Implement gradient thresholding for Canny filter

src/filter/canny/gradient_threshold.rs

@@ -0,0 +1,79 @@
+use super::*;
+
+use crate::*;
+
+#[derive(Clone, Copy, PartialEq)]
+struct Step {
+    x: isize,
+    y: isize,
+}
+
+impl Step {
+    fn of(mut theta: f32) -> Self {
+        theta /= std::f32::consts::PI;
+        if theta < 0.0 {
+            theta += 1.0;
+        }
+
+        if theta <= 0.125 {
+            Self { x: 1, y: 0 }
+        } else if theta <= 0.375 {
+            Self { x: 1, y: 1 }
+        } else if theta <= 0.625 {
+            Self { x: 0, y: 1 }
+        } else if theta <= 0.875 {
+            Self { x: -1, y: 1 }
+        } else if theta <= 1.0 {
+            Self { x: 1, y: 0 }
+        } else {
+            panic!("Angle out of expected range");
+        }
+    }
+
+    fn back(&self) -> Self {
+        Self {
+            x: -self.x,
+            y: -self.y,
+        }
+    }
+
+    fn from(&self, x: usize, y: usize) -> (isize, isize) {
+        (x as isize + self.x, y as isize + self.y)
+    }
+}
+
+impl Default for Step {
+    fn default() -> Self {
+        Step { x: 0, y: 0 }
+    }
+}
+
+pub fn gradient_threshold(img: Image<filter::sobel::Vector>) -> Image<f32> {
+    let mut result = image::Image::<f32>::with_extent(img.extent);
+    result.for_each(|x, y, p| {
+        let value = img.at(x, y);
+        let step = Step::of(value.direction);
+
+        let this = value.magnitude;
+        let next = get(&img, step.from(x, y))
+            .map_or(f32::NEG_INFINITY, |v| v.magnitude);
+        let prev = get(&img, step.back().from(x, y))
+            .map_or(f32::NEG_INFINITY, |v| v.magnitude);
+        if this > next && this > prev {
+            *p = this;
+        }
+    });
+    result
+}
+
+fn get<T: Copy>(img: &Image<T>, pos: (isize, isize)) -> Option<T> {
+    if pos.0 >= 0
+        && pos.0 < img.extent.x as isize
+        && pos.1 >= 0
+        && pos.1 < img.extent.y as isize
+    {
+        Some(img.at(pos.0 as usize, pos.1 as usize))
+    } else {
+        None
+    }
+}