2021-04-21 12:00:14 (UTC-03:00)

Marcel Rodrigues <marcelgmr@gmail.com>

add anti-aliasing

diff --git a/aa.lua b/aa.lua
new file mode 100644
index 0000000..0a93d18
--- /dev/null
+++ b/aa.lua
@@ -0,0 +1,98 @@
+local bit = require "bit"
+
+local surf = require "surf"
+
+local lshift, rshift =  bit.lshift,  bit.rshift
+
+-- ByteMaps (and GIFs) have a maximum of 256 colors
+-- we're anti-aliasing by mixing four pixels in one (k=4)
+-- the maximum n where C(n+k-1, k) < 256 is 7
+-- so we only need factorials up to 7+4-1=10
+local fact = {1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800}
+fact[0] = 1
+
+-- binomial coefficient
+-- returns zero if n < k
+local function C(n, k)
+    return n >= k and fact[n] / fact[k] / fact[n-k] or 0
+end
+
+-- index of a combination with repetition
+-- n must be ordered
+local function I(n, base)
+    local a, b, c, d = unpack(n)
+    local N = base + 3
+    return C(N, 4) - C(N-a-1, 4) - C(N-b-2, 3) - C(N-c-3, 2) - C(N-d-4, 1) - 1
+end
+
+-- helper to generate combinations with repetition in lexicographical order
+local function next_mix(n, base)
+    for i = 4, 1, -1 do
+        if n[i] < base-1 then
+            n[i] = n[i] + 1
+            for j = i+1, 4 do
+                n[j] = n[j-1]
+            end
+            break
+        end
+    end
+end
+
+local function test_combinations()
+    local base = 7
+    local n = {0, 0, 0, 0}
+    local i = 0
+    while i < C(base+3, 4) do
+        assert(I(n, base) == i)
+        i = i + 1
+        next_mix(n, base)
+    end
+    print(i.." tests passed")
+end
+
+local function antialias(bytemap, colors)
+    local base = #colors
+    assert(base >= 2 and base <= 7)
+    -- output has half dimensions
+    -- discard last row/column for odd dimensions
+    local w = rshift(bytemap.w, 1)
+    local h = rshift(bytemap.h, 1)
+    local outmap = surf.new_bytemap(w, h)
+    local n = {} -- indices of colors to be mixed
+    local dx, dy -- coordinates in input bytemap (d is for "double")
+    for y = 0, h-1 do
+        dy = lshift(y, 1)
+        for x = 0, w-1 do
+            dx = lshift(x, 1)
+            n[1] = bytemap:pget(dx  , dy  )
+            n[2] = bytemap:pget(dx+1, dy  )
+            n[3] = bytemap:pget(dx  , dy+1)
+            n[4] = bytemap:pget(dx+1, dy+1)
+            assert(math.max(unpack(n)) < base)
+            table.sort(n)
+            local v = I(n, base)
+            outmap:pset(x, y, v)
+        end
+    end
+    -- now just build the palette of mixed colors
+    local function mixed_rgb(n)
+        local c1 = colors[n[1]+1]
+        local c2 = colors[n[2]+1]
+        local c3 = colors[n[3]+1]
+        local c4 = colors[n[4]+1]
+        -- compute the average RGB of the four colors
+        local r = rshift(c1[1] + c2[1] + c3[1] + c4[1], 2)
+        local g = rshift(c1[2] + c2[2] + c3[2] + c4[2], 2)
+        local b = rshift(c1[3] + c2[3] + c3[3] + c4[3], 2)
+        return r, g, b
+    end
+    n = {0, 0, 0, 0}
+    local palette = {{mixed_rgb(n)}}
+    for i = 2, C(base+3, 4) do
+        next_mix(n, base)
+        table.insert(palette, {mixed_rgb(n)})
+    end
+    return outmap, palette
+end
+
+return {antialias=antialias}