Null Pointer exception when using resize Open Cv Java

Question

I was writing a program to resize an image while preserving the aspect ratio. Basically, I'm trying to get the source image into a 20x20 box with the aspect ratio preserved.

float ra= (float)20/image.width();
Size dsize = new Size((int)(image.height()*ra),20);
// System.out.println(image.height() + " "+image.width());
//System.out.println((int)(image.height()*ra)+" " + 20);
Imgproc.resize(image,output, dsize,ra,ra, Imgproc.INTER_LINEAR);

The input, image is a Mat. image.height()=10, image.width()=28. The new size would be 7,20. However when i run the code I get a null pointer exception at Imgproc.resize(). I am unable to find the reason.
I get the same error even if I run

Imgproc.resize(image,output,dsize);

I'm sure dsize is not null. image and output are also not null as it works if i put dsize=new Size(20,20); This will not preserve aspect ratio. — user3531996, Jun 10 '14 at 07:46

score 0 · Answer 1 · answered Jun 10 '14 at 06:53

0

use either dsize or a scale factor, not both.
you probably forgot:

output = new Mat(); // you can pass empty Mat's, but not null

answered Jun 10 '14 at 06:53

berak

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I had put Mat output = new Mat(); It works if i put dsize= new Size(20,20). But that won't serve the purpose. – user3531996 Jun 10 '14 at 06:56
I tried Imgproc.resize(image,output,size); Same issue.. – user3531996 Jun 10 '14 at 06:57
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^^ you seem to contradict yourself – berak Jun 10 '14 at 06:59
Sorry, where? I meant that I got the same error with Imgproc.resize(image,output,size); I simple tried declaring dsize as Size size = new Size(20,20); It worked, but this does preserve the aspect ratio. I apologise if I wasn't clear! – user3531996 Jun 10 '14 at 07:04
Size( 20, 20*img.height/img.width ); – berak Jun 10 '14 at 07:11
Unfortunately, thats what i did. Gave a nul pointer exception :( – user3531996 Jun 10 '14 at 07:36
Are there restrictions on the dimensions that can be passed to resize? – user3531996 Jun 10 '14 at 07:37

score 0 · Answer 2 · answered Jun 11 '14 at 07:53

0

Turns out I was interchanging the rows and columns while strong the image into Mat. I was using BufferedImage and mixed up the height and width. It works now.. Thanks everyone!!

answered Jun 11 '14 at 07:53

user3531996

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Null Pointer exception when using resize Open Cv Java

2 Answers2