Setting up depth for a cv::Mat

Question

I want to test a function that is looking for specific mat depth && number of channels

It has a test...

if (image.channels() == 1 && image.depth() == 8) ...
else if (image.channels() == 1 && image.depth() == 16)  ...
else if (image.channels() == 1 && image.depth() == 32)  ...
else
{  
  if ((image.channels() != 3) || (image.depth() != 8)) 
  {printf("Expecting rgb24 input image"); return false;}
  ...
}

I prefer to test with a made-up mat, to avoid using local resources:

cv::Mat M(255, 255, CV_8UC3, cv::Scalar(0,0,255));
printf("M: %d %d \n", M.channels(), M.depth());
cv::Mat M1(255, 255, CV_32F, cv::Scalar(0,0,255));
cv::Mat M2(255, 255, CV_32FC3, cv::Scalar(0,0,255));
cv::Mat M2(255, 255, CV_8SC3, cv::Scalar(0,0,255));

I have experimented with all kinds of combinations, but if I print, I get 0 depth.

I have also tried to load a png or a jpg file - with same result (I prefer not to use outside files... but I see no reason why this doesn't work)

cv::Mat M3 = cv::imread( "c:/my_image.png", CV_LOAD_IMAGE_COLOR );
cv::Mat M3 = cv::imread( "c:/my_image.jpg", CV_LOAD_IMAGE_COLOR );

They all seem to have depth = 0 ?

Is there something else I have to do ? I can't see anything in documentation.

Answer 1

When you call depth() on Mat, it returns depth values as defined below instead of number of bits:

#define CV_8U   0
#define CV_8S   1
#define CV_16U  2
#define CV_16S  3
#define CV_32S  4
#define CV_32F  5
#define CV_64F  6

And you can use cv::DataDepth::value to figure out which one is which. For example,

cv::DataDepth<unsigned char>::value == CV_8U;
cv::DataDepth<float>::value == CV_32F;

So you should get 0 on all CV_8UCX matrix, and when you load an image, it is usually loaded as CV_8UC3, so you will get 0 as well. But I am not sure why you got 0 on cv::Mat M(255, 255, CV_32FC3), I tested it on my computer, it returned 5.