Render HTML to an image

Question

There is a lot of options and they all have their pro and cons.

Option 1: Use an API

• ApiFlash (based on chrome)
• EvoPDF (has an option for html)
• Grabzit
• HTML/CSS to Image API
• ...

Pros

• Execute Javascript
• Near perfect rendering
• Fast when caching options are correctly used
• Scale is handled by the APIs
• Precise timing, viewport, ...
• Most of the time they offer a free plan

Cons

• Not free if you plan to use them a lot

Option 2: Use one of the many available libraries

• dom-to-image
• wkhtmltoimage (included in the wkhtmltopdf tool)
• IMGKit (for ruby and based on wkhtmltoimage)
• imgkit (for python and based on wkhtmltoimage)
• python-webkit2png
• ...

Pros

• Conversion is quite fast most of the time

Cons

• Bad rendering
• Does not execute javascript
• No support for recent web features (FlexBox, Advanced Selectors, Webfonts, Box Sizing, Media Queries, ...)
• Sometimes not so easy to install
• Complicated to scale

Option 3: Use PhantomJs and maybe a wrapper library

• PhantomJs
• node-webshot (javascript wrapper library for PhantomJs)
• ...

Pros

• Execute Javascript
• Quite fast

Cons

• Bad rendering
• No support for recent web features (FlexBox, Advanced Selectors, Webfonts, Box Sizing, Media Queries, ...)
• Complicated to scale
• Not so easy to make it work if there is images to be loaded ...

Option 4: Use Chrome Headless and maybe a wrapper library

• Chrome Headless
• chrome-devtools-protocol
• Puppeteer (javascript wrapper library for Chrome headless)
• ...

Pros

• Execute Javascript
• Near perfect rendering

Cons

• Not so easy to have exactly the wanted result regarding:
  • page load timing
  • viewport dimensions
• Complicated to scale
• Quite slow and even slower if the html contains external links

Disclosure: I'm the founder of ApiFlash. I did my best to provide an honest and useful answer.

Yes. HTML2Canvas exists to render HTML onto <canvas> (which you can then use as an image).

NOTE: There is a known issue, that this will not work with SVG

May I recommend dom-to-image library, that was written solely to address this problem (I'm the maintainer).
Here is how you use it (some more here):

var node = document.getElementById('my-node');

domtoimage.toPng(node)
    .then (function (dataUrl) {
        var img = new Image();
        img.src = dataUrl;
        document.appendChild(img);
    })
    .catch(function (error) {
        console.error('oops, something went wrong!', error);
    });

All the answers here use third party libraries while rendering HTML to an image can be relatively simple in pure Javascript. There is was even an article about it on the canvas section on MDN.

The trick is this:

• create an SVG with a foreignObject node containing your XHTML
• set the src of an image to the data url of that SVG
• drawImage onto the canvas
• set canvas data to target image.src

const {body} = document

const canvas = document.createElement('canvas')
const ctx = canvas.getContext('2d')
canvas.width = canvas.height = 100

const tempImg = document.createElement('img')
tempImg.addEventListener('load', onTempImageLoad)
tempImg.src = 'data:image/svg+xml,' + encodeURIComponent('<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100"><foreignObject width="100%" height="100%"><div xmlns="http://www.w3.org/1999/xhtml"><style>em{color:red;}</style><em>I</em> lick <span>cheese</span></div></foreignObject></svg>')

const targetImg = document.createElement('img')
body.appendChild(targetImg)

function onTempImageLoad(e){
  ctx.drawImage(e.target, 0, 0)
  targetImg.src = canvas.toDataURL()
}

Some things to note

• The HTML inside the SVG has to be XHTML
• For security reasons the SVG as data url of an image acts as an isolated CSS scope for the HTML since no external sources can be loaded. So a Google font for instance has to be inlined using a tool like this one.
• Even when the HTML inside the SVG exceeds the size of the image it wil draw onto the canvas correctly. But the actual height cannot be measured from that image. A fixed height solution will work just fine but dynamic height will require a bit more work. The best is to render the SVG data into an iframe (for isolated CSS scope) and use the resulting size for the canvas.

I know this is quite an old question which already has a lot of answers, yet I still spent hours trying to actually do what I wanted:

• given an html file, generate a (png) image with transparent background from the command line

Using Chrome headless (version 74.0.3729.157 as of this response), it is actually easy:

"/Applications/Google Chrome.app/Contents/MacOS/Google Chrome" --headless --screenshot --window-size=256,256 --default-background-color=0 button.html

Explanation of the command:

• you run Chrome from the command line (here shown for the Mac, but assuming similar on Windows or Linux)
• --headless runs Chrome without opening it and exits after the command completes
• --screenshot will capture a screenshot (note that it generates a file called screenshot.png in the folder where the command is run)
• --window-size allow to only capture a portion of the screen (format is --window-size=width,height)
• --default-background-color=0 is the magic trick that tells Chrome to use a transparent background, not the default white color
• finally you provide the html file (as a url either local or remote...)

You could use PhantomJS, which is a headless webkit (the rendering engine in safari and (up until recently) chrome) driver. You can learn how to do screen capture of pages here. Hope that helps!

You can use an HTML to PDF tool like wkhtmltopdf. And then you can use a PDF to image tool like imagemagick. Admittedly this is server side and a very convoluted process...