Code of the Ninja: Jump Height Calculator

If you haven't already, read the Code of the Ninja: Introduction

Hello again, Code Ninjas!

Today is something very simple: a formula that allows you to plug in variables for "jump force" and "gravity" and get the maximum height of a jump as the result.

When designing platformers, tweaking the physics until they're just right is very important. Having a formula such as this will be useful for zeroing in on exactly what values constants like gravity should have.

Another use might be this: say you already have established physics, for instance because you're hacking a Sonic game. But you want to add a new object, such as a new kind of bumper or spring, that bounces Sonic exactly 5 times his height. Determining the force at which Sonic should be impelled by the object in order to achieve that height would be a snap using the following formula.

The simplest way to write the formula is this:

g = gravity;
f = jumpForce;
h = 0;
t = 0;
while ( f > 0 )
{
  h += f;
  f -= g;
  t += 1;
}
return h;

g is your gravity, and f is the jump force, which can be set to anything. In Sonic, for example, gravity is 0.21875pps (pixels per step) and the jump force is 6.5pps.

h is the height value we are trying to find out - the number of pixels the character will travel given the jump force f with gravity g. t is time, which is optional - it will be the number of steps it takes to reach the apex of the jump. Both h and t are initialised at 0.

Then we run a loop while f is greater than 0. In the loop, first f is added to h, then g is subtracted from f. This simulates the jump: the force acts upon the character's position, then the gravity acts upon the force. t is then incremented by 1 in order to count the time.

When the loop is finished, h will be the ultimate height of the jump.

Note that this code assumes your physics to handle similarly to Sonic's. In Sonic, because of the particulars of the original code, the jump force is added once to the character's position before gravity acts upon it. If this is not the case in your game, then the loop should be restructured accordingly; g should be subtracted from f before f is added to h.

Earlier I said "the simplest way" to write the formula. The truth is that this, while simple, is a rather brute force method. Depending on the strength of the jump force and gravity, the loop could run dozens of times. Yes, modern computers can handle this without breaking a sweat, and yes, you probably won't even be using the formula in a running game, anyway. But for the sake of mathematical beauty, we can find a better way that doesn't employ a loop.

So let's build this new formula piece by piece. First, we need to find how many steps it will take for gravity to whittle the jump force away to 0 (or less). This will be time t again.

g = gravity;
f = jumpForce;
t = ceil(f/g);

We find t by dividing f by g and rounding up to the nearest 1. Why the rounding up? Well, if the jump force isn't perfectly divisible by gravity, the remainder would still count as upward velocity and the player would still move up by a little bit during that step. Since it counts toward the total, it must be taken into account.

Now that we know how long the jump will take to reach the apex, we can easily discover the distance the character would travel during that time, without gravity, merely by multiplying f by t.

h = f*t;

Of course this seems a little silly, because we're trying to find the height covered with gravity taken into account. But knowing this value is useful; 'cos if we can also determine how much force is deducted by gravity from the jump force over t steps, we can multiply gravity by that number, subtract it from h, and have our correct result.

In the first step (t = 0), the jump force is unaffected. In the next, it is lesser by gravity. In the next, it is lesser by gravity again, i.e. it is equal to the initial jump force value minus gravity times two. Next step, times three, then in the next, times four, and so on, until gravity overcomes the jump force in step t-1.

Visually represented, you might think of the amount of force lost to gravity as a triangular stack like this:

----- (t=0)
g---- (t=1)
gg--- (t=2)
ggg-- (t=3)
gggg- (t=4)
ggggg (t=5)
...

Fortunately it is easy to find the area of a triangle such as this by finding the area of a square the size of t*t-1 and then cutting that value in half. (If, as mentioned above, your physics subtract gravity before the character moves once, the square should have a size of t*t+1 instead).

h -= t*(t-1)*0.5*g;

Et voilà, you have the correct height of the jump, identical to the result of the while statement method used above.

Finally, because multiplication is a commutative process, the formula can be recast in a simpler way for our final code:

g = gravity;
f = jumpForce;
t = ceil(f/g);
h = (f-((t-1)*0.5*g))*t;
return h;

You can find a small example .gmk here that lets you play with the variables to get different jump heights. Until next time, happy coding!

If you use my code or scripts in your game or engine, no credit is necessary. But I'd love to hear about your project if you do! Just drop me a comment below, or e-mail me at us.mercurysilver@gmail.com

Code of the Ninja - Partially Erasing Surfaces in GM

If you haven't already, read the Code of the Ninja: Introduction

Hey there, Code Ninjas!

One of the cool things about Game Maker 8 is the ability to export PNGs with an alpha channel for transparency. There's no separate functions for doing so, though; if you want to be sure the exported image is in PNG format, you have to make sure the extension is explicitly ".png", like so:

Code:

sprite_save("sprite.png");
screen_save_part("screenie.png",320,224);

It's especially cool to create partially transparent surfaces and export them. After creating a surface, you can use the draw_clear_alpha() function to make it completely transparent:

Code:

surface_set_target(surface);

draw_clear_alpha(c_black,0); // clear the entire surface with fully transparent colour

surface_reset_target();

One thing that's annoying, though, is the way that drawing with a partial alpha to a surface works. Instead of blending with the colour underneath, the colour is completely replaced, alpha and all. Effectively this punches "holes" in the surface image.

You'd think this could exploited to create some kind of eraser tool. Draw pixels with an alpha of 0 to the surface to erase pixels that are already there, leaving fully transparent pixels behind.

This doesn't work, though, for some reason. Very low alpha values such as 0 or 0.01 function exactly as you'd normally expect when drawing to the screen, even though higher values such as 0.7 differ when using surfaces.

