请不要尝试旧的cv模块，使用cv2：

import cv2

cv2.rectangle(img, (x1, y1), (x2, y2), (255,0,0), 2)

x1,y1 ------
|          |
|          |
|          |
--------x2,y2

[edit]在下面附加后续问题：

cv2.imwrite("my.png",img)

cv2.imshow("lalala", img)
k = cv2.waitKey(0) # 0==wait forever

正如其他答案所说，您需要的函数是cv2.rectangle()，但请记住，如果边界框顶点在元组中，则它们的坐标需要是整数，并且它们需要按照(left, top)和(right, bottom)的顺序。或者，等价地，(xmin, ymin)和(xmax, ymax)。

可以使用cv2.rectangle()：

cv2.rectangle(img, pt1, pt2, color, thickness, lineType, shift)

Draws a simple, thick, or filled up-right rectangle.

The function rectangle draws a rectangle outline or a filled rectangle
whose two opposite corners are pt1 and pt2.

Parameters
    img   Image.
    pt1   Vertex of the rectangle.
    pt2    Vertex of the rectangle opposite to pt1 .
    color Rectangle color or brightness (grayscale image).
    thickness  Thickness of lines that make up the rectangle. Negative values,
    like CV_FILLED , mean that the function has to draw a filled rectangle.
    lineType  Type of the line. See the line description.
    shift   Number of fractional bits in the point coordinates.

我有一个PIL图像对象，我想在这个图像上绘制矩形。* * 我想使用opencv2绘制矩形，然后转换回PIL Image对象**。我是这样做的：

# im is a PIL Image object
im_arr = np.asarray(im)
# convert rgb array to opencv's bgr format
im_arr_bgr = cv2.cvtColor(im_arr, cv2.COLOR_RGB2BGR)
# pts1 and pts2 are the upper left and bottom right coordinates of the rectangle
cv2.rectangle(im_arr_bgr, pts1, pts2,
              color=(0, 255, 0), thickness=3)
im_arr = cv2.cvtColor(im_arr_bgr, cv2.COLOR_BGR2RGB)
# convert back to Image object
im = Image.fromarray(im_arr)

在图像上绘制矩形的代码是：

cv2.rectangle(image, pt1, pt2, color, thickness)

其中pt1是起点-->（x，y），pt2是终点-->（x+w，y+h）。