# Checking a point lies within a radius

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Basically i want to check if an entity lies within a grenades explosion radius. I have the origin of the grenade and the explosion radius and the list of possible entities it collides with. What is the possible check to see if the entity lies within the explosion radius?

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You need to find the distance between the two points (using Pythagorus' theorem), and then compare that distance to the explosion radius:
for e in entities:	diff = e.position - grenade.position	if sqrt(diff.x*diff.x + diff.y*diff.y) < explosion.radius:		# we are inside the eplosion radius

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I'd prefer to check the squared distance with the square of the explosion radius in order to avoid a call to sqrt for each entity.

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It all boils down to ye' old "speed or accuracy" comparision.
Depends on how critical the comparision is.

In the grenade-example presented by the OP, I would use a squared check.

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Quote:
 Original post by RasmadrakIt all boils down to ye' old "speed or accuracy" comparision.Depends on how critical the comparision is.In the grenade-example presented by the OP, I would use a squared check.
A distance squared check is just as accurate as a distance check. The only time distance would matter is if the OP wants to apply damage based on the distance to the explosion, then he could do a distance squared check to see if the player is within range, then use the real distance to compute damage.

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Quote:
 Original post by RasmadrakIt all boils down to ye' old "speed or accuracy" comparision.Depends on how critical the comparision is.In the grenade-example presented by the OP, I would use a squared check.

If all you need is a binary test of whether a point is within a distance/radius (which is what the OP asked for) then checking the squared distances results in exactly the same results. Speed vs. accuracy doesn't come into it.

Edit: too slow. [grin] Man gd.net is running sluggishly today.

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How many grenades per second, and how many entities to check against (realistically)? If this is for a FPS, I'd say that level of optimization (simple as it is) is probably wasted effort.

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 Original post by snakHow many grenades per second, and how many entities to check against (realistically)? If this is for a FPS, I'd say that level of optimization (simple as it is) is probably wasted effort.

It's no effort at all:

sqrt(diff.x*diff.x + diff.y*diff.y) < explosion.radius

This kind of optimization is so fast and easy there's no reason you should avoid it.

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