Raspberry Pi OS Bullseye 版本发布时弃用的 Picamera 以新面貌回归了，那就是 Picamera2 库。

下面介绍使用树莓派摄像头模块，用 Picamera2 库和 TensorFlow Lite 实现实时对象检测。

安装 Picamera2 库

因为目前 Picamera2 库还处于预览版，因此暂时只能通过 GitHub 编译安装。

$ sudo apt update
$ sudo apt install -y libboost-dev
$ sudo apt install -y libgnutls28-dev openssl libtiff5-dev
$ sudo apt install -y qtbase5-dev libqt5core5a libqt5gui5 libqt5widgets5
$ sudo apt install -y meson
$ sudo pip3 install pyyaml ply
$ sudo pip3 install --upgrade meson
$ sudo apt install -y libglib2.0-dev libgstreamer-plugins-base1.0-dev
$ git clone --branch picamera2 https://github.com/raspberrypi/libcamera.git
$ cd libcamera
$ meson build --buildtype=release -Dpipelines=raspberrypi -Dipas=raspberrypi -Dv4l2=true -Dgstreamer=enabled -Dtest=false -Dlc-compliance=disabled -Dcam=disabled -Dqcam=enabled -Ddocumentation=disabled -Dpycamera=enabled
$ ninja -C build
$ sudo ninja -C build install
$ cd ~
$ git clone https://github.com/tomba/kmsxx.git
$ cd kmsxx
$ git submodule update --init
$ sudo apt install -y libfmt-dev libdrm-dev
$ meson build
$ ninja -C build
$ cd ~
$ sudo pip3 install pyopengl
$ sudo apt install python3-pyqt5
$ git clone https://git@github.com:raspberrypi/picamera2.git
$ sudo pip3 install opencv-python==4.4.0.46
$ sudo apt install -y libatlas-base-dev
$ sudo pip3 install numpy --upgrade
$ cd ~
$ git clone https://github.com/RaspberryPiFoundation/python-v4l2.git

要跑起来还需要设置 PYTHONPATH 运行环境。比如你需要将下面的内容添加到 .bashrc 文件中。

export PYTHONPATH=/home/pi/picamera2:/home/pi/libcamera/build/src/py:/home/pi/kmsxx/build/py:/home/pi/python-v4l2

安装 TensorFlow Lite

由于我们将通过 Python 代码进行推理而不是训练，因此我们可以安装轻量级 TensorFlow Lite 运行时库以及我们需要的其他一些东西：

$ sudo apt install build-essentials
$ sudo apt install git
$ sudo apt install libatlas-base-dev
$ sudo apt install python3-pip
$ pip3 install tflite-runtime
$ pip3 install opencv-python==4.4.0.46
$ pip3 install pillow
$ pip3 install numpy

开始使用 TensorFlow Lite

安装完所有东西之后，下面我们构建一个演示程序：寻找图像中的苹果和香蕉。

代码会启用摄像头，并将采集到的图像不断传给 TensorFlow 的图像缓冲区。TensorFlow 随之在图像上进行对象检测。如果检测到任何对象，将用矩形框进行标注。

import tflite_runtime.interpreter as tflite

import sys
import os
import argparse

import cv2
import numpy as np
from PIL import Image
from PIL import ImageFont, ImageDraw

from qt_gl_preview import *
from picamera2 import *

normalSize = (640, 480)
lowresSize = (320, 240)

rectangles = []

def ReadLabelFile(file_path):
  with open(file_path, 'r') as f:
    lines = f.readlines()
  ret = {}
  for line in lines:
    pair = line.strip().split(maxsplit=1)
    ret[int(pair[0])] = pair[1].strip()
  return ret

def DrawRectangles(request):
   stream = request.picam2.stream_map["main"]
   fb = request.request.buffers[stream]
   with fb.mmap(0) as b:
       im = np.array(b, copy=False, dtype=np.uint8).reshape((normalSize[1],normalSize[0], 4))

       for rect in rectangles:
          print(rect)
          rect_start = (int(rect[0]*2) - 5, int(rect[1]*2) - 5)
          rect_end = (int(rect[2]*2) + 5, int(rect[3]*2) + 5)
          cv2.rectangle(im, rect_start, rect_end, (0,255,0,0))
          if len(rect) == 5:
            text = rect[4]
            font = cv2.FONT_HERSHEY_SIMPLEX
            cv2.putText(im, text, (int(rect[0]*2) + 10, int(rect[1]*2) + 10), font, 1, (255,255,255),2,cv2.LINE_AA)
       del im

def InferenceTensorFlow( image, model, output, label=None):
   global rectangles

   if label:
       labels = ReadLabelFile(label)
   else:
       labels = None

   interpreter = tflite.Interpreter(model_path=model, num_threads=4)
   interpreter.allocate_tensors()

   input_details = interpreter.get_input_details()
   output_details = interpreter.get_output_details()
   height = input_details[0]['shape'][1]
   width = input_details[0]['shape'][2]
   floating_model = False
   if input_details[0]['dtype'] == np.float32:
       floating_model = True

   rgb = cv2.cvtColor(image,cv2.COLOR_GRAY2RGB)
   initial_h, initial_w, channels = rgb.shape

   picture = cv2.resize(rgb, (width, height))

   input_data = np.expand_dims(picture, axis=0)
   if floating_model:
      input_data = (np.float32(input_data) - 127.5) / 127.5

   interpreter.set_tensor(input_details[0]['index'], input_data)

   interpreter.invoke()

   detected_boxes = interpreter.get_tensor(output_details[0]['index'])
   detected_classes = interpreter.get_tensor(output_details[1]['index'])
   detected_scores = interpreter.get_tensor(output_details[2]['index'])
   num_boxes = interpreter.get_tensor(output_details[3]['index'])

   rectangles = []
   for i in range(int(num_boxes)):
      top, left, bottom, right = detected_boxes[0][i]
      classId = int(detected_classes[0][i])
      score = detected_scores[0][i]
      if score > 0.5:
          xmin = left * initial_w
          ymin = bottom * initial_h
          xmax = right * initial_w
          ymax = top * initial_h
          box = [xmin, ymin, xmax, ymax]
          rectangles.append(box)
          if labels:
              print(labels[classId], 'score = ', score)
              rectangles[-1].append(labels[classId])
          else:
              print ('score = ', score)

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument('--model', help='Path of the detection model.', required=True)
    parser.add_argument('--label', help='Path of the labels file.')
    parser.add_argument('--output', help='File path of the output image.')
    args = parser.parse_args()

    if ( args.output):
      output_file = args.output
    else:
      output_file = 'out.jpg'

    if ( args.label ):
      label_file = args.label
    else:
      label_file = None

    picam2 = Picamera2()
    preview = QtGlPreview(picam2)
    config = picam2.preview_configuration(main={"size": normalSize},
                                          lores={"size": lowresSize, "format": "YUV420"})
    picam2.configure(config)

    stride = picam2.stream_configuration("lores")["stride"]
    picam2.request_callback = DrawRectangles

    picam2.start()

    while True:
        buffer = picam2.capture_buffer("lores")
        grey = buffer[:stride*lowresSize[1]].reshape((lowresSize[1], stride))
        result = InferenceTensorFlow( grey, args.model, output_file, label_file )

if __name__ == '__main__':
  main()

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