# Area of Movement in a game

Discussion in 'Public Game Developers Forum' started by Redoctober11, Apr 20, 2009.

1. ### Redoctober11 New Member

Apr 4, 2009
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#1
Hello,

I've been trying to figure this out for days now, and I'm starting to get pretty frustrated. Right now, I have a game that moves a clicked button (a game piece) to anywhere you touch on the iPhone screen. I set it up so it only allows you to move withen a specific square area around the button. This works great. But now I want to change that square into a circle, and I can't for the life of me figure out how it's done.

I'm thinking that I need to find all the x and Y points withen the area of the circle, throw them into an array, and check them against the x and y points of where the user is touching. But I don't know how to set up an equation that figures out every point in a circle. I also don't think this would be very efficient, seeing as how the computer would have to check tens of thousands of points every time the user touches.

I've just hit a wall and I can't figure out where else to go from here. Any help at all would be appreciated.

Jan 9, 2009
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#2
3. ### Redoctober11 New Member

Apr 4, 2009
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#3
Thanks!

That's exactly what I've been looking for! I guess I was too specific in my searches and just looking for obj-c code and general math equations. Thanks again!

4. ### Little White Bear Studios Well-Known Member Patreon Silver

Aug 27, 2008
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#4
You could also go a much much simplier route and create a circular path, and check if the point is in the path.

Code:
```		CGMutablePathRef path = CGPathCreateMutable();

if ( path )
{
CGPathAddArc(path, NULL, center.x, center.y, width, 0, 2*PI, 1);
CGPathCloseSubpath(path);

if ( CGPathContainsPoint(path, NULL, location, TRUE) )
found = YES;

CGPathRelease(path);
}```

5. ### Redoctober11 New Member

Apr 4, 2009
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#5
Thanks for the tip, but I ended using this equation in an if else statement: (x-center_x)^2 + (y - center_y)^2 < radius^2.

I do have one other question. Now I want to draw a circle around a button. I was going to use the drawRect: method, but the I relized that this method can only be used in a view, not a view controller. So I made a new view, and I can draw a circle using a radius and x and y points that I manually type in, but I want the circle to grab the x, y and radius values from the view controller. The problem I'm having is getting the values from the view controller to the view.

6. ### writingsama Well-Known Member

Dec 4, 2008
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#6
Last edited: Apr 25, 2009
wow...seriously? to check if it's inside a circle, just use the pythagorean theorem...

aa + bb = cc

Take the coordinates of the finger press, subtract the coordinates of the center of the circle.

User presses [20, 10]
Circle is at [5, 8] of radius 7

[20, 10] - [5, 8] = [15, 2]

15*15 + 2*2 = cc
225 + 4 = cc
sqrt(229) = 15.132....

EDIT: you're right, you don't need to do the sqrt(), compared against radius squared is "faster"

The user pressed at a distance of 15.132 from the center of the circle, whose radius is 7, so 15.132 > 7, so it's outside. Of course, make sure you abs() the value before the sqrt()...

As for drawing a circle, you tesselate it into line segments.

double i;
double last_x = 0;
for (i = 0; i < tesselate_amount; i+=1.0) {
new_x = last_x + (sin(pi*(i/tesselate_amount)) * circumference);
new_y = last_y + (cos(pi*(i/tesselate_amount)) * circumference);
draw_line(last_x,last_y, new_x, new_y);
last_x = new_x;
last_y = new_y;
}

You can determine the amount to tesselate by by getting the circumference; tesselating by this number (rounded to an integer) should give acceptable results. thats Pi * (radius * 2) here. and note, this assumes your math library sin() and cos() are taking fractions of pi, I'm not sure what the default is on the iPhone. and it only works pretty for circles with a radius larger than 10 or so, but I hope the basic concept was illustrated. the code isn't 100% correct. the point is to show the idea. This is NOT an optimal algorithm either; there are like 100 of those out there. just search for "circle drawing algorithm." if you're using GL, you're going to want to make a list of line segments using a method like this anyway; if a framebuffer, there are as I said alternatives.

7. ### Little White Bear Studios Well-Known Member Patreon Silver

Aug 27, 2008
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#7
Doesn't your view controller have access to the view? If so, you could assign the values to some variables in your view, and then tell the view to update. Then your drawRect could use those values.

8. ### Little White Bear Studios Well-Known Member Patreon Silver

Aug 27, 2008
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#8
Very interesting, although I have to ask why wouldn't you just draw a path? I don't know about the OpenGL world, but in 2D land, paths are the easiest way to draw vector data.

9. ### bovinedragon Well-Known Member

Sep 20, 2008
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#9
Last edited: Apr 25, 2009
OpenGL lives in the discrete hardware level world. In order to draw anything curvy, you have to break it down into strait lines.

10. ### Little White Bear Studios Well-Known Member Patreon Silver

Aug 27, 2008
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#10
Gotcha. Thanks for the clarification.

11. ### writingsama Well-Known Member

Dec 4, 2008
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#11
I'm sorry I'm not familiar with developing for the iPhone or mac os X. 'path' has lots of uses in lots of contexts, and seems like a Cocoa thing maybe in this case? I can't offer any help there

12. ### Redoctober11 New Member

Apr 4, 2009
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#12
Yup, that works fine. I can now get the x and y touch values from the view to the view controller. I've been making things more complicated then they need to be. But now I have another problem. I had a method in the touchesBegain:withEvent: that now can't be called because it's in a View. How do I tell the ViewController that the user touched the screen, so that a method can fired?