python - Converting Numpy Array to OpenCV Array

Question

I'm trying to convert a 2D Numpy array, representing a black-and-white image, into a 3-channel OpenCV array (i.e. an RGB image).

Based on code samples and the docs I'm attempting to do this via Python like:

import numpy as np, cv
vis = np.zeros((384, 836), np.uint32)
h,w = vis.shape
vis2 = cv.CreateMat(h, w, cv.CV_32FC3)
cv.CvtColor(vis, vis2, cv.CV_GRAY2BGR)

However, the call to CvtColor() is throwing the following cpp-level Exception:

OpenCV Error: Image step is wrong () in cvSetData, file /build/buildd/opencv-2.1.0/src/cxcore/cxarray.cpp, line 902
terminate called after throwing an instance of 'cv::Exception'
  what():  /build/buildd/opencv-2.1.0/src/cxcore/cxarray.cpp:902: error: (-13)  in function cvSetData

Aborted

What am I doing wrong?

Answer 1

Your code can be fixed as follows:

import numpy as np, cv
vis = np.zeros((384, 836), np.float32)
h,w = vis.shape
vis2 = cv.CreateMat(h, w, cv.CV_32FC3)
vis0 = cv.fromarray(vis)
cv.CvtColor(vis0, vis2, cv.CV_GRAY2BGR)

Short explanation:

1. np.uint32 data type is not supported by OpenCV (it supports uint8, int8, uint16, int16, int32, float32, float64)
2. cv.CvtColor can't handle numpy arrays so both arguments has to be converted to OpenCV type. cv.fromarray do this conversion.
3. Both arguments of cv.CvtColor must have the same depth. So I've changed source type to 32bit float to match the ddestination.

Also I recommend you use newer version of OpenCV python API because it uses numpy arrays as primary data type:

import numpy as np, cv2
vis = np.zeros((384, 836), np.float32)
vis2 = cv2.cvtColor(vis, cv2.COLOR_GRAY2BGR)