This post is about creating window sliding effect such as seen in my old program called GMConveter. Program itself was a bit of joke actually, since at the moment of it's publication the only actual difference between GM80 and GM81 formats was a version byte in file header, which GMConverter would change, making files compatible again. Not too bad of functional part for something that was downloaded over 4 thousand times over course of two years, right?
Apart from inner simplicity, it also had nice visuals, including a nice window sliding effect used on start and end of program. And that's what this article is about.

Basics

For start, we'll need a couple of variables to determine where the window will start moving from, where it will move to, and where exactly it is at the moment of time. For purpose of this example that code will look like this (goes into creation event):

slide_from = display_get_height() - 1 // slide in from bottom of screen
slide_to = window_get_y() // sliding target position
slide_now = 0 // position (0..1)

And a bit of code to actually move the window around (goes into step event):

slide_now = min(slide_now + 0.02, 1)
window_set_position(window_get_x(),
    slide_from + slide_now * (slide_to - slide_from))

If you'll run code at this time, you'll see window sliding out of bottom of the screen like this:

That works. Not a particularly interesting effect yet, but causes window to gradually slide into the screen after game has started.

Making it "interesting"

As you've probably noticed, uniform movement does not look particularly interesting. This is because used "tweening" method is just linear interpolation. Which is not exciting by itself. However, that can be fixed, for example, by replacing it with trigonometry-related formula based in sine or cosine functions. Following image illustrates looks of that:

After this minor replacement, step event will look like this:

slide_now = min(slide_now + 0.02, 1)
window_set_position(window_get_x(),
    slide_from + sin(slide_now * pi / 2) * (slide_to - slide_from))

And will behave like this:

Tweaks and extras

One thing that you may have noticed with earlier code, is that game appears at original position for a moment before snapping to bottom of screen and sliding from there. This can be fixed by copying window_set_position call to Create event too. Or moving that call into a user event, such as User Defined 0:

/// Updates window position
window_set_position(window_get_x(),
    slide_from + (slide_to - slide_from) * sin(slide_now * pi / 2))

Other thing to be paid attention is that sliding in isn't generally enough. Application may not last forever, and thus could use a "slide-out" effect too. For that we'll need an extra parameter to indicate current sliding direction (speed), which'll also go into Create event:

slide_from = display_get_height() - 1 // slide in from bottom of screen
slide_to = window_get_y() // sliding target position
slide_now = 0 // position (0..1)
slide_delta = 0.02 // sliding speed (_now change rate)
event_user(0) // call first update

For sake of optimization and handling of new situation, Step event code also undergoes some changes:

if (slide_delta > 0) { // sliding in
    if (slide_now < 1) {
        slide_now = min(slide_now + slide_delta, 1)
        event_user(0)
    }
} else { // sliding out
    slide_now += slide_delta
    if (slide_now <= 0) {
        // window disappears before closing
        if (window_get_visible()) window_set_visible(0)
    } else event_user(0)
    // this is where it actually closes:
    if (slide_now < -1) game_end()
}

And, finally, slide-out situation has to be triggered from some place, such as press Escape keyboard event (note that you'll need to disable automatic game termination on this keypress in Global Game Settings):

/// start sliding out, if didn't already
if (slide_delta > 0) {
    slide_to = window_get_y() // make sure coordinates didn't get broken
    slide_delta = -abs(slide_delta) // reverse sliding direction
    slide_now = 1 // again, making sure nothing gets broken
}

And, after all these changes, we get perfectly fine sliding behaviour, which would look like this:

You can also download final composed version of example to avoid "assembly errors":

Download GMK

As such, you can also download intermediate file, which I've used to make small demonstration animations for this post without having to do some extreme wizardry with capturing full-scale animation and downscaling it afterwards.

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