detection_head.h

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// SPDX-License-Identifier: AGPL-3.0-only
// Copyright (C) 2026 Artificial Intelligence Techniques, S.L. and Roberto López González
 
#pragma once
 
#include <array>
 
#include "opennn/core/opennn_types.h"
 
namespace opennn
{
 
enum class DetectionHeadKind { AnchorBased, AnchorFree };
enum class DetectionClassActivation { Softmax, Sigmoid };
 
struct DetectionHeadMetadata
{
    DetectionHeadKind kind = DetectionHeadKind::AnchorBased;
    Index boxes_per_cell = 1;
    Index classes_number = 0;
    Index regression_bins = 1;
    DetectionClassActivation class_activation = DetectionClassActivation::Softmax;
 
    bool is_anchor_free() const noexcept { return kind == DetectionHeadKind::AnchorFree; }
    bool uses_sigmoid_classes() const noexcept { return class_activation == DetectionClassActivation::Sigmoid; }
};
 
inline float yolo_box_iou(const std::array<float, 6>& a, const std::array<float, 6>& b)
{
    const float a_left = a[0] - 0.5f * a[2];
    const float a_top = a[1] - 0.5f * a[3];
    const float a_right = a[0] + 0.5f * a[2];
    const float a_bottom = a[1] + 0.5f * a[3];
 
    const float b_left = b[0] - 0.5f * b[2];
    const float b_top = b[1] - 0.5f * b[3];
    const float b_right = b[0] + 0.5f * b[2];
    const float b_bottom = b[1] + 0.5f * b[3];
 
    const float inter_w = max(0.0f, min(a_right, b_right) - max(a_left, b_left));
    const float inter_h = max(0.0f, min(a_bottom, b_bottom) - max(a_top, b_top));
    const float inter = inter_w * inter_h;
 
    const float area = a[2] * a[3] + b[2] * b[3] - inter;
 
    return area > 0.0f ? inter / area : 0.0f;
}
 
inline float dfl_decode(const float* logits, Index reg_max)
{
    float max_l = *max_element(logits, logits + reg_max);
    float sum = 0.0f;
    for (Index i = 0; i < reg_max; ++i) sum += expf(logits[i] - max_l);
    float d = 0.0f;
    for (Index i = 0; i < reg_max; ++i) d += float(i) * expf(logits[i] - max_l) / sum;
    return d;
}
 
class DetectionHeadEndpoint
{
public:
    virtual ~DetectionHeadEndpoint() = default;
 
    virtual DetectionHeadMetadata get_detection_head_metadata() const noexcept = 0;
};
 
}