I got to wondering whether you could rotate a sprite in PICO-8. Of course this facility is not provided, but it should be achievable, I thought.
How this works: 3 rotating objects. Arrows move the green cursor among them, and Z or X to toggle each object on or off.
The rotations are slightly ugly, but I think accurate. Smoothing could probably make them look better, but maybe not at reasonable cost. I don't know! Rotating the large sprite is CPU-intensive (and ugly!). I think for small objects this could be used in a game.
- Sprites are rotated about their top left corner. I think this is needed to guarantee that atan2 is one-to-one. But it would be better to rotate around an arbitrary center!
- I forgot what the other issues are.
You can calculate in reverse. From target position back to pixel of original sprite. I did that in "Pico Wheels" with the cars. Since we're doing this pixel by pixel for all pixels, it is also a good location to add realtime (per pixel) shadow in this loop.
Your method produces black pixels because of rounding pixel coordinate values. A rotated sprite might need more pixels than the original sprite has so iterating available pixels is not enough.
(Note: car.z is for jumping... it is used for shadow and car sprite offset during a jump animation)
car.drw=function() local r=flr(car.rot*20)/20 local s=sin(r) local c=cos(r) local b=s*s+c*c for y=-6,5 do for x=-6,5 do local ox=( s*y+c*x)/b local oy=(-s*x+c*y)/b local col=sget(ox+4,oy+4) if col>0 then pset(car.x+x-car.z,car.y+y-car.z,col) shadow(car.x+x+1+flr(car.z),car.y+y+1+flr(car.z)) end end end end
Yeah, if you have a limited number of things that need rotation, it might be a lot better to cache pre-rotated sprites. Keep in mind you can draw through the map and use its flags to flip, so a symmetrical object need only have 90 degrees worth of rotated images.
Also, if you were doing a game like Sega's old Afterburner, you'd simply rotate each sprite once at the start of the frame and then use them all as needed, since they all have the same rot and thus there's no need to rotate a hundred tree sprites individually.
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