Linear Approximation for the Smith's Visibility Term for GGX
運算式 2: "r" equals left parenthesis, cos left parenthesis, theta , right parenthesis times left parenthesis, 1 minus "a" , right parenthesis plus "a" , right parenthesis left brace, 0 less than or equal to theta less than or equal to StartFraction, pi Over 2 , EndFraction , right bracer=cosθ·1−a+a0≤θ≤π2
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定義域 \theta 最小值：
less than or equal to theta less than or equal to≤θ≤
