add example for PNG

png.js

@@ -0,0 +1,43 @@
+const { PNG } = require("pngjs");
+const {Encoder, Decoder} = require('./')
+
+const fs = require("node:fs");
+
+const pngFile = fs.readFileSync("raw.png");
+
+const pngData = PNG.sync.read(pngFile);
+
+
+const sampleBits = pngData.depth
+const codeBits = sampleBits - 2
+
+const encoders = {
+    red: new Encoder(sampleBits,codeBits),
+    green: new Encoder(sampleBits,codeBits),
+    blue: new Encoder(sampleBits,codeBits)
+}
+
+const decoders = {
+    red: new Decoder(sampleBits, codeBits),
+    green: new Decoder(sampleBits, codeBits),
+    blue: new Decoder(sampleBits, codeBits)
+}
+
+for (var y = 0; y < pngData.height; y++) {
+  for (var x = 0; x < pngData.width; x++) {
+    var idx = (pngData.width * y + x) << 2;
+
+    // invert color
+    const rawRed = pngData.data[idx] 
+    const rawGreen = pngData.data[idx + 1]
+    const rawBlue = pngData.data[idx + 2]
+
+    pngData.data[idx] = decoders.red.decode(encoders.red.encode(rawRed))
+    pngData.data[idx+1] = decoders.green.decode(encoders.green.encode(rawGreen))
+    pngData.data[idx+2] = decoders.blue.decode(encoders.blue.encode(rawBlue))
+
+  }
+}
+
+const out = PNG.sync.write(pngData)
+fs.writeFileSync('./out.png', out)