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Mat: Used to store values of an image... A Mat is the thing we call a "matrix" in Mathematics - a rectangular array of quantities set out by rows and columns. Those "quantities" represent pixels in the case of an image Mat, (e.g. every element of a matrix can be the color of each pixel in an image Mat).
So how would you go about iterating through a matrix? How about this:
for(int row=0;row<mat.rows();row++){
for(int col=0;col<mat.cols();col++){
//...do what you want..
//e.g. get the value of the 3rd element of 2nd row
//by mat.get(2,3);
}
}You get the value of an element of Mat by using its function get(x)(y), where x is the first coordinate (row number) and y is the second coordinate (column number) of the element. For example to get the 4th element of the 7th row of a BGR image Mat named bgrImageMat, use the get method of Mat to get an array of type double, which will have a size of 3, each array element representing each of the Blue, Green, and Red channels of the BGR image format.
double [] bgrColor = bgrImageMat.get();What is the purpose of CvType.CV_8UC4:
OpenCV types can be read the following:
- CV_
- 8U: Unsigned int 8-bit
- C4: Four channels.
- Thus mRgba = new Mat(height, width, CvType.CV_8UC4); creates a Matrix with four color channels and values in the range 0 to 255.
The reason there are 4 channels (like 4 slices that layer together to make an image of different colours) is to make up the image. The first 3 in this are R, G, B, and the last is Alpha, which is a value between 0 and 1 representing transparency. When these slices combine you get the correct combination of colours.
- CV_8U - 8-bit unsigned integers ( 0..255 )
- CV_8S - 8-bit signed integers ( -128..127 )
- CV_16U - 16-bit unsigned integers ( 0..65535 )
- CV_16S - 16-bit signed integers ( -32768..32767 )
- CV_32S - 32-bit signed integers ( -2147483648..2147483647 )
- CV_32F - 32-bit floating-point numbers ( -FLT_MAX..FLT_MAX, INF, NAN )
- CV_64F - 64-bit floating-point numbers ( -DBL_MAX..DBL_MAX, INF, NAN )
- 8-bit unsigned integer (uchar)
- 8-bit signed integer (schar)
- 16-bit unsigned integer (ushort)
- 16-bit signed integer (short)
- 32-bit signed integer (int)
- 32-bit floating-point number (float)
- 64-bit floating-point number (double)
- enum { CV_8U=0, CV_8S=1, CV_16U=2, CV_16S=3, CV_32S=4, CV_32F=5, CV_64F=6 };