…rations in

the blur shader.

Linear filtering allows us to evaluate the formula

    k₀c₀ + k₁c₁                 (Formula 1)

where c₀ and c₁ are the colors of adjacent texels and k₀ and k₁ are arbitrary
factors (in this case, the results of evaluating the Gaussian function) with a
single lookup. Linear filtering evaluates the following expression for some t
between 0 and 1:

    lerp(c₀, c₁, t)

It can be shown algebraically that Formula 1 is equivalent to:

                 ⎛           k₁  ⎞
    (k₀ + k₁)lerp⎜c₀, c₁, ───────⎟
                 ⎝        k₀ + k₁⎠

Which can be readily evaluated by letting `t = k₁/(k₀ + k₁)` and performing a
texture lookup.