Sungam:

Draw an invisible rectangle around your circles for collision checking. If needbe, you can easily modify a simple rectangle-maker that takes a midpoint and a radius.

If the distance between the centres of two circles is less than the sum of the radii of the circles, they overlap.

Edit: Doh, I was beaten to it. Need to pay more attention.

Quote:Originally posted by mcgrue

Sungam:

Draw an invisible rectangle around your circles for collision checking. If needbe, you can easily modify a simple rectangle-maker that takes a midpoint and a radius.

I'm probably just failing to see the obvious here, but could you elaborate a bit on this? If all you would do is draw a rectangle with the height and width of the circle's vertical and horizontal radii, centered around the center of the circle, then I expect it would be much too inaccurate to be useful.
If we're talking about drawing multiple rectangles inside the circle, and checking for each one (I think I've seen a graphical version of that somewhere...), I'm afraid I'll need a few more pointers.

Omni and Gayo, about two circles
Excellent. That, I understand. And the math rings faint bells from high school. I'm really starting to wish I'd paid just a little attention back then. I swear, if the teacher had suggested that I could use this stuff for game programming, I'd never have told her to mind her own business.
That distance formula also helps me with another problem I had, so you're looking at quadruple points, I think.

Yeah, the idea with a rectangle around the circles:

In order to get perfect circle-rectangle collision...well, I can't think of how I'd do that, and the perfect way would likely be very slow.

So, McGrue says, form a rectangle around your circles using their horizontal and vertical radii. I mean, unless your circles are like, 180x60 large, there's no major loss of accuracy for something small, like a 16x32 sprite against a 8x8 circle.

Now, if you are using incredibly large things...I'd say, either break it into smaller rects, or...I dunno. There's no simple way to do near perfect circular or pixel-detection that's also very fast and simple to explain, I think.

But then, what do I know?

By the way, for a brush up on all those trig math skills you thought you didn't need:

Sin and Cos: The Programmer's Pals!
(and a ton of other useful math techniques.)

Quote:Originally posted by Omni

Yeah, the idea with a rectangle around the circles:

In order to get perfect circle-rectangle collision...well, I can't think of how I'd do that, and the perfect way would likely be very slow.

So, McGrue says, form a rectangle around your circles using their horizontal and vertical radii. I mean, unless your circles are like, 180x60 large, there's no major loss of accuracy for something small, like a 16x32 sprite against a 8x8 circle.

Now, if you are using incredibly large things...I'd say, either break it into smaller rects, or...I dunno. There's no simple way to do near perfect circular or pixel-detection that's also very fast and simple to explain, I think.

But then, what do I know?

By the way, for a brush up on all those trig math skills you thought you didn't need:

Sin and Cos: The Programmer's Pals!
(and a ton of other useful math techniques.)

What I'm getting at is a tactical battle system which is pixel-based, not tile-based. I think La Pucelle (or one of those) does the same thing, though I haven't seen or played it. So for targeting stuff (say, choosing someone to attack) would have a set distance from the initiator - not just directly above, below or to the sides. In some cases (ranged attacks, magic, whatever), those ranges might become rather big, and with that comes a fairly high level of inaccuracy (I think.. maybe I'm overestimating it).
Anyway, it's only just occurred to me that I could really just treat the entity as a circle, slightly larger than it really is, and go for that. Or just decide that the range has to hit the center of the entity's hot spot. I think either of them would be fairly close, as long as the entities aren't huge.

Thanks much for the help, everyone, on all the topics. I think I can figure something out from this. And I'm reading up on that math stuff right now! I remember my calculator could do stuff like this. If I hadn't been too busy programming lame text-adventures in TI-Basic, maybe I'd bothered to use it for that ;)

I'm not sure how picky you want to be about the square-to-circle, but if you want to make a circle that is circumscribed about a square, the radius of the circle should be equal to half the diagonal distance across the square. That is to say, for an inscribed square with width w and height h, the circumscribed circle would have a radius of sqrt((w*w)+(h*h))/2. This will ensure that each vertex of the square is located on the circumference of the circle. I'm not sure if you care about it being that accurate, though.

That is the worst idea I have ever heard.

Wait!

Variation on Troupe's method.

Blit both objects to a testing buffer, then SCALE THE BUFFER DOWN to something small, like 20x20 pixels. The basic shape is there, so you can still check for obstructions, but extremely quickly.

Then do your testing! You can even say the size of the test buffer is the detail of the obstruction check. The less detail/resolution, the much faster the check is.

It's a lot faster than scanning through 320x240--use different sizes for varying levels of detail.

Well, I mean, that's what we're doing. We're just doing it in pixels instead of ...uh...

Heck, you know, GetPixel is probably slower than any type of looped math, so nevermind. If you're looking for a fast solution, don't use the graphical method.

Best solutions so far in my opinion:

if (R) is a rectangle and (C) is a circle.

1. if the (R) is far from a square shape, you could by some hacking split it up to two more square:ish rectangles or even at best, two squares and through some simple 'radius = sqr(x^2+y^2) / 2' math get circles for them, then do circle collision.

2. if you can lose some small accuracy, just make a square obstruction out of the (C) and do rectangle vs rectangle collision.

when you think of it, in most games rectangle and circle collision is not near as precise as you could be, but any cool programer knows you sacrifice accuracy for speed, because no one wants AWESOME collision code, but something that runs at 3 FPS. well, except retards.