Disney BSSRDF, sample scattering profile, lower

Percentage Accurate: 61.1% → 99.4%

Time: 8.0s

Precision: binary32

Cost: 3392

\[\left(0 \leq s \land s \leq 256\right) \land \left(2.328306437 \cdot 10^{-10} \leq u \land u \leq 0.25\right)\]

Math

\[s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right) \]

↓

\[\begin{array}{l} \\ \mathsf{log1p}\left(u \cdot -4\right) \cdot \left(-s\right) \end{array} \]

FPCore

(FPCore (s u) :precision binary32 (* s (log (/ 1.0 (- 1.0 (* 4.0 u))))))

↓

(FPCore (s u) :precision binary32 (* (log1p (* u -4.0)) (- s)))

C

float code(float s, float u) {
	return s * logf((1.0f / (1.0f - (4.0f * u))));
}

↓

float code(float s, float u) {
	return log1pf((u * -4.0f)) * -s;
}

Julia

function code(s, u)
	return Float32(s * log(Float32(Float32(1.0) / Float32(Float32(1.0) - Float32(Float32(4.0) * u)))))
end

↓

function code(s, u)
	return Float32(log1p(Float32(u * Float32(-4.0))) * Float32(-s))
end

Derivation

1. Initial program 61.4%

   \[s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right) \]

2. Simplified 99.3%

   \[\leadsto \color{blue}{\mathsf{log1p}\left(u \cdot -4\right) \cdot \left(-s\right)} \]
   Step-by-step derivation

   [Start] 61.4%	\[ s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right) \]
   *-commutative [=>] 61.4%	\[ \color{blue}{\log \left(\frac{1}{1 - 4 \cdot u}\right) \cdot s} \]
   log-rec [=>] 63.6%	\[ \color{blue}{\left(-\log \left(1 - 4 \cdot u\right)\right)} \cdot s \]
   distribute-lft-neg-out [=>] 63.6%	\[ \color{blue}{-\log \left(1 - 4 \cdot u\right) \cdot s} \]
   distribute-rgt-neg-in [=>] 63.6%	\[ \color{blue}{\log \left(1 - 4 \cdot u\right) \cdot \left(-s\right)} \]
   sub-neg [=>] 63.6%	\[ \log \color{blue}{\left(1 + \left(-4 \cdot u\right)\right)} \cdot \left(-s\right) \]
   log1p-def [=>] 99.3%	\[ \color{blue}{\mathsf{log1p}\left(-4 \cdot u\right)} \cdot \left(-s\right) \]
   *-commutative [=>] 99.3%	\[ \mathsf{log1p}\left(-\color{blue}{u \cdot 4}\right) \cdot \left(-s\right) \]
   distribute-rgt-neg-in [=>] 99.3%	\[ \mathsf{log1p}\left(\color{blue}{u \cdot \left(-4\right)}\right) \cdot \left(-s\right) \]
   metadata-eval [=>] 99.3%	\[ \mathsf{log1p}\left(u \cdot \color{blue}{-4}\right) \cdot \left(-s\right) \]

3. Final simplification 99.3%

   \[\leadsto \mathsf{log1p}\left(u \cdot -4\right) \cdot \left(-s\right) \]

Alternatives

Alternative 1
Accuracy	99.4%
Cost	3392

\[\mathsf{log1p}\left(u \cdot -4\right) \cdot \left(-s\right) \]

Alternative 2
Accuracy	91.0%
Cost	416

\[s \cdot \left(u \cdot \left(4 + u \cdot \left(8 + u \cdot 21.333333333333332\right)\right)\right) \]

Alternative 3
Accuracy	86.9%
Cost	352

\[u \cdot \left(u \cdot \left(s \cdot 8\right) + s \cdot 4\right) \]

Alternative 4
Accuracy	88.8%
Cost	352

\[u \cdot \frac{s \cdot 16}{4 + u \cdot -8} \]

Alternative 5
Accuracy	86.7%
Cost	288

\[s \cdot \left(u \cdot \left(4 + u \cdot 8\right)\right) \]

Alternative 6
Accuracy	73.8%
Cost	160

\[4 \cdot \left(u \cdot s\right) \]

Alternative 7
Accuracy	74.1%
Cost	160

\[u \cdot \left(s \cdot 4\right) \]

Alternative 8
Accuracy	16.5%
Cost	96

\[s \cdot 0 \]

Reproduce

herbie shell --seed 2023167 
(FPCore (s u)
  :name "Disney BSSRDF, sample scattering profile, lower"
  :precision binary32
  :pre (and (and (<= 0.0 s) (<= s 256.0)) (and (<= 2.328306437e-10 u) (<= u 0.25)))
  (* s (log (/ 1.0 (- 1.0 (* 4.0 u))))))