scaling

p8scale/readme.md

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+2x, 3x, 4x HQx scaling with transparency in PICO-8, enhanced with better curves by @CasualEffects.
+
+![](scale.png)

p8scale/scale.p8

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+pico-8 cartridge // http://www.pico-8.com
+version 15
+__lua__
+// scale up sprites 
+// 2x, 3x or 4x with 
+// transparency using 
+// extend hqx
+//
+// @casualeffects
+// http://casual-effects.com
+
+// num sprites
+local n = 12
+
+function _draw()
+ cls(0)
+ for i = 1,n do
+  spr(i, ((i-1) % 4) * 32, flr((i-1)/4) * 8)
+  scl_spr(i, ((i-1) % 4) * 32, flr((i-1)/4) * 32 + 24)  
+ end
+end
+
+
+
+
+-->8
+function get(a,x,y,w)
+ // return transparent for out of bounds
+ if x<0 or x>=w or y<0 or y>=w then return 0
+ else return a[x+y*w+1] end
+end
+
+// @casualeffects variant of exp/scale2x algorithm
+// https://en.wikipedia.org/wiki/pixel-art_scaling_algorithms#epx/scale2%c3%97/advmame2%c3%97
+function scl2x(src)
+ local w=sqrt(#src)
+ local dst={}
+ // rows are 2*w wide
+ dst[w*w*4] = 0
+
+ for y=0,w-1 do for x=0,w-1 do
+   local a, c, p, b, d =
+    get(src,x,y-1,w),
+    get(src,x-1,y,w),
+    get(src,x,y,w),
+    get(src,x+1,y,w),
+    get(src,x,y+1,w)
+
+   local e,f,g,h = p,p,p,p
+   if (c==a and c!=d and a!=b) e=a
+   if (a==b and a!=c and b!=d) f=b 
+   if (d==c and d!=b and c!=a) g=c 
+   if (b==d and b!=a and d!=c) h=d
+
+   // write four pixels
+   local i = x*2+y*4*w+1
+   dst[i],dst[i+1]=e,f
+   i+=2*w
+   dst[i],dst[i+1]=g,h
+ end end
+
+ if true then
+ // @casualeffects improvements
+ for y=0,w-1 do for x=0,w-1 do
+   local a,b,c, d,e,f, g,h,i=
+    get(src,x-1,y-1,w),
+    get(src,x,y-1,w),
+    get(src,x+1,y-1,w),
+
+    get(src,x-1,y,w),
+    get(src,x,y,w),
+    get(src,x+1,y,w),
+
+    get(src,x-1,y+1,w),
+    get(src,x,y+1,w),
+    get(src,x+1,y+1,w)
+
+
+    // maintain square corners
+    local j=1+x*2+y*4*w
+    if (g!=e and d!=e and a!=e and b!=e and c!=e) dst[j]=e
+
+    j+=1
+    if (a!=e and b!=e and c!=e and f!=e and i!=e) dst[j]=e
+
+    j+=2*w
+    if (g!=e and h!=e and i!=e and f!=e and c!=e) dst[j]=e
+
+    j-=1
+    if (a!=e and d!=e and g!=e and h!=e and i!=e) dst[j]=e
+
+    // smooth diagonals into transparent
+    // (also rounds curves while maintaining 2:1 slopes)
+    j=1+x*2+y*4*w
+    if (c!=0 and a==0 and b==0 and d==0 and g!=0) dst[j]=e
+
+    j+=1
+    if (a!=0 and b==0 and c==0 and f==0 and i!=0) dst[j]=e
+
+    j+=2*w
+    if (g!=0 and h==0 and i==0 and f==0 and c!=0) dst[j]=e
+
+    j-=1
+    if (a!=0 and d==0 and g==0 and h==0 and i!=0) dst[j]=e
+
+ end end
+ end
+
+ return dst
+end
+
+function scl4x(src)
