Mobile H5 page implements code to generate images

The content of this article is about the code for generating images on mobile H5 pages. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1. You can use canvas to generate images, but since there is already an open source library called html2canvas, I didn’t write it myself in order to save time.

    /**
     * 根据window.devicePixelRatio获取像素比
     */
    function DPR() {
        if (window.devicePixelRatio && window.devicePixelRatio > 1) {
            return window.devicePixelRatio;
        }
        return 1;
    }
    /**
     *  将传入值转为整数
     */
    function parseValue(value) {
        return parseInt(value, 10);
    };
    /**
     * 绘制canvas
     */
    async function drawCanvas (selector) {
        // 获取想要转换的 DOM 节点
        const dom = document.querySelector(selector);
        const box = window.getComputedStyle(dom);
        // DOM 节点计算后宽高
        const width = parseValue(box.width);
        const height = parseValue(box.height);
        // 获取像素比
        const scaleBy = DPR();
        // 创建自定义 canvas 元素
        var canvas = document.createElement('canvas');
        // 设定 canvas 元素属性宽高为 DOM 节点宽高 * 像素比
        canvas.width = width * scaleBy;
        canvas.height = height * scaleBy;
        // 设定 canvas css宽高为 DOM 节点宽高
        canvas.style.width = `${width}px`;
        canvas.style.height = `${height}px`;

        // 获取画笔
        const context = canvas.getContext('2d');

        // 将所有绘制内容放大像素比倍
        context.scale(scaleBy, scaleBy);

        let x = width;
        let y = height;
        return await html2canvas(dom, {canvas}).then(function () {
            convertCanvasToImage(canvas, x ,y)
        })
    }

    /**
     * 图片转base64格式
     */
    function convertCanvasToImage(canvas, x, y) {
        let image = new Image();
        let _container = document.getElementsByClassName('container')[0];
        let _body = document.getElementsByTagName('body')[0];
        image.width = x;
        image.height = y;
        image.src = canvas.toDataURL("image/png");
        _body.removeChild(_container);
        document.body.appendChild(image);
        return image;
    }
    drawCanvas('.container')

2. Since today’s mobile phones all have high-definition screens , so if you don’t handle it, there will be blurry situations. Why will there be blurry situations? This involves the device pixel ratio. devicePixelRatio js provides window.devicePixelRatio to obtain the device pixel ratio.

function DPR() {
        if (window.devicePixelRatio && window.devicePixelRatio > 1) {
            return window.devicePixelRatio;
        }
        return 1;
    }

This DPR function is to obtain the pixel ratio of the device. What should we do after obtaining the pixel ratio?

var canvas = document.createElement('canvas');
        // 设定 canvas 元素属性宽高为 DOM 节点宽高 * 像素比
        canvas.width = width * scaleBy;
        canvas.height = height * scaleBy;
        // 设定 canvas css宽高为 DOM 节点宽高
        canvas.style.width = `${width}px`;
        canvas.style.height = `${height}px`;

        // 获取画笔
        const context = canvas.getContext('2d');

        // 将所有绘制内容放大像素比倍
        context.scale(scaleBy, scaleBy);

3. After obtaining the device pixel ratio, multiply canavs.width and canvas.height by the device pixel ratio, which is scaleBy; At this time, set canvas.style.width and canvas.style.height are the width and height of the dom. Think about it, why do you write this? Finally, when drawing, the drawn content is enlarged by the pixel ratio.

For example, the device width and height of iphone6S are 375 ) So are the design drafts designers usually give you 750*1334?

So if you draw it on a one-to-one basis, it will be blurry on a high-definition screen. Look at the picture and speak 6S DPR=2

## 6plus DPR=3

4. Finally call canvas.toDataURL("image/png"); to assign the value to image.src, because it cannot be used in WeChat Save the picture, so you can only generate the picture file, call the long press function of WeChat to save the picture to the album, as shown in the picture:

# #Related recommendations:

html5 canvas is used to draw arc code implementation

html5 video how to implement real-time monitoring of the current playback time (code)

html5 mobile Meta settings

The above is the detailed content of Mobile H5 page implements code to generate images. For more information, please follow other related articles on the PHP Chinese website!