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float calcRGBAvg(in vec4 rgba)
{
    return (rgba.r + rgba.g + rgba.b) / 3.0;
}

vec3 defaultMaterialBumpNoLod(in sampler2D sampler, in float factor,
                              in vec2 texCoord, in vec3 tangent, in vec3 binormal,
                              in vec3 normal, in vec2 bumpSize)
{
    // invert factor
    float invFactor = -factor;

    vec2 unitStep = 1.0 / bumpSize;

    float du = calcRGBAvg(texture2D(sampler, vec2(texCoord.x + unitStep.x, texCoord.y)))
             - calcRGBAvg(texture2D(sampler, vec2(texCoord.x, texCoord.y)));
    float dv = calcRGBAvg(texture2D(sampler, vec2(texCoord.x, texCoord.y + unitStep.y)))
             - calcRGBAvg(texture2D(sampler, vec2(texCoord.x, texCoord.y)));

    vec3 n = normalize(vec3(invFactor * du, invFactor * dv, 1.0));
    n = n.x * normalize(tangent) + n.y * normalize(binormal) + n.z * normal;
    return normalize(normal + n);
}