Get portion of an image using rotated Shape

Question

From the above image if I want a portion behind the RED Rectangle I can easily get it, but the issue I cannot get the portion behind the Yellow Rectangle because it is rotated.

So how can I get a portion of an image from a rotated shape on it?

For example my goal is to get a portion of an Image where the rectangle is located on the image. if someone rotates this rectangle by an x degree [in whatever direction] then it is getting difficult to extract the exact portion of an image after applying rotation.

Any suggestions?

Answer 1

Here a more lengthy description of a possible approach. I do not know the Java2D drawing API very well but if I remember correctly it has the capabilities to do what is required.

First you have to figure out the translation and rotation of the subregion you want compared to an equally size rectangle located straight in the upper left corner in the image. Then invert this transformation.

Make a graphics context which is backed by a bitmap in memory. This one should have the size of the subimage you want. Setup the inverse transformation you calculated earlier on the context and draw your image at position 0,0. As Java2D will take the transformation into account you should now get the sub image you want in the memory bitmap.

Answer 2

Mihir, I think you might be getting distracted by the rotation/AffineTransform aspects of this challenge and it is leading you down the wrong road. Also keep in mind that I don't totally know what you mean by "get" here -- do you want to save out the highlighted region to an image? Do you want to render it as a watermark on another image? etc... I'll just try and answer in the general case to get you down the right track.

What you want is the content from the image defined by the polygon in yellow in your image above; ignoring the fact that it looks like a rotated rectangle.

It is late and I am missing a step in here, but I think this will get you 90% of the way there and clarify the last piece (Graphics2D.setClip) that you need.

• Create a java.awt.Polygon that defines the region around the area you want.
• Use getBounds() or getBounds2D() to get the width/height of the bounding box required to hold this Polygon when rendered out into a rectangle. (e.g. boundingBox)
• Create a new BufferedImage with these width/height values.
• Get the Graphics2D from the new BufferedImage (e.g. newG2)
• newG2.drawImage(originalImage, boundingBox.x, boundingBox.y, boundingBox.width, boundingBox.height)

NOTE This is where my memory is failing me; at some point you need to set the clip on newG2 (newG2.setClip(someShape)) so when the bounding box is rendered into it, you don't get the full bounding box of graphics rendered in, but instead some subset as defined by the yellow outline.

One easy way to do this is to create two Polygon's:

1. poly1 = a java.awt.Polygon that defines the yellow selection in the ORIGINAL image.
2. poly2 = a java.awt.Polygon that defines the exact same shape of Polygon, but shifted to a 0,0 origin point.

poly1 is used to get the bounding box to copy out the full bounding box that encompasses the content selected in yellow (and extra content around it)

poly2 is used to set the clip on the target Graphics2D (newG2) so when the bounding box is rendered into it, we clip back out everything outside of that Yellow shape so we just get the content in Yellow. You'll likely want to use an ARGB image type and set the background of the target image as transparent otherwise you'll get a black fill color.

I think this is the right direction for the clips; I was up to my eyeballs in Java2D for years and years but have been out of it for a while and forget if this will give you exactly what you want or not; you might need to tweak it around, but these are all the tools you need.