CSS – Local File System Paths On Windows

When working with Windows you probably have noticed that local file paths use the backslash as the path separator. Although this is the case, even on Windows, in CSS you can use the forward-slash as a path separator to reference an image. If you want to use the forward-slash though, you must escape it with another backslash. Here are two examples:

#elem1 {
  background-image: url("relative-path\\to\\file.ext");
#elem2 {
  background-image: url("C:\\path\\to\\file.ext");
#elem3 {
  background-image: url("relative-path/to/file.ext");
#elem4 {
  background-image: url("C:/path/to/file.ext");

As you can see above I have three different ways to add a background image:

  1. url("relative-path\\to\\file.ext")
    For the element of ID “elem1” I am specifying that the background image is found at the relative path of “relative-path\to\file.ext”. It is important to note that I am escaping the backslashes (path separators) with an additional backslash.
  2. url("C:\\path\\to\\file.ext")
    Just as in the previous example, if we need to reference something using an absolute path while maintaining backslashes as path separators, we need to escape all backslashes with a preceeding backslash.
  3. url("relative-path/to/file.ext")
    For the element of ID “elem2” I am specifying that the background image is found at the relative path of “relative-path/to/file.ext”. Even though the normal path separator on Windows is a backslash you can use a forward slash.
  4. url("C:/path/to/file.ext")
    Just as in the previous example, if we need to reference something using an absolute path we can use a forward slash as the path separator.

This may seem like a minor issue but when developing Electron apps it may come in handy. Have fun and as always, happy coding! 😎

JavaScript – Create Gradient Image Using Canvas

The <canvas> can be quite useful, especially if you want to modify how images look on your site. For example, if you want to make an image appear as a gradient starting at the top with an alpha of 0 and then linearly fading to the image’s full alpha value you can use JavaScript along with the <canvas>:

  1. Make Sure The Image Is On Your Domain

    If the image that you are trying to modify is not on your site’s domain you will not be able to modify it using the canvas unless the image is CORS enabled or unless you are pulling the image via an <input type="file">.
  2. Create the Canvas & Add the Image To It

    var image = document.getElementById('id_of_image');
    var canvas = document.createElement('canvas');
    canvas.width = image.width;
    canvas.height = image.height;
    var context = canvas.getContext('2d');
    context.drawImage(image, 0, 0);
  3. Retrieve & Modify Alpha Values

    The canvas’ 2D context object provides a function called getImageData() which will return another object. This object’s data property is an array of the rgba values that make up the image. In other words, the first four values in the array represent the top-most and left-most pixel in the image while the second four values represent the next pixel going from left-to-right and top-to-bottom. Since we want to create a fading gradient effect we need to determine the y value at all times to then figure out the alpha which will depend on the maxY value. NOTE: the alpha value ranges from 0 to 255.
    var imageData = context.getImageData(0, 0, image.width, image.height);
    var d = imageData.data;
    var maxY = image.height;
    for (var y, alpha, i = 3, l = d.length; i < l; i += 4) {
      y = Math.floor(i / 4 / image.width);
      alpha = Math.round(255 * y / maxY);
      d[i] = Math.min(d[i], alpha);
  4. Reset Canvas & Set Updated Image Data

    Since we have played around with the alpha values we don't want to simply draw the image data on top of the canvas. Instead we need to clear that canvas and then draw the updated image:
    context.clearRect(0, 0, image.width, image.height);
    context.putImageData(imageData, 0, 0);
  5. Use Canvas Data URL As Image's Source

    The canvas has a handy function called toDataURL() which among other things can be used to pull a PNG version of our image which will preserve the transparency. All we need to do is assign this data URL to image's src property and then we are done:
    image.src = canvas.toDataURL();
  6. Try It Out

    Feel free to try this code out on your site or you can see how it all works together by using the button below to choose an image to convert to a gradient. After the image is created you should be able to click on it to open it up in a new tab.
So now you may be wondering how the above example works, right? Click here to see the source code. Of course, that is only a very brief example of what you can do. Just as this post from the maker of watermark.js inspired me, I am sure that you can think of many more ways to use the canvas to add amazing effects to your images. As always, feel free to make the code your own. Happy coding! 😎

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! 😎