Building a Google Plus inspired image gallery

I recently built a Google Plus inspired image Gallery as an extension for the Alfresco ECM product which won first place in Alfresco developer challenge. Now I’d like to share my insights, explain how I built it and give you a starting point to build your own galleries based on the great G+ gallery design.

Screenshot of Alfresco Gallery Plus extension

The G+ image grid disected

The main focus of this article will be on the image grid. Let’s start with a look on the features of the original G+ image grid to find out what it takes to recreate it. To illustrate the features I am using Thomas Hawk’s G+ gallery which has some amazing images (check it out).

  1. Justifed alignment: All images are displayed in rows and aligned to left and right borders. 
  2. Even spacing: All Images are evenly spaced (6px spacing, i.e. 3px margin around each image).  
  3. Natural order: Images don’t have a special ordering to better fit in the grid. I first thought the perfect alignment was solved with some sort of bin packing algorithm, but that is not the case und is not needed. Images are usually sorted by date from latest to oldest.
  4. Cropped thumbnails: To achieve the justified layout some portions of the images are hidden (cropped). When I first checked out the G+ gallery I saw most of the cropping done with HTML but now it looks like Google generates thumbnails in any arbitrary resolution or cropping on the server-side.
    Both images have a similar URL which only differs in a part which specifies the resolution. The left image contains the path /w300-h150-n-k/ whereas the right image contains the path /w150-h150-n-k/ and therefore is cropped differently on the server side.
  5. Big title image: There is a “title” image or (album cover image) which spans two rows.  
  6. Comment badges: The number of comments is displayed on each image thumbnail. 
  7. Dynamic resolution: The size of the images depends on the width of the browser window. G+ uses small images for smaller browser windows and larger images for wider browser windows, i.e. the image resolution adapts to the screen size. This is espacially useful for mobile devices which have lower resultion than desktop browsers.  
  8. Window resize: The image sizes change and the grid images are re-aligned dynamically when the browser window is resized.  
  9. Dynamic row height: This is a really subtle feature: Depending on the dimensions of the images in one row (e.g. if a lot of images in portrait format) some rows are rendered bigger in height than others. This helps portrait shots not to become too small.
  10. Popup Preview: When you hover over an image an animated popup appears and displays the image in a larger rendition.
  11. Infinite scroll: When you scroll to the bottom of the page more images are loaded automatically.

As far I have seen G+ servers generate different sizes of thumbnails that are used for the window specific thumbnail size, dynamic row height and the preview popup. At times your browser will load 10-20 differently sized thumbnails of an image.

I bet the feature list above is not even complete but it shows the design and engineering effort that went into this seemingly simple gallery grid.

What if you don’t have Google’s infrastructure and manpower?

For my one man competition contribution I couldn’t possibly implement all those features. I axed the double-row-spanning title image, different thumbnail sizes and the preview popup:

  • Only one thumbnail size (120 pixel height, width depending on image aspect ratio)
  • No server side cropping, only a single static thumbnail for each image. 

Still I came pretty close to the original with all the other features.

How does the cropping in the Browser work?

To better understand how the G+ image grid worked I studied the HTML source, which basically looks like this:

The main insight I took from this is they way the images can be cropped: using a DIV with overflow:hidden and changing margin-left of the image to define the x-offset.

How to align the images?

Once I started experimenting I realized that the image alignment is not that hard to do. Let’s say we have three images and are using a fixed thumbnail height of 120 pixel.

image 1: 200x120
image 2: 180x120
image 3: 120x120

Let’s also assume we want to fit these images in a row with a given width of 460 pixels.

I started to build rows of images by adding image by image until the window width is met or surpassed.

1 image: 200 OK (<460)
2 images: 200 + 180 = 380 OK (<460)
3 images: 380 + 120 = 500 surpassed (break >460, 40 pixels too long)

So putting all three images in a row would make it 40 pixel too long. The only thing that needs to be done now is to crop all the images by overall 40 pixels to perfectly align them with the window width.

My naive approach was to simply devide the excess pixels, so 40 pixel / 3 images = 13 pixels/image You have to be careful with the integer division though. To perfectly distribute the 40 pixels you need to crop one additional pixel from one of the images:

cropped image 1: 200 - 14 = 186x120
cropped image 2: 180 - 13 = 167x120
cropped image 3: 120 - 13 = 107x120

If you sum up the witdh now you get exactly 460 pixel. Combining these values with the cropping method from the previous section we get the following HTML code for the three images.

<div style="overflow:hidden; width:186px; height:120px;">
  <img src="image1.jpg" style="width:200px; height:120px";
<div style="overflow:hidden; width:167px; height:120px;">
  <img src="image2.jpg" style="width:180px; height:120px";
<div style="overflow:hidden; width:107px; height:120px;">
  <img src="image3.jpg" style="width:120px; height:120px";

As you can see the negative margin-left is used to center the cropped area horizontally. On thing I left out here is the spacing between images. We have 6 pixels of spacing between each image that have to be added in the inital sum.

This naive approch worked pretty well even with only a single thumbnail size (120 pixels height). In a later iteration I changed the distribution of the crop-pixels though, because when equally distributing the excess pixels, portrait images tend to get too small in width. That’s why I based the crop amount on the image width, with the result that wider images are cropped more than narrow ones.

The server side

As you see above the image layout algorithm needs the widths of the individual images to do it’s mojo. In my Alfresco extension I leveraged the metadata extraction capabilties to get to the thumbnail resolution. The resulting JSON data that is finally generated on the server side is a simple array with all the image data (here is a single image as an example):

{ "thumbs" : [
    "thumbUrl" : "api/node/workspace/SpacesStore/c2f5e06f-ee15-4b0f-9e6c-1f734b4db45f/content/thumbnails/galpThumb120",
    "title" : "2010-05-02, 2 images, IMG_2173 - IMG_2174 - 5096x2320 - SCUL-Smartblend.jpg",
    "twidth" : 300,
    "theight" : 120,
    "description" : "",
    "author" : "Administrator",
    "nodeRef" : "workspace://SpacesStore/c2f5e06f-ee15-4b0f-9e6c-1f734b4db45f",
    "name" : "2010-05-02, 2 images, IMG_2173 - IMG_2174 - 5096x2320 - SCUL-Smartblend.jpg"

The most important information here are the thumbnail dimensions and the thumbUrl that are used for the gallery grid. If you are interested in the details of the server side code in my Gallery+ extension you can have a look in the webscript.

Standalone JQuery demo version

To demonstrate the grid rendering independently of the full the Alfresco extension I moved the grid generation code to a standalone website with the Javascript logic based on jQuery (the original extension is based on YUI). You can see the example in action at the following URL:

If you try it out, make sure to resize your browser to see the dynamic resizing in action. This is not a full jQuery gallery component, sorry – it is just meant to demonstrate the rendering but you are welcome to further extend the code to a full blown jQuery gallery.

Note that the example gallery is not backed by a REST or thumbnail server, it uses this static JSON file as image source for the purpose of demonstration. The page is built using html5 boilerplate and therefore my complete implementation can be found in the scripts.js file.


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