So much for the ability to erase pixels from a surface using that method... But there is another way.

Set the blend mode to bm_subtract before drawing to the surface and you'll effectively be able to erase from the image:

Code:

surface_set_target(surface);

draw_set_blend_mode(bm_subtract);
draw_set_color(c_white); // color doesn't actually matter
draw_set_alpha(1); // alpha must be 1 to fully erase pixels

// erase an "X" across the surface
draw_line(0,0,surface_get_width(surface),surface_get_height(surface));
draw_line(surface_get_width(surface),0,0,surface_get_height(surface));

draw_set_blend_mode(bm_normal);

surface_reset_target();

This trick may come in handy on occasion, especially when making games where the user is allowed to paint custom textures for things - an erase tool is essential.

Until we meet again, happy coding!

If you use my code or scripts in your game or engine, no credit is necessary. But I'd love to hear about your project if you do! Just drop me a comment below, or e-mail me at us.mercurysilver@gmail.com

Code of the Ninja: Looking Up and Down

If you haven't already, read the Code of the Ninja: Introduction

Hello again, Code Ninjas!

In most 2D platformers the player has the ability to shift the camera a short way by holding up or down on the D-pad. In Sonic games, it works great but there is minor flaw that's hard to notice and really doesn't cause any problems, but I thought it would nice to show how to fix it anyway.

The problem arises when the player looks up or down near the top or bottom boundaries of the level (or whatever current boundaries the camera is limited to). In order to understand the problem, we need to look at the basic idea behind shifting the camera.

If there were no ability to look up or down, the process would be really simple. The camera would simply follow the player's position directly, with a simple check to make sure it doesn't leave the level boundaries. In order to shift the camera up and down, though, an extra step is needed. An offset is added to the player's Y position before the camera follows it. By increasing or decreasing the offset value when the up or down buttons are pressed, the camera will shift vertically relative to the player's position.

Clearly there need to be maximum and minimum values that the offset can't exceed, otherwise the player could continue to scroll the camera freely until it reached the top or bottom of the level. Normally this behaviour is not desired, so limits are set that will keep the player visible on the screen.

Imagine for a moment, though, that there aren't. Say the player is looking up - the offset decreases and decreases. It will continue to decrease even after the camera stops at the top of the level, because the offset doesn't know when to quit. The camera is limited, sure, and the player never sees beyond the boundary of the level, but the offset value is still decreasing away.

Now what would happen if the player stops pressing up? The offset value will start increasing until it returns to 0 (no shift at all). But depending on how long you've been pressing up, you'll have to wait a few moments before the camera starts to visibly scroll back to the neutral position. After all, the offset has been invisibly counting away for an undetermined amount of time, and that extra time has to be made up for when it tries to return.

This example with no upper/lower limit for the offset illustrates the problem I'm talking about, and that we're going to fix. It's less noticable when limits are present for the offset, because the value can't continue to increase or decrease indefinitely, but it's still there. If you've got a copy of any Genesis Sonic game, try looking up or down near the top or bottom of a zone and see for yourself. Scandalous, isn't it?

It may not be the worst problem in the world, but it can be fixed, so let's give it a go.

There are actually two solutions. One would be to reduce the offset by the appropriate amount when the player lets up from pressing up or down, but that's not the solution I'll describe. Why not? Because that method would require the detection of when up or down is released, which - while certainly possible - is harder to slot right into the way the code already works in the original Sonic.

Let's look at some basic code for handling the offset, and then we'll apply the fix to it.

cam_step()

shiftMode = 0; // reset shift mode
if ( joy( UP ) ) shiftMode = -1; // player is looking up
if ( joy( DOWN ) ) shiftMode = 1; // player is looking down

switch ( shiftMode )
{
case 0: // camera is recentring
  if ( shiftOffset < 0) shiftOffset += 2; // scroll down if too high
  if ( shiftOffset > 0) shiftOffset -= 2; // scroll up if too low
  break;
case -1: // camera is shifting up
  if ( shiftOffset > -shiftLimit ) shiftOffset -= 2; // scroll up until reach negative limit
  break;
case 1: // camera is shifting down
  if ( shiftOffset < shiftLimit ) shiftOffset += 2; // scroll down until reach positive limit
  break;
}

The problem occurs in case -1 and case 1: stopping at the shiftLimit isn't good enough, because at the top and bottom of the level, we need to stop increasing or decreasing early.

How close to the top or bottom of the level must one be in order for the undesired behaviour to occur? Close enough that the distance between the top/bottom of the camera and the top/bottom of the level is less than the shiftLimit.

This suggests the solution. Instead of using the shiftLimit alone, we should use whichever happens to lesser - the shiftLimit or the difference between the view boundary and the level boundary.

cam_step()

case -1: // camera is shifting up
  r = min( shiftLimit, view_yview - shiftOffset );
  if ( shiftOffset > -r ) shiftOffset -= 2; // scroll up until reach negative limit
  break;
case 1: // camera is shifting down
  r = min( shiftLimit, levelBottom - ( ( view_yview - shiftOffset ) + view_hview ) );
  if ( shiftOffset < r ) shiftOffset += 2; // scroll down until reach positive limit
  break;

(The reason why shiftOffset has to be subtracted from view_yview is so the difference won't change once the screen starts scrolling and the offset starts to change.)

And that's all that's needed. It may not be much, but obsessive compulsives will enjoy the game more!

If you enjoyed this and the previous Code of the Ninja, be sure to come back tomorrow for one more before I slip back into the shadows. =P

If you use my code or scripts in your game or engine, no credit is necessary. But I'd love to hear about your project if you do! Just drop me a comment below, or e-mail me at us.mercurysilver@gmail.com