條形碼解碼首先要做定位,找到條形碼在圖像中的區域。但是有時候受圖像質量影響,算法找尋的區域可能產生偏差,最後導致解碼失敗。這個時候,可以嘗試手動選擇區域,來輔助條形碼的識別。
Web Canvas框選識別條形碼
需求:
- 在網頁中加載條形碼圖像。
- 使用鼠標框選出條形碼的區域。
- 識別區域中的條形碼。
Web Canvas圖像顯示
創建input
選擇文件。獲取文件對象之後,使用FileReader
讀取並繪製到canvas
上。如果讀取的圖像太大,可以設定一個最大縮放值:
<input type="file" id="barcode-file" onchange="loadfile()" accept=".jpg,.jpeg,.png,.bmp" />
function loadfile() {
let img = new Image();
var reader = new FileReader();
reader.onload = function (evt) {
img.onload = function () {
if (img.width > maxLength) {
img.height = maxLength * img.height / img.width
img.width = maxLength;
}
else if (img.height > maxLength) {
img.width = maxLength * img.width / img.height
img.height = maxLength;
}
canvas.width = img.width;
canvas.height = img.height;
overlay.width = canvas.width;
overlay.height = canvas.height;
context.drawImage(img, 0, 0, img.width, img.height);
};
img.src = evt.target.result;
};
reader.readAsDataURL(name.files[0]);
}
鼠標框選矩形區域
要在圖像上面繪製出一個矩形框,需要創建另外一個canvas,並把它放置到圖像的上面:
<style>
#container {
position: relative;
}
#imageCanvas {
position: relative;
z-index: 1;
}
#overlay {
position: absolute;
top: 0;
left: 0;
z-index: 2
}
</style>
<div id="container">
<canvas id="imageCanvas"> </canvas>
<canvas id="overlay"></canvas>
</div>
接下來監聽鼠標事件,在鼠標移動的時候繪製矩形:
function clearOverlay() {
overlayCtx.clearRect(0, 0, overlay.width, overlay.height);
overlayCtx.strokeStyle = '#ff0000';
overlayCtx.lineWidth = 5;
}
overlay.addEventListener('mousedown', e => {
startX = e.offsetX;
startY = e.offsetY;
isDrawing = true;
clearOverlay();
overlay.style.cursor = "crosshair";
});
overlay.addEventListener('mousemove', e => {
if (isDrawing) {
clearOverlay();
overlayCtx.beginPath();
overlayCtx.rect(startX, startY, e.offsetX - startX, e.offsetY - startY);
overlayCtx.stroke();
}
mousePosition.innerHTML = "Cursor: (" + e.offsetX + ", " + e.offsetY + ")";
});
overlay.addEventListener('mouseup', e => {
if (isDrawing) {
isDrawing = false;
mousePosition.innerHTML = "Cursor: (" + e.offsetX + ", " + e.offsetY + ")";
region.innerHTML = "Decode a region: (" + startX + ", " + startY + ", " + e.offsetX + ", " + e.offsetY + "). ";
overlay.style.cursor = "default";
}
});
條形碼識別
在鼠標左鍵擡起的時候,可以獲得矩形最終的區域。把這個區域設置到接口中:
overlay.addEventListener('mouseup', e => {
if (isDrawing) {
isDrawing = false;
mousePosition.innerHTML = "Cursor: (" + e.offsetX + ", " + e.offsetY + ")";
region.innerHTML = "Decode a region: (" + startX + ", " + startY + ", " + e.offsetX + ", " + e.offsetY + "). ";
overlay.style.cursor = "default";
// Decode a region of the barcode image
(async () => {
let settings = await barcodereader.getRuntimeSettings();
settings.region.regionLeft = startX * 100 / overlay.width;
settings.region.regionTop = startY * 100 / overlay.height;
settings.region.regionRight = e.offsetX * 100 / overlay.width;
settings.region.regionBottom = e.offsetY * 100 / overlay.height;
settings.region.regionMeasuredByPercentage = 1;
barcodereader.updateRuntimeSettings(settings);
try {
let decodingStart = Date.now();
await barcodereader.decode(name.files[0]).then((results) => {
let decodingEnd = Date.now();
let txts = [];
try {
for (let i = 0; i < results.length; ++i) {
txts.push(results[i].BarcodeText);
}
let barcoderesults = txts.join(', ');
catch (e) {
}
});
} catch (error) {
alert(error);
}
})();
}
});
現在可以很方便的選中一塊條形碼的區域來查看解碼結果。
源碼
https://gist.github.com/yushulx/b21d0919a1e92e0a320929799a99a5de