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src/services/zplService.js

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+import { createCanvas, loadImage } from 'canvas';
+
+/**
+ * Converts a PNG image buffer into Zebra ZPL II Graphic Field (^GFA) format.
+ * Uses Zebra's alternative compression scheme (ACS run-length encoding on hex)
+ * to minimize payload size over the network and serial/USB ports.
+ *
+ * @param {Buffer} pngBuffer - Rendered 1-bit or RGBA PNG buffer.
+ * @param {number} [quantity=1] - Number of copies to print (^PQ).
+ * @returns {Promise<string>} Full ZPL string ready to send to a Zebra printer.
+ */
+export async function pngToZpl(pngBuffer, quantity = 1) {
+    const img = await loadImage(pngBuffer);
+    const canvas = createCanvas(img.width, img.height);
+    const ctx = canvas.getContext('2d');
+    ctx.drawImage(img, 0, 0);
+    const imgData = ctx.getImageData(0, 0, img.width, img.height);
+    const { data, width: w, height: h } = imgData;
+
+    const bytesPerRow = Math.ceil(w / 8);
+    const totalBytes = bytesPerRow * h;
+    const buffer = new Uint8Array(totalBytes);
+
+    for (let y = 0; y < h; y++) {
+        for (let x = 0; x < w; x++) {
+            const idx = (y * w + x) * 4;
+            const r = data[idx];
+            const g = data[idx + 1];
+            const b = data[idx + 2];
+            const a = data[idx + 3];
+            // Ink threshold: non-transparent and dark pixels are black (1)
+            const isBlack = a > 128 && (r * 0.299 + g * 0.587 + b * 0.114) < 128;
+            if (isBlack) {
+                const byteIdx = y * bytesPerRow + Math.floor(x / 8);
+                const bitIdx = 7 - (x % 8);
+                buffer[byteIdx] |= 1 << bitIdx;
+            }
+        }
+    }
+
+    const hexMap = Array.from({ length: 256 }, (_, i) => i.toString(16).padStart(2, '0').toUpperCase());
+    let hex = '';
+    for (let i = 0; i < totalBytes; i++) {
+        hex += hexMap[buffer[i]];
+    }
+
+    // Zebra ACS run-length encoding:
+    // Repeated characters are replaced by letters:
+    // 'G'..'Y' = 1..19 repeats
+    // 'g'..'y' = 20..380 repeats (step 20)
+    // 'z' = 400 repeats
+    let acs = '';
+    const re = /([0-9A-F])\1{2,}/g;
+    let match = re.exec(hex);
+    let offset = 0;
+    while (match) {
+        acs += hex.substring(offset, match.index);
+        let l = match[0].length;
+        while (l >= 400) {
+            acs += 'z';
+            l -= 400;
+        }
+        if (l >= 20) {
+            acs += '_ghijklmnopqrstuvwxy'[Math.floor(l / 20)];
+            l = l % 20;
+        }
+        if (l) {
+            acs += '_GHIJKLMNOPQRSTUVWXY'[l];
+        }
+        acs += match[1];
+        offset = re.lastIndex;
+        match = re.exec(hex);
+    }
+    acs += hex.substring(offset);
+
+    const qty = Number(quantity);
+    const pq = Number.isInteger(qty) && qty > 1 ? `^PQ${qty}\n` : '';
+    return `^XA^PW${w}^LL${h}^LS0^FT0,0^GFA,${totalBytes},${totalBytes},${bytesPerRow},${acs}\n${pq}^XZ`;
+}
+