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atan2: clarify sharing bits between atan argument and constant factors

This commit is contained in:
Matt Huszagh 2020-12-17 09:31:38 -08:00
parent 5717991ada
commit cb38c3e3bd
1 changed files with 12 additions and 7 deletions
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19
dsp/src/trig.rs
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@ -47,16 +47,21 @@ pub fn atan2(y: i32, x: i32) -> i32 {
return 0;
}
let ratio = (min << 15) / max;
// We need to share the 31 available non-sign bits between the
// atan argument and constant factors used in the atan
// approximation. Sharing the bits roughly equally between them
// gives good accuracy.
const ATAN_ARGUMENT_BITS: usize = 15;
let ratio = (min << ATAN_ARGUMENT_BITS) / max;
let mut angle = {
const K1: i32 =
((1_f64 / 4_f64 + 0.285_f64 / PI) * (1 << 16) as f64) as i32;
const K2: i32 = ((0.285_f64 / PI) * (1 << 16) as f64) as i32;
const K1: i32 = ((1. / 4. + 0.285 / PI)
* (1 << (31 - ATAN_ARGUMENT_BITS)) as f64)
as i32;
const K2: i32 =
((0.285 / PI) * (1 << (31 - ATAN_ARGUMENT_BITS)) as f64) as i32;
let ratio_squared = shift_round(ratio * ratio, 15);
ratio * K1 - K2 * ratio_squared
ratio * K1 - K2 * shift_round(ratio * ratio, ATAN_ARGUMENT_BITS)
};
if uy > ux {