Even better (high speed) moving platforms

There’s an oversight in my previous article (i.e. there’s a bug) where if you increase the xvel or yvel of the moving platforms above 1, strange things start to happen. Let’s fix high speed moving platforms.

Why is this happening

This is happening because, as I said in the article, xprevious and yprevious get automatically set before the begin step event and they get set just once, never to be updated with the new positions we give to the players during the platform’s moving cycle.

So if the platform is cycling 10 times to move upward (yvel = -10, cycling pixel by pixel) and it stops right after the 5th cycle iteration (due to a collision like in the image above), it resets the players position back to where they were at the very beginning of the cycle, even if they successfully moved for 5 whole cycles, effectively putting the instances back inside the platform itself.

That’s because we forgot to update the xprevious and yprevious upon each successful movement cycle completion.

How to fix it

Let’s create a simple script called instances_update_previous_position, very similar to instances_reset_position.

///@func instances_update_previous_position(instances_to_move)

var instances_to_move    = argument[0]
var num_of_instances     = ds_list_size(instances_to_move)
var index                = 0

repeat(num_of_instances)
{
    var instance              = instances_to_move[| index]
    instance.xprevious        = instance.x
    instance.yprevious        = instance.y
    index++
}

This script accepts a list and it cycles the list to update the xprevious and yprevious variable.

Now inside the move_platform_y and move_platform_x scripts, just before destroying the ds lists, call this script passing a list of successfully moved objects. In our case we should already have one. It’s called instances_to_move but if you look into the code you’ll realize that, at this point in the code, it holds the instances that successfully moved. Instances for which we should update the xprevious and yprevious variables.

This should really give us pixel perfect movement. If you find any other bug, please don’t hesitate to leave a comment. Thanks for your patience 🙂