How to retain the size of image when we rotate image?

I was using the imrotate to rotate an image B = imrotate( image,20) and the image was rotated. After certain manipulations, I wanted to make it back to its original image, so I used C = imrotate(B,-20). But I find the size of image C is changed, which is bigger than original image. And the original image lies in the center of image C .Does anyone know how to ensure the size of the image exactly same as the original image after the twice rotation?

[binbin cui]

In fact, I wanna get the exact original image after rotation.

Answer 1

[Image Analyst]

When it rotates, it enlarges the canvas. You'll have to store the original size so that you can crop it after a round trip rotation. Like if it was 1000 rows, then after rotation it was 1200 rows tall, then when you rotate it back you need to crop off 100 rows from the top and bottom. Same for the right and left sides. I'm sure you can handle the math. Let me know if you can't.

[binbin cui]

So you mean the real size of original image will not change; it is just expanded by certain pixels. And what we need is only to crop the center region based on the original size and image size after rotation? In fact we can get the difference by simple subtraction ?

[Image Analyst]

Yes. And no. It does get expanded. But you can't subtract different sized matrices. You'll have to crop it back to the original size.

[binbin cui]

I do crop it. But the result is totally out of expectation. I find most of the original image is removed after the crop.

[binbin cui]

I get it, thank you very much. I should directly retrieve the target matrix from the matrix C but not to crop it.