From 0c99ce8227ea99f7a8eea20f9799d31c64e74478 Mon Sep 17 00:00:00 2001
From: Nathan Vegdahl <cessen@cessen.com>
Date: Wed, 31 Jul 2019 22:46:06 +0900
Subject: [PATCH] Playing around with surface patch implementation.  Trying out
 ideas.

---
 src/surface/bilinear_patch.rs | 82 ++++++++++++++++++++++++++++++++---
 src/surface/mod.rs            | 37 +++++++++++++++-
 2 files changed, 112 insertions(+), 7 deletions(-)

diff --git a/src/surface/bilinear_patch.rs b/src/surface/bilinear_patch.rs
index dcacdf9..c85fbaf 100644
--- a/src/surface/bilinear_patch.rs
+++ b/src/surface/bilinear_patch.rs
@@ -1,10 +1,8 @@
-use super::Splitable;
-use crate::math::Point;
+use super::{point_order, PointOrder, Splitable, MAX_EDGE_DICE};
+use crate::{lerp::lerp, math::Point};
 
 #[derive(Debug, Copy, Clone)]
 pub struct BilinearPatch<'a> {
-    time_sample_count: usize,
-
     // The control points are stored in clockwise order, like this:
     //  u ----->
     // v  0  1
@@ -25,6 +23,12 @@ pub struct BilinearPatch<'a> {
     must_split: [bool; 4],
 }
 
+fn bilerp_point(patch: [Point; 4], uv: (f32, f32)) -> Point {
+    let a = lerp(patch[0], patch[1], uv.0);
+    let b = lerp(patch[3], patch[2], uv.0);
+    lerp(a, b, uv.1)
+}
+
 #[derive(Debug, Copy, Clone)]
 pub struct BilinearSubPatch<'a> {
     original: &'a BilinearPatch<'a>,
@@ -33,7 +37,73 @@ pub struct BilinearSubPatch<'a> {
 }
 
 impl<'a> Splitable for BilinearSubPatch<'a> {
-    fn split(&self /* TODO: splitting criteria. */) -> Option<(Self, Self)> {
-        unimplemented!()
+    fn split<F>(&self, metric: F) -> Option<(Self, Self)>
+    where
+        F: Fn(Point, Point) -> f32,
+    {
+        // Get the points of the sub-patch.
+        let patch = self.original.control_points[0];
+        let points = [
+            bilerp_point(patch, self.clip[0]),
+            bilerp_point(patch, self.clip[1]),
+            bilerp_point(patch, self.clip[2]),
+            bilerp_point(patch, self.clip[3]),
+        ];
+
+        // Calculate edge metrics.
+        let edge_metric = [
+            metric(points[0], points[1]),
+            metric(points[1], points[2]),
+            metric(points[2], points[3]),
+            metric(points[3], points[0]),
+        ];
+
+        // Do the split, if needed.
+        for i in 0..4 {
+            if self.must_split[i] || edge_metric[i] > MAX_EDGE_DICE as f32 {
+                let edge_1 = (i, (i + 1) % 4);
+                let edge_2 = ((i + 2) % 4, (i + 3) % 4);
+                let new_must_split = {
+                    let mut new_must_split = self.must_split;
+                    new_must_split[edge_1.0] = false;
+                    new_must_split[edge_2.0] = false;
+                    new_must_split
+                };
+
+                let midpoint_1 = lerp(self.clip[edge_1.0], self.clip[edge_1.1], 0.5);
+                let midpoint_2 = {
+                    let alpha = if self.must_split[edge_2.0]
+                        || edge_metric[edge_2.0] > MAX_EDGE_DICE as f32
+                    {
+                        0.5
+                    } else {
+                        let edge_2_dice_rate = edge_metric[edge_2.0].ceil();
+                        (edge_2_dice_rate * 0.5).floor() / edge_2_dice_rate
+                    };
+
+                    match point_order(points[edge_2.0], points[edge_2.1]) {
+                        PointOrder::AsIs => lerp(self.clip[edge_2.0], self.clip[edge_2.1], alpha),
+                        PointOrder::Flip => lerp(self.clip[edge_2.1], self.clip[edge_2.0], alpha),
+                    }
+                };
+
+                // Build the new sub-patches
+                let mut patch_1 = BilinearSubPatch {
+                    original: self.original,
+                    clip: self.clip,
+                    must_split: new_must_split,
+                };
+                let mut patch_2 = patch_1;
+                patch_1.clip[edge_1.1] = midpoint_1;
+                patch_1.clip[edge_2.0] = midpoint_2;
+                patch_2.clip[edge_1.0] = midpoint_1;
+                patch_2.clip[edge_2.1] = midpoint_2;
+
+                return Some((patch_1, patch_2));
+            }
+        }
+
+        // No splitting needed to be done.
+        None
     }
 }
diff --git a/src/surface/mod.rs b/src/surface/mod.rs
index 65d9114..9c6f18d 100644
--- a/src/surface/mod.rs
+++ b/src/surface/mod.rs
@@ -16,6 +16,8 @@ use crate::{
     shading::SurfaceShader,
 };
 
+const MAX_EDGE_DICE: u32 = 128;
+
 pub trait Surface: Boundable + Debug + Sync {
     fn intersect_rays(
         &self,
@@ -29,7 +31,40 @@ pub trait Surface: Boundable + Debug + Sync {
 
 pub trait Splitable: Copy {
     /// Splits the surface into two pieces if necessary.
-    fn split(&self /* TODO: splitting criteria. */) -> Option<(Self, Self)>;
+    fn split<F>(&self, metric: F) -> Option<(Self, Self)>
+    where
+        F: Fn(Point, Point) -> f32;
+}
+
+#[derive(Debug, Copy, Clone)]
+pub enum PointOrder {
+    AsIs,
+    Flip,
+}
+
+pub fn point_order(p1: Point, p2: Point) -> PointOrder {
+    let max_diff = {
+        let v = p2 - p1;
+        let v_abs = v.abs();
+
+        let mut diff = v.x();
+        let mut diff_abs = v_abs.x();
+        if v_abs.y() > diff_abs {
+            diff = v.y();
+            diff_abs = v_abs.y();
+        }
+        if v_abs.z() > diff_abs {
+            diff = v.z();
+        }
+
+        diff
+    };
+
+    if max_diff <= 0.0 {
+        PointOrder::AsIs
+    } else {
+        PointOrder::Flip
+    }
 }
 
 #[derive(Debug, Copy, Clone)]