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How to get and modify the pixel of Mat in Java?

asked 2012-06-08 08:47:00 -0500

Kirill Kornyakov gravatar image

I want to read and modify some pixels in my matrix. How can I do that in Java? Is there any equivalent for the C++ Mat::at method?

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answered 2012-06-08 14:02:54 -0500

Andrey Pavlenko gravatar image

updated 2012-06-08 14:03:38 -0500

In Java you can use Mat::get() and Mat::put() methods. One pixel can be read by

double[] Mat::get(int row, int col)

One or few pixels can be written by

Mat::put(int row, int col, double... data)

But if you're going to work with many pixels, it's better to get all the Mat data at once to Java primitive array, work with it in Java, and then put it back to Mat with a single JNI call. The corresponding Java primitive array type depends on the Mat type:

CV_8U and CV_8S -> byte[],
CV_16U and CV_16S -> short[],
CV_32S -> int[],
CV_32F -> float[],
CV_64F-> double[].

So the code will look like following:

Mat m = ...  // assuming it's of CV_8U type
byte buff[] = new byte[ * m.channels()];
m.get(0, 0, buff);
// working with buff
// ...
m.put(0, 0, buff);
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Thanks for the answer. However I don't quite understand the use of the byte array. I tried, based on the above suggestion, using m.get(0, 0, buff), and m.get(0,0)[0] (for a grayscale image). The latter resulted in the expected value; the former however (using the byte array) gave a large value (exceeding 255) that I don't understand. My image is of type CV_8U.

user1446598 gravatar imageuser1446598 ( 2013-03-10 12:51:16 -0500 )edit

Not working for me too....

jishnu gravatar imagejishnu ( 2015-02-10 05:04:23 -0500 )edit

Role of an array in Mat Pixel access/modify:

Consideran Mat 2*2 with single channel of type byte.Represent it with by m

get the image data m.rows->2; m.cols->2; m.typ->8uc1;

suppose elements in m are,

5 10 15 20

m.get(0,0)[0] -->5;;//returns only specified location element.

Means we have to call m.get() 4 times to get all elements in m.

Use af array in accessing;

Suppose byte [] b=new byte[2];//size is 2 here

m.get(0,b)--> {5,10};//retuns the row of length specified by byte b

where 0--> id for row of mat ..To get next row we should use 1

now, b[0]-->5; b[1]-->10;

Means we have to call m.get() 2 times to get all elements in m.

"m.get() 2 times" is speeder than "m.get() 4 times ...(more)

Venky009 gravatar imageVenky009 ( 2015-11-18 00:36:42 -0500 )edit

Is there a way to just actually use the underlying native array instead of making a copy to a new byte array?

nic gravatar imagenic ( 2016-09-19 13:48:00 -0500 )edit
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Asked: 2012-06-08 08:47:00 -0500

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Last updated: Jun 08 '12