"gas giant" tweetjam explanation

-- set the screen's color-palette
-- ?"hello" is a shorthand for print("hello") in pico8.
-- p8 also has a feature called "P8SCII" where
-- you can print special escape-codes to do stuff.
-- \^! is the "blit data to memory" command
-- 5f10 is the target memory address (palette location)
-- each character after that is one color-value to write
?"\^!5f100█ˇ☉8🅾️◆?7"


-- begin main loop
-- underscore is our goto-label...could be any word,
-- but this looks funny to me in tweetjam-land
::_::

    -- main idea: calculate color for one random pixel at a time

    -- pick an (x,y) position from -2 to +2,
    -- we'll consider (0,0) to be the center of the screen
    x = rnd(4) - 2
    y = rnd(4) - 2

    -- square distance to center
    d = x*x + y*y

    -- "c" is the color we'll use if we're not touching the planet
    c = max(2 - d/4)

    -- if the planet has radius=1, are we touching it?
    if (d<1) then
        -- solve for a z position which makes (x,y,z) into a unit-vector.
        -- rearranged version of the pythagorean theorem: x^2+y^2+z^2 = length^2,
        -- where length is assumed to be 1, and x and y are known. solve for z.
        -- (remember, d is x^2+y^2)
        z=sqrt(1-d)

        -- the next part where we pick a color is all one huge horrible line of
        -- code in the original, so i've split it apart into intermediate values.

        -- lambert-shading term:
        -- assume light-vector is (1,-1,1), and treat (x,y,z) as our surface normal,
        -- because on the surface of a unit sphere centered at the world origin,
        -- your position is also the (unit length) normal.
        -- standard lambert is dot(normal, lightDir), or nX*lX + nY*lY + nZ*lZ
        -- ...but we're assuming that lightDir is (1,-1,1), so this simplifies down
        -- to just nX - nY + nZ. multiply by 2 at the end just expands the range of
        -- possible brightness-values.
        light = (x-y+z) * 2

        -- this value is 1 in the center and drops to zero at the outer edges
        -- of the planet. we'll multiply this into our color to make the
        -- planet fade to dark around the edges.
        fresnel = 1 - (1-z)^2

        -- prepare some values for a stripey pattern across the surface:
        angle = atan2(x, z) -- current angle around the y axis
        rotate = t() / 8 -- t() is current time in seconds
        wiggle = sin(angle*8 + rotate) / 14 -- add a wiggle as we move around the planet

        -- finally, time for stripes!
        stripes = abs((y + wiggle - z/4)%.4 - .2) * 5

        -- combine the above stuff into a final color value
        c = 2 + (1.5 + light + stripes) * fresnel
    end

    -- convert to screenspace (0 to 127), write our chosen color.
    -- the rnd() at the end applies white-noise dithering
    pset(64 + x*32, 64 + y*32, c + rnd())

-- we're done with this pixel! back to the start, do it again
goto _

-- this is an infinite loop with no waiting...which would
-- usually crash an application, but pico8 has built-in
-- limits for how much work can be done per frame, so
-- it'll interrupt our work periodically on its own, and
-- the final result is that we update "as many pixels as
-- we can" per frame.