I have draw several polygons in a canvas. I am now interested in how to find the coordinates (vertices) of the selected polygon. I can find the selected polygon (region) using mouse coordinates, but I need to redraw it if it is selected, which is why I need its coordinates to do that.
A similar answer that I found so far is this one https://help.geogebra.org/topic/-solved-how-do-i-get-a-list-of-coordinates-from-a-polygon- using GeoGebra, but I am using HTML5 and JavaScript!
Once you draw something on a canvas, you can not get the coordinates of the object you drew unless you have some image processing algorithm.
Instead, you need need to keep the polygon object you drew on the canvas in memory and you need to define it's boundaries on the canvas.
This way, when the user clicks on the canvas, you simply get the mouse coordinates on the canvas and you calculate which polygon was clicked by checking where you drew your polygon and checking if the mouse clicks enters the boundaries of this polygon.
Sounds like you are trying to solve a couple different problems:
1) Figure out which polygon the user clicked. As was mentioned, if you are keeping an array of your polygons in memory, and can implement an isPointInPath method, then you can loop through your in-memory polygons and check if any of them were clicked. Don't forget that to get an accurate check, you may need to take your given "mouse x" and "mouse y" coordinates (from a click event, for example) and subtract the canvas's page X and Y coordinate.
(See the SO answer How do I get the coordinates of a mouse click on a canvas element? for more detail.)
2) Once you've determined the user has clicked polygon P, you want to redraw it. You should be able to redraw it no problem, by simply redrawing the same polygon using the same coordinates as before (for example, you could change your strokeStyle or fillStyle to a different color to represent the selection).
If you want to make the polygon larger and in the same position, that's a different question - basically, you need to translate the canvas to the center of the polygon, then scale the canvas, then draw your polygon. If this is new to you, there's a tutorial available at http://codetheory.in/canvas-rotating-and-scaling-images-around-a-particular-point/ (uses images instead of polygons, but same concept).
Note that if your polygon is complex, even determining the right point to scale around may be difficult. The question "what is the center of a complex polygon" has several different answers; a simple answer is to average all of the polygon's point's X and Y values (Center X = avg(X), Center Y = avg(Y)). If that doesn't look right, there are more complicated approaches available in another SO question, What is the fastest way to find the "visual" center of an irregularly shaped polygon? .
Hope this points you in the right direction!
