Move the yolo shared bits to a common place. (#548)

* Move the yolo shared bits to a common place.

* Share more code.

* Configurable thresholds.

candle-examples/src/object_detection.rs

@@ -0,0 +1,44 @@
+/// A bounding box around an object.
+#[derive(Debug, Clone, Copy)]
+pub struct Bbox {
+    pub xmin: f32,
+    pub ymin: f32,
+    pub xmax: f32,
+    pub ymax: f32,
+    pub confidence: f32,
+}
+
+/// Intersection over union of two bounding boxes.
+pub fn iou(b1: &Bbox, b2: &Bbox) -> f32 {
+    let b1_area = (b1.xmax - b1.xmin + 1.) * (b1.ymax - b1.ymin + 1.);
+    let b2_area = (b2.xmax - b2.xmin + 1.) * (b2.ymax - b2.ymin + 1.);
+    let i_xmin = b1.xmin.max(b2.xmin);
+    let i_xmax = b1.xmax.min(b2.xmax);
+    let i_ymin = b1.ymin.max(b2.ymin);
+    let i_ymax = b1.ymax.min(b2.ymax);
+    let i_area = (i_xmax - i_xmin + 1.).max(0.) * (i_ymax - i_ymin + 1.).max(0.);
+    i_area / (b1_area + b2_area - i_area)
+}
+
+pub fn non_maximum_suppression(bboxes: &mut [Vec<Bbox>], threshold: f32) {
+    // Perform non-maximum suppression.
+    for bboxes_for_class in bboxes.iter_mut() {
+        bboxes_for_class.sort_by(|b1, b2| b2.confidence.partial_cmp(&b1.confidence).unwrap());
+        let mut current_index = 0;
+        for index in 0..bboxes_for_class.len() {
+            let mut drop = false;
+            for prev_index in 0..current_index {
+                let iou = iou(&bboxes_for_class[prev_index], &bboxes_for_class[index]);
+                if iou > threshold {
+                    drop = true;
+                    break;
+                }
+            }
+            if !drop {
+                bboxes_for_class.swap(current_index, index);
+                current_index += 1;
+            }
+        }
+        bboxes_for_class.truncate(current_index);
+    }
+}