Resize and lazy load images in Rich Text fields in Sitecore

Often the imagery that is uploaded in Sitecore is of way higher resolution than what is required, in an attempt to produce high quality pages.
While these are uploaded onto image fields in sitecore, we can resize the images when we output them using suitable html helpers. Refer:Render Responsive Image Glass Html Helper with Sitecore Image Processor Module.

We also needed a way to achieve this for images inserted in the rich text field. Especially seeing as we had content authors adding images sized 3000×3000 in there, and page sizes soaring up to 80mb each at times!
We implemented a solution with the bLazy plugin.
The plugin documentation will tell you, that once we include the script and initialize it with the right parameters, all it takes is adding the right attributes and class into our html, to enable lazy load of images – resized if need be for each breakpoint as we determine.

Here’s what we implemented for this:

  • We added a patch to the renderField pipeline, which would transform all img tags as we needed – add the required class which is to be used as the selector for the blazy plugin, and also the breakpoint specific resized image url attributes. In addition, we also replaced the img tag source with a base64 encoded transparent gif so it wouldn’t do any extra requests.
  • We added a reference to the bLazy plugin, and initialized it with the parameters suitable to our requirements.

The pipeline code:

using HtmlAgilityPack;
using sbdshared._Classes.SBDShared.Helpers;
using Sitecore;
using Sitecore.Diagnostics;
using Sitecore.Pipelines.RenderField;
using System;
using System.Web;

namespace MySite.Customizations.Customized_Sitecore
    public class ProcessRichTextImages
        /// <summary>
        /// Processes the specified arguments.
        /// </summary>
        /// <param name="args">The arguments.</param>
        public void Process(RenderFieldArgs args)
                if (args != null && (args.FieldTypeKey != "rich text" || string.IsNullOrEmpty(args.FieldValue) || !Context.PageMode.IsNormal))

                if (args != null && args.Result != null && !string.IsNullOrWhiteSpace(args.Result.FirstPart))
                    HtmlDocument doc = new HtmlDocument { OptionWriteEmptyNodes = true };

                    if (doc.DocumentNode != null)
                        // Search for all img tags
                        HtmlNodeCollection imgTag = doc.DocumentNode.SelectNodes("//img");
                        if (imgTag == null || imgTag.Count == 0)

                        foreach (HtmlNode node in imgTag)
                            if (node.Attributes["src"] != null && node.ParentNode != null)
                                string imgUrl = node.Attributes["src"].Value;

                                node.Attributes.Add("data-responsive-bkg-desktop", ImageHelper.GetResizedExternalUrl(imgUrl, 580, 0));
                                node.Attributes.Add("data-responsive-bkg-tablet", ImageHelper.GetResizedExternalUrl(imgUrl, 350, 0));
                                node.Attributes.Add("data-responsive-bkg-mobile", ImageHelper.GetResizedExternalUrl(imgUrl, 200, 0));
                                node.Attributes["src"].Value = "/_Images/loader.gif";
                                string currentClass = node.Attributes["class"] != null ? node.Attributes["class"].Value : "";
                                node.Attributes.Add("class", (string.IsNullOrWhiteSpace(currentClass) ? "" : currentClass + " ") + "responsive-bkg");
                                if (node.Attributes["style"] != null && !string.IsNullOrWhiteSpace(node.Attributes["style"].Value)
                                    && (node.Attributes["style"].Value.Contains("width") || node.Attributes["style"].Value.Contains("height")))

                        // Replace the Rich Text content with the modified content
                        args.Result.FirstPart = doc.DocumentNode.OuterHtml;
            catch (Exception ex)
            { Log.Error("Error in MySite.Customizations.Customized_Sitecore.ProcessRichTextImages:" + ex.Message, ex); }

We used the Sitecore Image Processor Module to get the resized sitecore images.

        public static string GetResizedSitecoreUrl(string imageUrl, int width, int height, bool centerCrop = true)
            if (string.IsNullOrWhiteSpace(imageUrl)) return string.Empty;

            if (height > 0)
                imageUrl = imageUrl + (imageUrl.Contains('?') ? "&h=" : "?h=") + height;
            if (width > 0)
                imageUrl = imageUrl + (imageUrl.Contains('?') ? "&w=" : "?w=") + width;
            if (centerCrop)
                imageUrl = imageUrl + (imageUrl.Contains('?') ? "&" : "?") + "usecustomfunctions=1&centercrop=1";

            return imageUrl;

Additionally – we also removed any height / width attributes added to the images in the rich text field – since the design in our case allowed for it.
We added this to the renderField processor:

        <processor type="MySite.Customizations.Customized_Sitecore.ProcessRichTextImages, MySite" />

Now that we had the html the way we required it for the blazy plugin, all that was needed was to include the script and initialize it:

<script src="blazy.js"></script>

And the following needed to go into the $(document).Ready() method.

var bLazy = new Blazy({
	selector: '.responsive-bkg',
    offset: 200,
    breakpoints: [{
			width: 768,
			src: 'data-responsive-bkg-mobile'
		      width: 980
		    , src: 'data-responsive-bkg-tablet'
	src: 'data-responsive-bkg-desktop'

The images would now load in a lazy-load fashion based on the current viewport breakpoint, when the cursor was within 200px of the image iteself. (Refer bLazy Documentation)

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