Curve intersection, scale width based on ribbon orientation

Average Error: 0.9 → 0.4

Time: 17.3s

Precision: binary32

\[\left(\left(\left(0 \leq normAngle \land normAngle \leq \frac{\pi}{2}\right) \land \left(-1 \leq n0_i \land n0_i \leq 1\right)\right) \land \left(-1 \leq n1_i \land n1_i \leq 1\right)\right) \land \left(2.328306437 \cdot 10^{-10} \leq u \land u \leq 1\right)\]

Math

\[\left(\sin \left(\left(1 - u\right) \cdot normAngle\right) \cdot \frac{1}{\sin normAngle}\right) \cdot n0_i + \left(\sin \left(u \cdot normAngle\right) \cdot \frac{1}{\sin normAngle}\right) \cdot n1_i \]

↓

\[\left(\sin \left(\left(1 - u\right) \cdot normAngle\right) \cdot \frac{1}{\sin normAngle}\right) \cdot n0_i + \mathsf{fma}\left(\frac{u}{\sin normAngle}, normAngle, \frac{{u}^{3} \cdot {normAngle}^{3}}{\sin normAngle} \cdot -0.16666666666666666\right) \cdot n1_i \]

FPCore

(FPCore (normAngle u n0_i n1_i)
 :precision binary32
 (+
  (* (* (sin (* (- 1.0 u) normAngle)) (/ 1.0 (sin normAngle))) n0_i)
  (* (* (sin (* u normAngle)) (/ 1.0 (sin normAngle))) n1_i)))

↓

(FPCore (normAngle u n0_i n1_i)
 :precision binary32
 (+
  (* (* (sin (* (- 1.0 u) normAngle)) (/ 1.0 (sin normAngle))) n0_i)
  (*
   (fma
    (/ u (sin normAngle))
    normAngle
    (*
     (/ (* (pow u 3.0) (pow normAngle 3.0)) (sin normAngle))
     -0.16666666666666666))
   n1_i)))

C

float code(float normAngle, float u, float n0_i, float n1_i) {
	return ((sinf((1.0f - u) * normAngle) * (1.0f / sinf(normAngle))) * n0_i) + ((sinf(u * normAngle) * (1.0f / sinf(normAngle))) * n1_i);
}

↓

float code(float normAngle, float u, float n0_i, float n1_i) {
	return ((sinf((1.0f - u) * normAngle) * (1.0f / sinf(normAngle))) * n0_i) + (fmaf((u / sinf(normAngle)), normAngle, (((powf(u, 3.0f) * powf(normAngle, 3.0f)) / sinf(normAngle)) * -0.16666666666666666f)) * n1_i);
}

Error

Bits error versus normAngle

Bits error versus u

Bits error versus n0_i

Bits error versus n1_i

Derivation

1. Initial program 0.9

   \[\left(\sin \left(\left(1 - u\right) \cdot normAngle\right) \cdot \frac{1}{\sin normAngle}\right) \cdot n0_i + \left(\sin \left(u \cdot normAngle\right) \cdot \frac{1}{\sin normAngle}\right) \cdot n1_i \]

2. Taylor expanded in u around 0 0.9

   \[\leadsto \left(\sin \left(\left(1 - u\right) \cdot normAngle\right) \cdot \frac{1}{\sin normAngle}\right) \cdot n0_i + \color{blue}{\left(\frac{u \cdot normAngle}{\sin normAngle} - 0.16666666666666666 \cdot \frac{{u}^{3} \cdot {normAngle}^{3}}{\sin normAngle}\right)} \cdot n1_i \]

3. Simplified 0.4

   \[\leadsto \left(\sin \left(\left(1 - u\right) \cdot normAngle\right) \cdot \frac{1}{\sin normAngle}\right) \cdot n0_i + \color{blue}{\mathsf{fma}\left(\frac{u}{\sin normAngle}, normAngle, \frac{{u}^{3} \cdot {normAngle}^{3}}{\sin normAngle} \cdot -0.16666666666666666\right)} \cdot n1_i \]

4. Final simplification 0.4

   \[\leadsto \left(\sin \left(\left(1 - u\right) \cdot normAngle\right) \cdot \frac{1}{\sin normAngle}\right) \cdot n0_i + \mathsf{fma}\left(\frac{u}{\sin normAngle}, normAngle, \frac{{u}^{3} \cdot {normAngle}^{3}}{\sin normAngle} \cdot -0.16666666666666666\right) \cdot n1_i \]

Reproduce

herbie shell --seed 2021280 
(FPCore (normAngle u n0_i n1_i)
  :name "Curve intersection, scale width based on ribbon orientation"
  :precision binary32
  :pre (and (and (and (and (<= 0.0 normAngle) (<= normAngle (/ PI 2.0))) (and (<= -1.0 n0_i) (<= n0_i 1.0))) (and (<= -1.0 n1_i) (<= n1_i 1.0))) (and (<= 2.328306437e-10 u) (<= u 1.0)))
  (+ (* (* (sin (* (- 1.0 u) normAngle)) (/ 1.0 (sin normAngle))) n0_i) (* (* (sin (* u normAngle)) (/ 1.0 (sin normAngle))) n1_i)))