplerTransform(image, d + vec2(1.0, 1.0));
    vec2 topLeftTextureCoordinate = samplerTransform(image, d + vec2(-1.0, 1.0));
    vec2 bottomRightTextureCoordinate = samplerTransform(image, d + vec2(1.0, -1.0));

    vec2 leftTextureCoordinate = samplerTransform(image, d + vec2(-1.0, 0.0));
    vec2 rightTextureCoordinate = samplerTransform(image, d + vec2(1.0, 0.0));
    vec2 bottomTextureCoordinate = samplerTransform(image, d + vec2(0.0, -1.0));
    vec2 topTextureCoordinate = samplerTransform(image, d + vec2(0.0, 1.0));

    float bottomLeftIntensity = sample(image, bottomLeftTextureCoordinate).r;
    float topRightIntensity = sample(image, topRightTextureCoordinate).r;
    float topLeftIntensity = sample(image, topLeftTextureCoordinate).r;
    float bottomRightIntensity = sample(image, bottomRightTextureCoordinate).r;

    float leftIntensity = sample(image, leftTextureCoordinate).r;
    float rightIntensity = sample(image, rightTextureCoordinate).r;
    float bottomIntensity = sample(image, bottomTextureCoordinate).r;
    float topIntensity = sample(image, topTextureCoordinate).r;

    float h = -topLeftIntensity - 2.0 * topIntensity - topRightIntensity + bottomLeftIntensity + 2.0 * bottomIntensity + bottomRightIntensity;
    float v = -bottomLeftIntensity - 2.0 * leftIntensity - topLeftIntensity + bottomRightIntensity + 2.0 * rightIntensity + topRightIntensity;

    float mag = 1.0 - (length(vec2(h, v))*strength);

    return vec4(vec3(mag), 1.0);
}