Javacv: Crop leaning rectangle

[nlebreton]

Hi,

I know there is straightforward way to crop an horizontal rectangle. I need to crop a set of rectangles (up to 100). These rectangles are in the same image, are not horizontal and not necessarily have the same angle with the horizontal axis. As a consequence, rotating the image 100 times to crop 100 horizontal rectangles is not efficient.

Do you know a simple way to do that? Does it exist a method like "crop ( new Point(x1, y1), new Point(x2, y2), new Point(x3, y3), new Point(x4, y4) )"?

[saudet]

We can apply the reverse transformation directly into an image of the right
size to avoid the cropping.

[nlebreton]

Great! Do you an example or the key methods to be used?

[saudet]

Just warpAffine() will do that:
https://docs.opencv.org/3.4.1/d4/d61/tutorial_warp_affine.html

[saudet]

Though if you have 4 points, you probably have a perspective transformation instead, and you'd use getPerspectiveTransform() instead of getAffineTransform() and warpPerspective() instead of warpAffine(). If you have any issues translating that to Java, let me know!

[nlebreton]

Hi,

Yesterday, I followed your recommandations and started with warpAffine(srcMat,outMat,rotMat,size) and the following data:

<center x="753" y="203" />
<size w="43" h="36" />
<angle d="-88" />

The image rotates around the center of the leaning rectangle but I have not yet understood how to use the "size" argument of the method warpAffine(...) to "crop" the resulting horizontal rectangle based on its center, its width and its height.

[nlebreton]

I can also have the following data for each leaning rectangle:

<point x="736" y="182" />
<point x="772" y="183" />
<point x="769" y="223" />
<point x="735" y="225" />

Based on what I already did and the data I have, which approach seems to you finally to be the best?

[saudet]

Like I said, if you have 4 points, I'd just use a perspective transform.

[nlebreton]

Ok. Thanks. I will work in this direction and keep you informed.

[nlebreton]

I have tried to use both getPerspectiveTransform() and warpPerspective() based on few examples I found. The resulting image has the good size but is completly dark.

https://gist.github.com/nlebreton/21ce05a0078fa2b0f86f5d1b213b8ba7

[saudet]

You'll need to call it that way:
getPerspectiveTransform(srcPts.position(0), dstPts.position(0))

[nlebreton]

I have already tried that but the result is the same: an image with the good size but completly dark.

For information,

• in srcPts and dstPts, the points are in the following order.

        1st-------2nd
         |                |
         |                |
         |                |
        3rd-------4th
- I have check that the coordinates of the corners are correct by drawing lines on the source image

[saudet]

I'm pretty sure that works. You're just forgetting the calls to postition(0).

[nlebreton]

Unfortunately, there still is the same issue after adding position(0).

[nlebreton]

When I increase de size parameter of warpPerspective(), I can see a part of the rotated image at the bottom right corner. It explains the black rectangle.

[nlebreton]

Our last exchange helped me to understand that the issue comes from bad coordinates for the destination:

BAD:
dstPts.position(0).x(x1).y(y1);
dstPts.position(1).x(x1+srcWidth).y(y1);
dstPts.position(2).x(x1).y(y1+srcLength);
dstPts.position(3).x(x1+srcWidth).y(y1+srcLength);

GOOD:

    dstPts.position(0).x(0).y(0);
    dstPts.position(1).x(srcWidth).y(0);
    dstPts.position(2).x(0).y(srcHeight);
    dstPts.position(3).x(srcWidth).y(srcHeight);

Thx for your help.