问题

What is the standard way to access and modify individual elements of a Mat in OpenCV4Android? Also, what is the format of the data for BGR (which is the default, I think) and grayscale?

edit: Let's make this more specific. mat.get(row, col) returns a double array. What is in this array?

回答1:

What is the standard way to access and modify individual elements of a Mat in OpenCV4Android?

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). From this tutorial:

in the above image you can see that the mirror of the car is nothing

more than a matrix containing all the intensity values of the pixel

points.

So how would you go about iterating through a matrix? How about this:

for (int row=0; row

for (int col=0; col

//...do what you want..

//e.g. get the value of the 3rd element of 2nd row

//by mat.get(2,3);

}

}

What is the standard way to access and modify individual elements of a Mat in OpenCV4Android?

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();

Also, what is the format of the data for BGR (which is the default, I think) and grayscale? edit: Let's make this more specific. mat.get(row, col) returns a double array. What is in this array?

You can read about BGR color format and grayscale from the web. e.g. BGR and Grayscale.

In short, BGR color format has 3 channels: Blue, Green and Red. So the double array returned by mat.get(row, col), when mat is a BGR image, is an array of size 3, and each of its elements contains the values of each of the Blue, green and red channels respectively.

Likewise, Grayscale format is a 1 channel color format, so the double returned will have a size of 1.

回答2:

If you just want to access some pixels do it by using double[] get(int row, int col) and writing using put(int row, int col, double... data). If you are thinking in accessing the whole image or iterate the data of the image in a loop the best thing you should do is copy the Mat data into a Java primitive data type. When you're done with the data operations just copy the data back into a Mat structure.

Images use CV_8U, if you have a grayscale image it will use CV_8UC1 if you have a RGB image it will use a Mat of CV_8UC3 (3 channels of CV_8U). CV_8U is the equivalent of byte in java. :)

I can give you and example of a method I use in Java (Android platform) to binarize a grayscale image:

private Mat featuresVectorBinarization(Mat fv){

int size = (int) fv.total() * fv.channels();

double[] buff = new double[size];

fv.get(0, 0, buff);

for(int i = 0; i < size; i++)

{

buff[i] = (buff[i] >= 0) ? 1 : 0;

}

Mat bv = new Mat(fv.size(), CvType.CV_8U);

bv.put(0, 0, buff);

return bv;

}

Hope that helps.

回答3:

What I could understand from learning about OpenCV Mat object is that Mat is object that can represent any image of W x H pixels.

Now lets say you want to access center pixel of your image then

X = W/2

Y = H/2

Then you can access pixel data as follows

double[] data = matObject.get(x,y);

Now what does data represent and what is size of data array.That depends on image type.

if image is grayscale then data.length = 1 , since there is one channel only , and data[0] represents color value of that pixel ie. 0(black) - 255(white)

if image is color image then data.length = 4(rgba) , since there are four channels , and data[0-n] represent color value of that pixel

来源：https://stackoverflow.com/questions/12394364/opencv-for-android-access-elements-of-mat