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//! Provides an implementation of the Mitchell-Netravali reconstruction filter.
//! See [Reconstruction Filters in Computer Graphics](http://www.cs.utexas.edu/~fussell/courses/cs384g-fall2013/lectures/mitchell/Mitchell.pdf).

use std::f32;

use linalg;
use film::filter::Filter;

/// A Mitchell-Netravali reconstruction filter.
/// Recommended parameters to try: w = 2.0, h = 2.0, b = 1.0 / 3.0, c = 1.0 / 3.0
#[derive(Copy, Clone, Debug)]
pub struct MitchellNetravali {
    w: f32,
    h: f32,
    inv_w: f32,
    inv_h: f32,
    b: f32,
    c: f32,
}

impl MitchellNetravali {
    pub fn new(w: f32, h: f32, b: f32, c: f32) -> MitchellNetravali {
        if b < 0.0 || b > 1.0 {
            println!("Warning! Mitchell-Netravali b param = {} is out of bounds, clamping in range", b);
        }
        if c < 0.0 || c > 1.0 {
            println!("Warning! Mitchell-Netravali c param = {} is out of bounds, clamping in range", c);
        }
        MitchellNetravali { w: w, h: h, inv_w: 1.0 / w, inv_h: 1.0 / h,
            b: linalg::clamp(b, 0.0, 1.0), c: linalg::clamp(c, 0.0, 1.0)
        }
    }
    /// Compute a 1d weight for the filter. Note that the Mitchell-Netravali
    /// filter is defined on [-2, 2] so x should be in this range
    fn weight_1d(&self, x: f32) -> f32 {
        let abs_x = f32::abs(x);
        if x >= 2.0 {
            0.0
        } else if x >= 1.0 {
            1.0 / 6.0 * ((-self.b - 6.0 * self.c) * f32::powf(abs_x, 3.0)
                         + (6.0 * self.b + 30.0 * self.c) * f32::powf(abs_x, 2.0)
                         + (-12.0 * self.b - 48.0 * self.c) * abs_x
                         + (8.0 * self.b + 24.0 * self.c))
        } else {
            1.0 / 6.0 * ((12.0 - 9.0 * self.b - 6.0 * self.c) * f32::powf(abs_x, 3.0)
                         + (-18.0 + 12.0 * self.b + 6.0 * self.c) * f32::powf(abs_x, 2.0)
                         + (6.0 - 2.0 * self.b))
        }
    }
}

impl Filter for MitchellNetravali {
    fn weight(&self, x: f32, y: f32) -> f32 {
        self.weight_1d(2.0 * x * self.inv_w) * self.weight_1d(2.0 * y * self.inv_h)
    }
    fn width(&self) -> f32 { self.w }
    fn inv_width(&self) -> f32 { self.inv_w }
    fn height(&self) -> f32 { self.h }
    fn inv_height(&self) -> f32 { self.inv_h }
}