如何在 Chrome 扩展程序中捕获单个 HTML 元素的屏幕截图？

我知道有一个捕获可见选项卡 http://developer.chrome.com/extensions/tabs.html#method-captureVisibleTab，但是如何剪切选项卡的屏幕截图，以便只留下一个 HTML 元素？

为此你需要消息上 https://developer.chrome.com/extensions/runtime#event-onMessage, 发信息 https://developer.chrome.com/extensions/tabs#method-sendMessage, and 捕获可见选项卡 https://developer.chrome.com/extensions/tabs#method-captureVisibleTab. The onMessage是一种方法chrome 运行时 https://developer.chrome.com/extensions/runtime, sendMessage, and captureVisibleTab都是方法tabs https://developer.chrome.com/extensions/tabs.

Manifest

在清单文件中，您必须添加tabs, and <all_urls>给你的权限manifest https://developer.chrome.com/extensions/manifest文件。如果没有权限，这将无法工作。

"permissions": [
    "tabs",
    "<all_urls>"
],

内容脚本

在您的内容脚本文件中，您需要添加以下内容。这允许您与后台页面通信以获取活动选项卡的屏幕截图。

chrome.runtime.sendMessage({ chromeAction: "screenshot" }, function (imageString) {
    console.log(imageString);
});

背景脚本/页面

这就是奇迹发生的地方。

chrome.runtime.onMessage.addListener(function (request, sender, sendResponse) {
    if (request.chromeAction === "screenshot") {
        createScreenshot(function (dataURL) {
            createImage(dataURL);
        });
        return true;
    }
});

// here we create a new image
function createImage(dataURL) {
    // create a canvas
    var canvas = createCanvas(500, 500);
    // get the context of your canvas
    var context = canvas.getContext('2d');
    // create a new image object
    var croppedImage = new Image();

    croppedImage.onload = function() {
        // this is where you manipulate the screenshot (cropping)
        // parameter 1: source image (screenshot)
        // parameter 2: source image x coordinate
        // parameter 3: source image y coordinate
        // parameter 4: source image width
        // parameter 5: source image height
        // parameter 6: destination x coordinate
        // parameter 7: destination y coordinate
        // parameter 8: destination width
        // parameter 9: destination height
        context.drawImage(croppedImage, 10, 10, 300, 300, 0, 0, 250, 250);

        // canvas.toDataURL() contains your cropped image
        console.log(canvas.toDataURL());
    };
    croppedImage.src = dataURL; // screenshot (full image)
}

// creates a canvas element
function createCanvas(canvasWidth, canvasHeight) {
    var canvas = document.createElement("canvas");

    // size of canvas in pixels
    canvas.width = canvasWidth;
    canvas.height = canvasHeight;
    return canvas;
}

// calling the captureVisibleTab method takes a screenhot
function createScreenshot(callback) {
    // you can have two image formats (jpeg and png)
    // for jpeg use { format: "jpeg", quality: 100 } (you can adjust the jpeg image quality from 0-100) 
    // for png use { format: "png" }
    chrome.tabs.captureVisibleTab(null, { format: "png" }, callback);
}

Cropping

为了更好地了解drawImage画布方法 阅读全文文档 https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D#drawImage.