Interactive demo. Mouseover to activate, drag planets around.

This is an example of making an instance orbit around other instance in GameMaker.

In contrast to other similar examples (and commonly given advice of the kind), it allows to define orbiting speed in a common way (pixels/frame instead of degrees/frame).

This is accomplished by calculating the distance to orbit' center (from current position), calculating the orbit' length, and thus converting pixel speed to degree speed.

New position is then calculated and instance' speed is adjusted for it to reach it the next frame.
This allows to use direction in collision calculations as per usual.

Overall, the code is simple enough but documented regardless.

varcx, cy, crad, clen, cdir, cnext, nx, ny;
// retrieve variables for later use:cx=obj_sun.x;
cy=obj_sun.y;
crad=point_distance(cx, cy, x, y);
// find "length" of circle:clen=crad*pi*2;
// find current angle on circle:cdir=point_direction(cx, cy, x, y);
// find next angle on circle:cnext=cdir+vel*360/clen;
// find coordinates of next point:nx=cx+lengthdir_x(crad, cnext);
ny=cy+lengthdir_y(crad, cnext);
// set speed vector to reach that point in next step:hspeed=nx-x;
vspeed=ny-y;

Where vel is the object's velocity (px/step) and obj_sun is the thing to orbit around.
Code is to be included in Step event.

if mouse_check_button_pressed(mb_left) {
instance_create_layer(mouse_x,mouse_y,”Instances”,target);
}
if mouse_check_button_released(mb_left) {
instance_destroy(target);
}

if instance_exists(target) {
var cx, cy, crad, clen, cdir, cnext, nx, ny;
// retrieve variables for later use:
cx = target.x;
cy = target.y;
crad = point_distance(cx, cy, x, y);
// find “length” of circle:
clen = crad * pi * 2;
// find current angle on circle:
cdir = point_direction(cx, cy, x, y);
// find next angle on circle:
cnext = cdir + vel * 360 / clen;
// find coordinates of next point:
nx = cx + lengthdir_x(crad, cnext);
ny = cy + lengthdir_y(crad, cnext);
// set speed vector to reach that point in next step:
hspeed = nx – x;
vspeed = ny – y;
}

however the object is always orbiting counter-clockwise.
how could i make it rotate clockwise if needed?
it looks kinda odd when target is above the orbiting object.

the problem is that in the GMC forums they all work by modifing de x y variables directly, instead of using hspeed and vspeed.
And what do you mean by “swing arc”? is it the angle in that moment?

In simplest case you would modify step event code to set next angle to be 270 + sin(t) * c (where “t” is changed over time and “c” is swing arc). For actual pendulum physics you would need to incorporate some extra code to handle velocity and acceleration while still modifying that “next angle” variable in the end. I believe that GameMaker Community forums did contain at least a couple of such examples.

i got a code like this in my step event:

if mouse_check_button_pressed(mb_left) {

instance_create_layer(mouse_x,mouse_y,”Instances”,target);

}

if mouse_check_button_released(mb_left) {

instance_destroy(target);

}

if instance_exists(target) {

var cx, cy, crad, clen, cdir, cnext, nx, ny;

// retrieve variables for later use:

cx = target.x;

cy = target.y;

crad = point_distance(cx, cy, x, y);

// find “length” of circle:

clen = crad * pi * 2;

// find current angle on circle:

cdir = point_direction(cx, cy, x, y);

// find next angle on circle:

cnext = cdir + vel * 360 / clen;

// find coordinates of next point:

nx = cx + lengthdir_x(crad, cnext);

ny = cy + lengthdir_y(crad, cnext);

// set speed vector to reach that point in next step:

hspeed = nx – x;

vspeed = ny – y;

}

however the object is always orbiting counter-clockwise.

how could i make it rotate clockwise if needed?

it looks kinda odd when target is above the orbiting object.

“vel” variable is what defines the direction of rotation. Have it negative, and it’ll orbit clockwise.

the problem is that in the GMC forums they all work by modifing de x y variables directly, instead of using hspeed and vspeed.

And what do you mean by “swing arc”? is it the angle in that moment?

Awesome website, dude!

Quick question: How can I turn this into some kind pendulum?

In simplest case you would modify step event code to set next angle to be

270 + sin(t) * c(where “t” is changed over time and “c” is swing arc). For actual pendulum physics you would need to incorporate some extra code to handle velocity and acceleration while still modifying that “next angle” variable in the end. I believe that GameMaker Community forums did contain at least a couple of such examples.