ubuntu22.04@laptop OpenCV Get Started: 004_cropping_image

1. 源由

使用OpenCV进行裁剪相当于对NumPy数组做类似切片的动作。

每个图像都存储在2D阵列中，区域的高度和宽度（以像素为单位）就是一张对应的图片。

2. Cropping应用Demo

004_cropping_image是OpenCV调整图像比例的示例程序。

确认OpenCV安装路径：

$ find /home/daniel/ -name "OpenCVConfig.cmake"
/home/daniel/OpenCV/installation/opencv-4.9.0/lib/cmake/opencv4/
/home/daniel/OpenCV/opencv/build/OpenCVConfig.cmake
/home/daniel/OpenCV/opencv/build/unix-install/OpenCVConfig.cmake


$ export OpenCV_DIR=/home/daniel/OpenCV/installation/opencv-4.9.0/lib/cmake/opencv4/

3 image_crop

3.1 C++应用Demo

C++应用Demo工程结构：

004_cropping_image/CPP$ tree .
.
├── CMakeLists.txt
├── image_crop.cpp
├── Patching
│   ├── saved_patches
│   ├── CMakeLists.txt
│   ├── patching.cpp
│   └── test_cropped.jpg
└── test.jpg

2 directories, 6 files

C++应用Demo工程编译执行：

$ mkdir build
$ cd build
$ cmake ..
$ cmake --build . --config Release
$ cd ..
$ ./build/image_crop

$ cd Patching
$ mkdir build
$ cd build
$ cmake ..
$ cmake --build . --config Release
$ cd ..
$ ./build/image_patch

3.2 Python应用Demo

Python应用Demo工程结构：

004_cropping_image/Python$ tree .
.
├── saved_patches
├── image_crop.py
├── patching.py
├── requirements.txt
├── test_cropped.jpg
└── test.jpg

1 directory, 5 files

Python应用Demo工程执行：

$ workoncv-4.9.0
$ python image_crop.py
$ python patching.py

3.3 重点过程分析

3.3.1 裁剪图像

• img(Range(start_row, end_row), Range(start_col, end_col))
• cropped = img[start_row:end_row, start_col:end_col]

第一个参数：行 (高度, 自上而下递增)
第二个参数：列 (宽度, 自左往右递增)

C++:

// Crop image
// First Range(20,500) is for y coordinates and the second is for x respectively
Mat cropped_image = img(Range(80,280), Range(150,330));

Python:

# Cropping an image
cropped_image = img[80:280, 150:330]

3.3.2 描绘矩形

• rectangle(img, pt1, pt2, color, thickness, lineType, shift)

C++:

rectangle(img, Point(x,y), Point(x1,y1), Scalar(0,255,0), 1);  

Python:

cv2.rectangle(img, (x, y), (x1, y1), (0, 255, 0), 1)

4. 总结

通过对NumPy二维数组操作，对图像进行剪裁，使用rectangle函数进行矩形框的绘制。

5. 参考资料

【1】ubuntu22.04@laptop OpenCV Get Started
【2】ubuntu22.04@laptop OpenCV安装
【3】ubuntu22.04@laptop OpenCV定制化安装