非极大值抑制用C语言实现

非极大值抑制（Non-Maximum Suppression，NMS）是一种常用于目标检测中的算法，其目的是从一组重叠的候选框中选择出最佳的框。

在C语言中，实现非极大值抑制的步骤如下：

1. 定义一个结构体来表示候选框，包括左上角坐标、右下角坐标、得分等信息。

typedef struct {
    int x1, y1, x2, y2;
    float score;
} Box;

2. 定义一个比较函数，用于按照得分从高到低排序。

int compare(const void *a, const void *b) {
    Box *box1 = (Box *)a;
    Box *box2 = (Box *)b;
    if (box1->score < box2->score) {
        return 1;
    } else if (box1->score > box2->score) {
        return -1;
    } else {
        return 0;
    }
}

3. 对所有候选框按照得分从高到低排序。

qsort(boxes, num_boxes, sizeof(Box), compare);

4. 定义一个数组来保存每个候选框是否被保留的标志，初始值为0。

int *keep = (int *)calloc(num_boxes, sizeof(int));

5. 遍历所有候选框，对于每个框，如果其与已经被保留的框的重叠面积大于一定阈值，则将其标记为不保留。

for (int i = 0; i < num_boxes; i++) {
    if (keep[i]) {
        continue;
    }
    for (int j = i + 1; j < num_boxes; j++) {
        if (iou(boxes[i], boxes[j]) > thresh) {
            keep[j] = 0;
        }
    }
}

6. 将所有被保留的框保存到一个新的数组中。

Box *result_boxes = (Box *)malloc(num_boxes * sizeof(Box));
int count = 0;
for (int i = 0; i < num_boxes; i++) {
    if (keep[i]) {
        result_boxes[count++] = boxes[i];
    }
}

完整的C语言实现代码如下：

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    int x1, y1, x2, y2;
    float score;
} Box;

int compare(const void *a, const void *b) {
    Box *box1 = (Box *)a;
    Box *box2 = (Box *)b;
    if (box1->score < box2->score) {
        return 1;
    } else if (box1->score > box2->score) {
        return -1;
    } else {
        return 0;
    }
}

float iou(Box box1, Box box2) {
    int x_overlap = fmin(box1.x2, box2.x2) - fmax(box1.x1, box2.x1);
    int y_overlap = fmin(box1.y2, box2.y2) - fmax(box1.y1, box2.y1);
    if (x_overlap > 0 && y_overlap > 0) {
        float intersection = x_overlap * y_overlap;
        float area1 = (box1.x2 - box1.x1) * (box1.y2 - box1.y1);
        float area2 = (box2.x2 - box2.x1) * (box2.y2 - box2.y1);
        float union_ = area1 + area2 - intersection;
        return intersection / union_;
    } else {
        return 0.0;
    }
}

Box *nms(Box *boxes, int num_boxes, float thresh, int *num_out) {
    qsort(boxes, num_boxes, sizeof(Box), compare);
    int *keep = (int *)calloc(num_boxes, sizeof(int));
    for (int i = 0; i < num_boxes; i++) {
        if (keep[i]) {
            continue;
        }
        for (int j = i + 1; j < num_boxes; j++) {
            if (iou(boxes[i], boxes[j]) > thresh) {
                keep[j] = 0;
            }
        }
    }
    Box *result_boxes = (Box *)malloc(num_boxes * sizeof(Box));
    int count = 0;
    for (int i = 0; i < num_boxes; i++) {
        if (keep[i]) {
            result_boxes[count++] = boxes[i];
        }
    }
    *num_out = count;
    free(keep);
    return result_boxes;
}

int main() {
    Box boxes[] = {
        {10, 10, 50, 50, 0.9},
        {20, 20, 60, 60, 0.8},
        {30, 30, 70, 70, 0.7},
        {40, 40, 80, 80, 0.6},
        {50, 50, 90, 90, 0.5},
    };
    int num_boxes = 5;
    int num_out;
    Box *result_boxes = nms(boxes, num_boxes, 0.5, &num_out);
    for (int i = 0; i < num_out; i++) {
        printf("(%d, %d, %d, %d, %.2f)\n", result_boxes[i].x1, result_boxes[i].y1, result_boxes[i].x2, result_boxes[i].y2, result_boxes[i].score);
    }
    free(result_boxes);
    return 0;
}