If you decide to go ahead and run the code, you'll see one of the quirks I was talking about. If you draw a quick sketch and then go into corner mode (press '2') you'll see that you can select most of the points (bar the last one) by hovering near them. You'll notice you can also select them if you move the mouse along the bottom of the window and move it sideways.

Now, if you click a point and drag it to create a corner, try and select a point above this afterwards. It won't work. Strange, huh? Or maybe not, but I'm very confused.

Thanks!

That's also handy to find a formula that's easier to use, thanks for that.

The else selectedPoint = 0 part was just included to show another peculiarity - that the selectedPoint didn't flick back to the first one if the mouse wasn't over a different point.

I'd agree that's probably a better way of calculating distance, unless the OP specifically wanted a rectangular area; but your code can be optimised a little further:

1. if (sq(xCoords[i]-mouseX)+sq(yCoords[i]-mouseY) < 49) { //where 49 is the radius squared from the points center
2.       selectedPoint = i;
3. }

By all accounts calculating the square root of a number is an 'expensive' process, so best avoided if possible; particularly if it's a calculation that's going to be frequently repeated.  Actually I'd also tend to avoid making unnecessary function calls (so put in the long calculation for the square of deltaX and deltaY); though that might just be a matter of habit.

At the moment the size of the window was the only space being worked on. Extending the code above, I wanted to load a background image and use the program to trace on top of it. This worked great first time. 

Most of the time the background image would be bigger than the window size so I looked into examples of scrolling the image. Because the coordinates of the points drawn are important and relative to the picture behind them, I wanted everything to scroll when scrolling the view (bg image, points drawn, lines drawn, etc), with the size of the window only being for the sake of viewing the area I'm working on (think most drawing apps). I found references to creating a buffer PImage and went about doing this, calling it 'buf'.

I thought I could draw to this PImage rather than the screen by adding 'buf' in front of a drawing function like 'line' etc. but this is giving me a NullPointerException the moment the program tries to draw a line for creating the first point.

Am I doing this wrong and why do the arrays and mouseX and mouseY throw up an error in relation to drawing to 'buf' but not the screen. For testing, the 'buf' PImage is the same size as the screen and hasn't got as far as scrolling yet so all coordinates should be equal? Thanks.

1. buf.line(0,0,200,200);

It gives the same error. I checked the buf for width and height and it threw back the right figures. Is there a step I'm missing?