問題描述

我正在嘗試使用 OpenCV 找到棋盤的角.

I'm trying to find the corners of a chessboard using OpenCV.

我使用的圖像包含兩個棋盤，但我只對其中一個的子區域感興趣.下圖為原圖.

The image I'm using contains two chessboards, but I'm interested only in a sub-region of one of those. The following image shows the original image.

使用 GIMP，然后我選擇了感興趣的區域，并將所有其他像素設置為默認值.

Using GIMP, I've then selected the area of interest and I've set all the other pixel to a default value.

我實際上并沒有裁剪圖像，因為我已經使用此圖像尺寸校準了相機并且我不想更改它.該操作應該等同于更改圖像矩陣中的值，但我更喜歡使用 GIMP 進行操作.這是一個一次性的實驗，使用圖形工具而不是使用代碼執行該操作更快.

I haven't actually cropped the image because I've already calibrated the camera using this image size and I didn't want to change it. The operation should be equivalent to change the values in the image matrix, but I preferred to do it with GIMP. It is a one time experiment and it is faster to do that operation with a graphic tool instead of using the code.

生成的圖像包含一個 24x5 角的棋盤，但函數 findChessboardCorners 找不到任何東西.

The resulting image contains a chessboard with 24x5 corners, but the function findChessboardCorners is not able to find anything.

這是我正在使用的 Python 代碼:

Here is the Python code I'm using:

>>> img = cv2.imread('C:\Path\To\C4-Cropped.png', 0)
>>> cv2.findChessboardCorners(img, (24, 5))
(False, None)
>>> cv2.findChessboardCorners(img, (5, 24))
(False, None)

我也試過設置自適應閾值，但還是不行

I also tried to set the adaptive threshold, but it is still not working

>>> cv2.findChessboardCorners(img, (24, 5), flags=cv2.cv.CV_CALIB_CB_ADAPTIVE_THRESH)
(False, None)

這看起來很奇怪.我過去曾多次使用過 OpenCV 的這個功能，它總是有效，即使圖像看起來比這個復雜得多.復雜.該區域的照明不是均勻的，但函數應該足夠強大以處理它.

That seems really strange. I used this function of OpenCV many times in the past and it always worked, even with images that looked much more complicated than this one. The illumination of the area is not homogeneous but the function should be robust enough to handle that.

使用 GIMP 臨時創建的 人工 圖像有什么問題嗎?我怎樣才能找到角落?

Is there any problem with the artificial image created ad hoc with GIMP? How can I find the corners?

任何建議將不勝感激.

推薦答案

需要進行兩項更改才能使該圖像為非常挑剔的 cv2.findChessboardCorners 函數所接受.首先，棋盤需要白色背景.我只是通過調整圖像的對比度來獲得這個.其次，我還必須將連接棋盤頂部和底部黑色方塊的黑色水平線涂成白色.這是生成的圖像:

There are two changes needed to make that image acceptable to the very finicky cv2.findChessboardCorners function. First, the chess board needs a white background. I obtained this simply by adjusting the contrast on your image. Second, I also had to white-out the dark horizontal line that connects the black squares at the top and bottom of your chess board. This is the resulting image:

通過這些增強功能，cv2.findChessboardCorners 可以成功分析圖像.結果是:

With these enhancements, cv2.findChessboardCorners can successfully analyze the image. The results were:

camera matrix =
    [[  1.67e+04   0.00e+00   1.02e+03]
    [  0.00e+00   1.70e+04   5.45e+02]
    [  0.00e+00   0.00e+00   1.00e+00]]

distortion coefficients = [ -4.28e+00   1.38e+03  -8.59e-03  -1.49e-02   6.93e+00]

(圖像增強方式的微小變化可以極大地改變上述結果.只有一張小棋盤的圖像，這些結果是不可信的.)

(Small changes to how the image is enhanced can change the above results greatly. With only one image of a small chess board, these results are not to be trusted.)

如您所述，cv2.findChessboardCorners 接受旨在幫助識別棋盤的標志(自適應閾值、filter_quads 和規范化).我嘗試了所有方法，但它們在這里沒有任何區別.

As you noted, cv2.findChessboardCorners accepts flags (adaptive threshold, filter_quads, and normalization) that are intended to help with chess board recognition. I tried all but they made no difference here.

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