只能在多边形内部进行平移（Pygame）

Question

Is it possible to have red circles to only appear inside a polygon. For a rectangle, you can determine the height and width.

I'm planning to make like a virus simulator, and that there are red circles that only appear only inside the countries. But the countries aren't rectangles, but polygons/images.

I was wondering if it was possible to have only circles to be blit inside a polygon or image. Thanks

Answer 1

Let's say you have a map as an image, and you want to determine if a random point is on land or not. What you need for that is an image redrawn to have only black and white pixels. You can then use Pygame's Surface.get_at() command to see what color the pixel there is, and make a decision based on that.

Alternately, let's say you have a map as a bunch of polygons, and you want to determine if a random point is on land or not. The logic there is called ray tracing, and is explained better over here: How can I determine whether a 2D Point is within a Polygon?

Answer 2

Use matplotlib for that. A very simple example (Give values to the vertices of the square):

  from matplotlib.path import Path

  verts = [
             left_bottom,
             left_top,
             right_top,
             right_bottom,
             (0, 0),
            ]

    codes = [
             Path.MOVETO,
             Path.LINETO,
             Path.LINETO,
             Path.LINETO,
             Path.CLOSEPOLY,
             ]

    path = Path(verts, codes)

   if path.contains_point((x, y)):
       print("True")