断言失败 (scn == 3 || scn == 4) in void cv::cvtColor(cv::InputArray, cv::OutputArray, int, int), file /../Linux/./../src/ color.cpp，第 8000 行

Question

I am trying to learn openCv with native code and I am taking reference from here .
I successfully build the project using ndk-build.
Now I want to make change in scan.cpp file which is responsible for getting point for image, crop it, scan it and set color.
I want to give different argument for line 321 in file which is
cvtColor(mbgra, dst, CV_BGR2GRAY);
Can I give any other argument for CV_BGR2GRAY .
If yes HOW? If no WHY?

Please guide me and tell me if I am missing anything. Thank you.

Answer 1

Yes, you can give any argument you want. Would you get a reasonable output? It depends. CV_BGR2GRAY expects BGR (3 channel) input and will output gray (1 channel).

If you input is 3 channel BGR (and even if it isn't really BGR, opencv won't care) you can use any 3 channel conversions, for example CV_BGR2HSV which would result in 3 channel HSV output.

If your input is 1 channel - than you won't be able to use BGR 2 GRAY obviously.

Mat bgraImage = imread("BGRA_IMAGE.png", -1);  // 4 channel input image
Mat grayImage = imread("GRAY_IMAGE.png", CV_LOAD_IMAGE_GRAYSCALE);  // 1 channel input image
Mat result;

cvtColor(bgraImage, result, CV_BGRA2GRAY);    // CORRECT, input 4 channel, output will be 1 channel
cvtColor(bgraImage, result, CV_BGR2GRAY);    // ALSO CORRECT
cvtColor(grayImage, result, CV_BGR2GRAY);    // INCORRECT & will crash, input is 1 channel, expecting 3 or 4
cvtColor(grayImage, result, CV_GRAY2BGR);    // CORRECT, input is 1 channel, output is 3 channel

You can see all possible color conversions here and read more about them here