Hey. I'm working on a destructable terrain program. Most of it is working however there is a problem that Im stumped on. How would you get the x and y of every point inside of a circle, using only the x,y of the center, and a radius?
Right now my explosions are a square (not very realistic). using this: maps->destroy(int x, int y) destroys a certain pixel of the map.

Can anyone help me turn the explosions into circles?

Well you have the general idea down as to how to destroy a circle. Now the problem is really a question of increasing how many points you destroy as you go about the circle. It is rather hard to guess what your radius field represents but i will assume it is not representing a pixel. Circles are essentially impossible to mimic in a game since you would need an infinite resolution and an infinite amount of deleted pixels to have a perfect circle. Rahter in games whe just delete enough pixels to give the illusion that there is a circle.

The radius is an actual pixel value.

And I am not concerned about an exact circle, I just want something that looks more realistic than a square explosion.

Well then you need to scrap all that code. As they say it is rather hard to make a circle with squares.

Now you really have a couple options.
1). Use a polar coordinate algorithm that uses sin/cos to draw a circle.
Something like:
dx = x + r.cos(theta)
dy = y + r.sin(theta)
where dx is the x coordinate to destroy, r is the radius and theta is the angle in radians
2). Use an algorithm such as Bresenham's to draw a circle.
x = r*cos(theta)
y = r*sin(theta)
where x is the coordinate to destroy, r is the radius and theta the angle in radians.

So it will be harder then I though. Only in grade 11, and didnt do to good in math last year

Just a quick idea for a simpler algorithm:

From the center point of your circle, we'll perform the destroys in 4 quarters, dividing the areas along the compass axis (eg. NSEW)

Eg bottom right quarter:
Start deleting pixels in a horizontal row from the center until you reach limit().
Then start on the next row, delete along until limit() which will be slightly less.
Then start on the next row, delete along until limit() which will have decreased greater than it did the first time.
Repeat until you're radius rows down/at the bottom of the final circle.

Limit() is a function which when given the row number and radius returns the horizontal distance from the center (eg distance along the current row) at which the edge of the circle is reached.

This can be calculated by the fact that the traced curved edge is sinusoidal/in this case follows the value of the second quarter of a sine curve, between 90 and 180 degrees. You'd just have to do a bit of division to get what fraction of the way down the radius each row is (equal to how far between the 90 and 180 limits we are), and probably use integer calculations so you're rounding to the nearest whole pixel.

Ok thanks.

Another suggestion (and it partly depends on your implementation, which sounds like a large bitmap if you are working on a per-pixel basis).

You could just generate one or more binary (or even greater) mask of whatever shape you want your explosions to look like and use that ('AND'ing with your terrain or whatever calculation you would like for more complicated effects).

Im using a raw image format (.pnm) for maps. the filesize is huge, but it can be compressed via gz or bz2 really well..

Problem with using the masks is that its harder to control the size of the explosion.

I'll consider it though. It seems to have a lot of advantages that I might want (irregular shapes specifically)