Add a simple PNG image encoder with Lua API (#11485)

* Add a simple PNG image encoder with Lua API
Add ColorSpec to RGBA converter
Make a safety wrapper for the encoder
Create devtest examples

Co-authored-by: hecktest <>
Co-authored-by: sfan5 <sfan5@live.de>

games/devtest/mods/testnodes/textures.lua

@@ -65,3 +65,78 @@ for a=1,#alphas do
 	})
 end
 
+-- Generate PNG textures
+
+local function mandelbrot(w, h, iterations)
+	local r = {}
+	for y=0, h-1 do
+		for x=0, w-1 do
+			local re = (x - w/2) * 4/w
+			local im = (y - h/2) * 4/h
+			-- zoom in on a nice view
+			re = re / 128 - 0.23
+			im = im / 128 - 0.82
+
+			local px, py = 0, 0
+			local i = 0
+			while px*px + py*py <= 4 and i < iterations do
+				px, py = px*px - py*py + re, 2 * px * py + im
+				i = i + 1
+			end
+			r[w*y+x+1] = i / iterations
+		end
+	end
+	return r
+end
+
+local function gen_checkers(w, h, tile)
+	local r = {}
+	for y=0, h-1 do
+		for x=0, w-1 do
+			local hori = math.floor(x / tile) % 2 == 0
+			local vert = math.floor(y / tile) % 2 == 0
+			r[w*y+x+1] = hori ~= vert and 1 or 0
+		end
+	end
+	return r
+end
+
+local fractal = mandelbrot(512, 512, 128)
+local checker = gen_checkers(512, 512, 32)
+
+local floor = math.floor
+local abs = math.abs
+local data_mb = {}
+local data_ck = {}
+for i=1, #fractal do
+	data_mb[i] = {
+		r = floor(fractal[i] * 255),
+		g = floor(abs(fractal[i] * 2 - 1) * 255),
+		b = floor(abs(1 - fractal[i]) * 255),
+		a = 255,
+	}
+	data_ck[i] = checker[i] > 0 and "#F80" or "#000"
+end
+
+local textures_path = minetest.get_modpath( minetest.get_current_modname() ) .. "/textures/"
+minetest.safe_file_write(
+	textures_path .. "testnodes_generated_mb.png",
+	minetest.encode_png(512,512,data_mb)
+)
+minetest.safe_file_write(
+	textures_path .. "testnodes_generated_ck.png",
+	minetest.encode_png(512,512,data_ck)
+)
+
+minetest.register_node("testnodes:generated_png_mb", {
+	description = S("Generated Mandelbrot PNG Test Node"),
+	tiles = { "testnodes_generated_mb.png" },
+
+	groups = { dig_immediate = 2 },
+})
+minetest.register_node("testnodes:generated_png_ck", {
+	description = S("Generated Checker PNG Test Node"),
+	tiles = { "testnodes_generated_ck.png" },
+
+	groups = { dig_immediate = 2 },
+})