+ return scl2x(scl2x(src))
+end
+
+// scale3x algorithm
+// https://en.wikipedia.org/wiki/pixel-art_scaling_algorithms#scale3%c3%97/advmame3%c3%97_and_scalefx
+function scl3x(src)
+ local w=sqrt(#src)
+ local dst={}
+ // rows are 3*w wide
+ dst[w*w*9] = 0
+
+ for y=0,w-1 do for x=0,w-1 do
+   local a,b,c, d,e,f, g,h,i=
+    get(src,x-1,y-1,w),
+    get(src,x,y-1,w),
+    get(src,x+1,y-1,w),
+
+    get(src,x-1,y,w),
+    get(src,x,y,w),
+    get(src,x+1,y,w),
+
+    get(src,x-1,y+1,w),
+    get(src,x,y+1,w),
+    get(src,x+1,y+1,w)
+
+    local d1,d2,d3, d4,d5,d6, d7,d8,d9=e,e,e, e,e,e, e,e,e
+
+    if (d==b and d!=h and b!=f) d1=d
+    if ((d==b and d!=h and b!=f and e!=c) or (b==f and b!=d and f!=h and e!=a)) d2=b
+    if (b==f and b!=d and f!=h) d3=f
+    if ((h==d and h!=f and d!=b and e!=a) or (d==b and d!=h and b!=f and e!=g)) d4=d
+    if ((b==f and b!=d and f!=h and e!=i) or (f==h and f!=b and h!=d and e!=c)) d6=f
+    if (h==d and h!=f and d!=b) d7=d
+    if ((f==h and f!=b and h!=d and e!=g) or (h==d and h!=f and d!=b and e!=i)) d8=h
+    if (f==h and f!=b and h!=d) d9=f
+
+    // write 9 pixels
+    local i = x*3+y*9*w+1
+    dst[i],dst[i+1],dst[i+2]=d1,d2,d3
+    i+=3*w
+    dst[i],dst[i+1],dst[i+2]=d4,d5,d6
+    i+=3*w
+    dst[i],dst[i+1],dst[i+2]=d7,d8,d9
+ end end
+
+ return dst
+end
+
+function scl_spr(i, x, y)
+ local tmp = {}
+ tmp[8*8] = 0
+
+ // copy the source to temp memory
+ for v=0,7 do for u=0,7 do
+   tmp[1+u+v*8]=sget((i%16)*8+u,flr(i/16)*8+v)
+ end end
+
+ // enlarge to 4x the original size
+ // (32x32 pixels)
+ tmp = scl4x(tmp)
+
+ // debug visualization
+ local w=sqrt(#tmp)
+ for v=0,w-1 do for u=0,w-1 do
+   pset(x+u,y+v,tmp[1+u+v*w])
+ end end
+end
+
+// rotate source pixels
+// by -1 < a < 1, centered
+// at (x, y) by gathering
+// at the dest on an enlarged
+// image. this is the @casualeffects
+// version of "rotsprite" using
+// improved scaling and simplified
+// sampling
+function rot_spr(i, x, y, a)
+ local tmp = {}
+ tmp[8*8] = 0
+
+ // copy the source to temp memory
+ for v=0,7 do for u=0,7 do
+   tmp[1+u+v*8]=sget((i%16)*8+u,flr(i/16)*8+v)
+ end end
+
+ // enlarge to 4x the original size
+ // (32x32 pixels)
+ tmp = scl2x(scl4x(tmp))
+ local k = 8
+
+ local c, s = cos(-a) * k, sin(-a) * k
+
+ for v = -2, 9 do
+  local b = v - 4
+  for u = -2, 9 do
+   local a = u - 4
+
+   local z, w =
+    flr(4*k + c * a + s * b),
+    flr(4*k - s * a + c * b)
+
+   // todo: better filtering
+   if (z >= 0) and (z < 8*k) and (w >= 0) and (w < 8*k) then
+    local q = tmp[z + w*k*8 + 1]
+    pset(x + u - 4, y + v - 4, q)
+   end
+  end
+ end	
+end
+
+
+
+__gfx__
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