Android, Java: Stretch Bitmap or Path to arbitrary polygon

Question

As the title implies I need an algorithm, code or a library that would help me to stretch a Bitmap (or a Path in Android) to an arbitrary polygon. Polygon is given with a list of x, y coordinates. Actually I need to transform/stretch a Path object in Android which is also given by x, y coordinates. I mentioned Bitmap because it is more likely that someone had similar problem and I assume that both will be transformed my a Matrix I tried to use Matrix.setPolyToPoly(...) but it doesn't seem to help since it is transforming to square like area (only 4 points) not to an arbitrary polygon.

For better illustration what I need please check out image bellow. It is not exact transformation but something close. Note that whole image is stretched to star shaped polygon, it is not a mask and not a trim, just pixel transition.

Answer 1

I saw your question a few days ago, then yesterday I ran across this:

Canvas#drawBitmapMesh | Android Developers

It's kind of hard to grasp, but the way I understand it you start with an imaginary elastic grid over your bitmap. The way you want to warp the bitmap can be expressed by moving the x,y points of the grid to alternate locations.

Here's an article with a diagram and here's an article with some sample code.

Obviously, the hard part now is to take your frame polygon and use it to generate the warped vertices in the mesh. That may take some fancy mathematics. But I thought this would be a step in the right direction.

This is what I was envisioning: I'm looking at the star polygon and I'm picturing a circle as the starting point (not the square). The star could be seen as taking the circle and stretching points on it toward and away from the center. Whichever way it was stretched would create some vectors, from zero at the center to strongest at the stretch point.

For a Path, you could then just apply the vectors to the points in the path, but the lines would also need to be bent so this would be some pretty convoluted math with Bezier curves (convoluted at least for me, I'm not any sort of mathematician).

But if you drew the Path onto a Bitmap you might be in a better position. You could just alter the mesh vertices using the different vectors then use Canvas.drawBitmapMesh() to render the final result.