Disney BSSRDF, sample scattering profile, lower

Percentage Accurate: 61.4% → 61.4%
Time: 1.6s
Alternatives: 3
Speedup: 1.0×

Specification

?
\[\left(0 \leq s \land s \leq 256\right) \land \left(2.328306437 \cdot 10^{-10} \leq u \land u \leq 0.25\right)\]
\[\begin{array}{l} \\ s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right) \end{array} \]
(FPCore (s u) :precision binary32 (* s (log (/ 1.0 (- 1.0 (* 4.0 u))))))
float code(float s, float u) {
	return s * logf((1.0f / (1.0f - (4.0f * u))));
}

real(4) function code(s, u)
    real(4), intent (in) :: s
    real(4), intent (in) :: u
    code = s * log((1.0e0 / (1.0e0 - (4.0e0 * u))))
end function

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

function tmp = code(s, u)
	tmp = s * log((single(1.0) / (single(1.0) - (single(4.0) * u))));
end

\begin{array}{l}

\\
s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right)
\end{array}

Sampling outcomes in binary32 precision:

Local Percentage Accuracy vs ?

The average percentage accuracy by input value. Horizontal axis shows value of an input variable; the variable is choosen in the title. Vertical axis is accuracy; higher is better. Red represent the original program, while blue represents Herbie's suggestion. These can be toggled with buttons below the plot. The line is an average while dots represent individual samples.

Accuracy vs Speed?

Herbie found 3 alternatives:

AlternativeAccuracySpeedup
The accuracy (vertical axis) and speed (horizontal axis) of each alternatives. Up and to the right is better. The red square shows the initial program, and each blue circle shows an alternative.The line shows the best available speed-accuracy tradeoffs.

Initial Program: 61.4% accurate, 1.0× speedup?

\[\begin{array}{l} \\ s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right) \end{array} \]
(FPCore (s u) :precision binary32 (* s (log (/ 1.0 (- 1.0 (* 4.0 u))))))
float code(float s, float u) {
	return s * logf((1.0f / (1.0f - (4.0f * u))));
}

real(4) function code(s, u)
    real(4), intent (in) :: s
    real(4), intent (in) :: u
    code = s * log((1.0e0 / (1.0e0 - (4.0e0 * u))))
end function

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

function tmp = code(s, u)
	tmp = s * log((single(1.0) / (single(1.0) - (single(4.0) * u))));
end

\begin{array}{l}

\\
s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right)
\end{array}

Alternative 1: 61.4% accurate, 1.0× speedup?

\[\begin{array}{l} \\ s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right) \end{array} \]
(FPCore (s u) :precision binary32 (* s (log (/ 1.0 (- 1.0 (* 4.0 u))))))
float code(float s, float u) {
	return s * logf((1.0f / (1.0f - (4.0f * u))));
}

real(4) function code(s, u)
    real(4), intent (in) :: s
    real(4), intent (in) :: u
    code = s * log((1.0e0 / (1.0e0 - (4.0e0 * u))))
end function

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

function tmp = code(s, u)
	tmp = s * log((single(1.0) / (single(1.0) - (single(4.0) * u))));
end

\begin{array}{l}

\\
s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right)
\end{array}
Derivation

  1. Initial program 59.8%

    \[s \cdot \log \left(\frac{1}{1 - 4 \cdot u}\right) \]
  2. Add Preprocessing
  3. Add Preprocessing

Alternative 2: 13.4% accurate, 1.0× speedup?

\[\begin{array}{l} \\ \log \left(\frac{1}{1 - 4 \cdot u}\right) \end{array} \]
(FPCore (s u) :precision binary32 (log (/ 1.0 (- 1.0 (* 4.0 u)))))
float code(float s, float u) {
	return logf((1.0f / (1.0f - (4.0f * u))));
}

real(4) function code(s, u)
    real(4), intent (in) :: s
    real(4), intent (in) :: u
    code = log((1.0e0 / (1.0e0 - (4.0e0 * u))))
end function

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

function tmp = code(s, u)
	tmp = log((single(1.0) / (single(1.0) - (single(4.0) * u))));
end

\begin{array}{l}

\\
\log \left(\frac{1}{1 - 4 \cdot u}\right)
\end{array}
Derivation

  1. Initial program 59.8%

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

  3. Taylor expanded in u around 0

    \[\leadsto \color{blue}{4 \cdot \left(s \cdot u\right)} \]

  5. Applied rewrites13.1%

    \[\leadsto \color{blue}{\log \left(\frac{1}{1 - 4 \cdot u}\right)} \]
  6. Add Preprocessing

Alternative 3: 11.6% accurate, 20.8× speedup?

\[\begin{array}{l} \\ 4 \cdot u \end{array} \]
(FPCore (s u) :precision binary32 (* 4.0 u))
float code(float s, float u) {
	return 4.0f * u;
}

real(4) function code(s, u)
    real(4), intent (in) :: s
    real(4), intent (in) :: u
    code = 4.0e0 * u
end function

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

function tmp = code(s, u)
	tmp = single(4.0) * u;
end

\begin{array}{l}

\\
4 \cdot u
\end{array}
Derivation

  1. Initial program 59.8%

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

  3. Taylor expanded in u around 0

    \[\leadsto \color{blue}{u \cdot \left(4 \cdot s + u \cdot \left(8 \cdot s + u \cdot \left(\frac{64}{3} \cdot s + 64 \cdot \left(s \cdot u\right)\right)\right)\right)} \]

  5. Applied rewrites11.2%

    \[\leadsto \color{blue}{4 \cdot u} \]
  6. Add Preprocessing

Reproduce

?
herbie shell --seed 2024318 
(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))))))