JavaScript – Use Canvas To Watermark Images

Did you know that you can add a watermark to all of your images that are on the same domain as your site just using JavaScript? Thanks to the <canvas> we can modify images and present in various ways as if they were originally available that way all via JavaScript. The following function is a quick-and-dirty example of how to leverage the canvas to add a watermark to an <img>:

Now let’s use this watermarkImage() function on an image found under the same domain:

Original Watermarked

By using the code below we end up getting the watermarked image above on the right showing the neighborhood of Ensanche Espaillat (Santiago de los Caballeros, Dominican Republic):

var elemImg = document.getElementById('imgToWatermark');
watermarkImage(elemImg, 'Ensanche Espaillat');

Of course, this simple function is not a fully featured solution for adding watermarks to our images. For example the following cases may occur:

  • We just want to create a watermarked image from an image URL.
  • We want to change the background color of the watermark.
  • We want to use a logo image as the watermark.
  • etc., etc.

If the watermarkImage() function works the watermark will be added to the image, but if not the image will not be changed. This is done on purpose to avoid strange behaviour on failure. There are two main reasons why this would fail:

  1. The image you want to watermark is not under the same domain as the page running the JavaScript code.
  2. The browser running the code doesn’t support adding images and/or SVGs to a <canvas>.

There is actually a great solution out there called watermark.js which will probably provide all of the options that you need to watermark your images.

Of course, if you would like to write your own code, my gist at the beginning of this post should be a pretty good start. Have fun and happy coding! 😎

JavaScript Snippet – Get Video Frame As An Image

Lately I have dedicated a little more time to developing an electron app which saves a frame of every video it encounters as PNG images:

JW Videos with images captured via a video element and a canvas element.
JW Videos with images captured via a video element and a canvas element.

Believe it or not the code to turn a frame of a video into an image element is not terribly complicated. Below is a simple function that can be used to get the an image element corresponding to a specific frame in a video:

This getVideoImage() function takes three arguments:

  1. path {string}:
    The path to the video. In the case that you are using this code in a web page this must be a video within the same domain.
  2. secs {number | function(duration): number}:
    If this is a non-negative number this will indicate the time of the frame to capture in seconds. If this is a negative number this will indicate the time of the frame from the end of the video to capture in seconds. If this is a function it will be passed the duration as a number and the return value should be a number (positive or negative) which indicates the time of the frame that should be captured.
  3. callback {function(img, currentTime, event)}:
    The function which is called either after loading the frame’s image successfully or after getting an error. The first argument passed will be the Image object that is created (if no error occurred). The second argument passed will be the actual time in seconds used to pull the frame’s image (if no error occurred).
    The third argument will either be a seeked event or an error event.

This function can be used as is or can be modified. It is important to note that unless the video is in the exact same domain as the web page this code will NOT work due to the canvas being tainted. You can paste the following code into the code editor on this W3Schools sandbox page to see an example of getting multiple frames:

Let me know if you have any questions or suggestions and as usual, enjoy the free code! 😎

JavaScript Snippet – toggleFullscreen()

Making your web app toggle back and forth between full screen mode is a simple enough task as long as you do some feature testing. You can also use this handy function which will step through the 4 different feature tests:

This code is based off of documentation found here on MDN. I personally am using this code on my Big It! (a small web app that simply shows user entered text as big as possible). Feel free to use this function in any of your projects. 😎