Trig origin

[Degensquared]

I have just created a program that allows the user to rotate a box, w, (drawn with 4 lines) using trig, but it only rotates from the top left hand corner, I know how to make it rotate from another corner, but I'm not sure how to change the origin that it rotates around to the center, not one of the points for the lines.

code:

setscreen ("graphics:800;600,offscreenonly") Mouse.ButtonChoose ("multibutton") var angle, x, y, b, anglev : int := 0 var left, middle, right : int var linex : array 1 .. 3 of int var liney : array 1 .. 3 of int anglev := 1 loop     Mouse.Where (x, y, b)     left := b mod 10     middle := (b - left) mod 100     right := b - middle - left     if left = 1 then         angle += anglev     end if     if right = 100 then         angle -= anglev     end if     if angle >= 360 then         angle := anglev     end if     linex (1) := x     liney (1) := y     linex (2) := linex (1) - 25 + round (cosd (angle - 90) * 50)     liney (2) := liney (1) + 25 + round (sind (angle - 90) * 50)     linex (3) := linex (1) - 25 + round (cosd (angle) * 50)     liney (3) := liney (1) + 25 + round (sind (angle) * 50)     drawline (linex (1) - 25, liney (1) + 25, linex (1) - 25 + round (cosd (angle) * 50), liney (1) + 25 + round (sind (angle) * 50), black)     drawline (x - 25, y + 25, x - 25 + round (cosd (angle - 90) * 50), y + 25 + round (sind (angle - 90) * 50), black)     drawline (linex (2), liney (2), linex (2) + round (cosd (angle) * 50), liney (2) + round (sind (angle) * 50), black)     drawline (linex (3), liney (3), linex (3) + round (cosd (angle - 90) * 50), liney (3) + round (sind (angle - 90) * 50), black)     Time.DelaySinceLast (2)     View.Update     cls end loop

[richcash]

Are you having trouble with finding out the coordinates of the center of a rectangle or are you having trouble with rotating a box around a point (which is basically the same as rotating 4 points around a point of rotation)?

Do the following for all four points and join the new points to get the new box.

1) Find the angle of the line joining each point and the point of rotation by using arctan

code:

angle(1) = arctand ((cy - y(1)) / (cx - x(1))) if angle(1) < 0 then angle(1) += 180 if y(2) < y(1) then angle(1) += 180

where cx and cy are the coordinates of your point of rotation and x(1) and y(1) is your first point from the box

2) Find the distance between each point and the point of rotation by using pythagorean theorem :

code:

dist(1) = ((cx - x(1))^2 + (cy - y(1))^2)^0.5

3)To rotate a point by n degrees, use :

code:

new_x(1) = cosd (angle(1) + n) * dist(1) + cx new_y(1) = sind (angle(1) + n) * dist(1) + cy

*The above is pseudocode.