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76 lines
2.7 KiB
Lua
76 lines
2.7 KiB
Lua
; Animate a zooming star field.
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; Rather than doing complicated 3d stuff, this simply uses polar coordinates.
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; Every star lies along the circle and has a radius.
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; Movement is dictated by a random accelleration, which increases the speed, which increases the radius.
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; TODO extend the radius to a larger maximum (0-319 or perhaps 0-511, instead of 0-255) so that we can fill the whole screen.
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; TODO do something with the colors palette: make stars darker in the distance
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%import math
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%import palette
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%import gfx_lores
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%option no_sysinit
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main {
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%option verafxmuls
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sub start() {
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const ubyte NUM_STARS = 128 ; maximum is 128.
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const ubyte CIRCLE_SKIP = 256/NUM_STARS
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ubyte[NUM_STARS] color
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uword[NUM_STARS] @split radius
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uword[NUM_STARS] @split accel
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uword[NUM_STARS] @split speed
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uword[NUM_STARS] @split prev_x
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ubyte[NUM_STARS] prev_y
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gfx_lores.graphics_mode()
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; init the star positions
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ubyte star
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for star in 0 to NUM_STARS-1 {
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color[star] = star
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accel[star] = (math.rnd() & 15) | 1
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radius[star] = mkword(math.rnd() & %11111000, 0)
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speed[star] = 0 ; radius[star] >> 6 ; a slow buildup of movement at the start is kinda nice I think, so we start with speed zero
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}
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const uword angle = 0 ; if you make this a variable and increase it, the stars rotate.
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repeat {
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sys.waitvsync()
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;;palette.set_color(0, $400)
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for star in NUM_STARS-1 downto 0 {
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gfx_lores.plot(prev_x[star], prev_y[star], 0)
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cx16.r2L = star*CIRCLE_SKIP + msb(angle)
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uword sx = (msb(math.sin8(cx16.r2L) as word * msb(radius[star])) + 128) as uword + 32
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ubyte sy = (msb(math.cos8(cx16.r2L) as word * msb(radius[star])) + 120)
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prev_x[star] = sx
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if sy<240 {
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prev_y[star] = sy
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gfx_lores.plot(sx, sy, color[star])
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} else
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prev_y[star] = 0
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if radius[star] > $fffe - speed[star] {
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; reset star to center
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accel[star] = (math.rnd() & 15) | 1
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radius[star] = $0400 + accel[star] * 128
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speed[star] = 0 ; radius[star] >> 6
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color[star] = math.rnd()
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} else {
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; can still move more.
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radius[star] += speed[star]
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speed[star] += accel[star]
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}
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}
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;; angle += $0040
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;;palette.set_color(0, $000)
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}
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}
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}
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