Silly Snow

Link (seems better for mobile)

(update: added 2 new methods that incorporate gusts of wind)

Controls

• UP/DOWN chooses different behaviour for the snow flakes
• X changes the colour of the falling snow so you can see the "trick" a bit easier
• O/Z toggles drawing the non-snow bits of the graphics each frame on and off

What it is

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TL;DR

Only track falling snow, add lying snow just as pixels to the background.

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As it's that time of year again, I thought I'd write some winter-themed stuff.

This is my take on ye olde faithful falling snow demo, which I'm using for the intro screen for another cart (on its way soon). I've cleaned it up a bit, added way more comments than I normally do and added some options to show what's happening a bit and show some variations. Click "Code" below the display above if you're interested.

I first typed in a version of this demo from a magazine back in the mid 80s on an Atari 800XL. In the 35ish years since then I'm not sure I've ever implemented it again though.

The gist of how it works:

Falling snow is represented by a collection of simple particles that are represented by a position (x,y). Each frame that position is updated (mostly downwards i.e. y+=some_dist).

If the snow flake encounters an obstacle (i.e. a pixel that isn't empty or the bottom of the screen) then the snow flake has landed.

Tracking snow flakes after they've landed isn't practical (even with PICO-8 and it certainly wasn't with an 800XL) and is also a bit pointless since once a snow flake has landed, it aint going anywhere. Well, at least not in this demo. Instead, the particle is destroyed and the pixel where it landed is set to white and becomes part of the background (in this demo that means it has a colour greater than 1).

This is an incredibly basic version of how a lot of "physics" engines work: updating only moving or otherwise interesting parts of the simulation and "parking" the rest. Usually they'd check the stationary particles in some way for whether they need to be re-activated too. If you wanted to simulate collapsing drifts etc. there's nothing really to stop you keeping a record of where the snow has fallen or scanning for white pixels and perhaps checking to see if a condition to collapse has been met. Then you could find all the snow pixels affected by the collapsing pixel and "wake" them up. You would probably need to check at a slower than per frame rate for performance reasons.

As with most sims the fun is in playing about with it, hence the 9 method variations in the cart e.g.

• the exact behaviour of the flakes in the air
• whether they land or slide to the side on encountering the background can be tweaked.
• wind gusts...
• Blustery Blizzards

Methods 8 and 9 add a wind factor to the updated position for each flake generated by a value derived from complex and deep trigonometric knowledge (aka trial and error). The nice thing about this is it's only calculated once per frame and only added per snow flake so it's very cheap. Adding sin values per flake based on the flake's position gives some odd results.

Limitations

• For there to be stationary, landed snow relies on not clearing the screen. If you clear the screen then your pretty snow drifts all disappear. You can still have snow flakes falling, but they will disappear a frame after they land...
• The methods in this cart are fairly non-destructive of the background, but it's fairly easy to make a version that "eats" into what's already on the screen (acid snow!). I've included an option to re-draw the background each frame. As long as your background doesn't move then it works fine (remember, no screen clear).
• "tunneling" through thin parts of the background varies in frequency depending on your method. Generally, the more random the snow flake movement, the more problems you'll have.

On the Atari ST there was a game based around this that I got from a magazine cover disk called "Downfall". I could only find one video of it and the uploader doesn't seem to really understand how to play it sadly.

You were supposed to compete against a friend to draw lines to funnel snow from your side out of a hole at the bottom that would then be sent to their side (and vice versa). You could also funnel snow to the sides to give you energy for "special powers" that let you e.g. draw lines on your opponent's side.

I expect that explanation's as clear as a blizzard; hopefully the code isn't as bad.

Happy holidays :)