Image Metadata

Introduction

"A picture is worth a thousand words" is a common idiom that we all understand instinctively - a picture is often a better way to convey a message than words. Is it any wonder that the web and mobile applications extensively use images to convey feelings, moods, ideas or stories?

Images are a wonderful way to convey data and information, but they can take longer to download, and due to their size, can use a lot of data. In this best practice, we are looking at ways to reduce the data cost of images – without harming the image quality delivered to your end users.

Background

If you can remove 3-10% of the image transmission size, images will typically download faster, and be seen faster – making your application appear to be faster. How can you improve image download without harming image quality? Many photos contain extensive metadata inserted by the digital camera. For example, this photo of Bern, Switzerland:

This image is 3 MB, but has a large amount of metadata contained in the image that has nothing to do with the quality of the photograph.

The Issue

Digital cameras record a great deal of information about the photograph. For example, the photograph above contains the following metadata:

Camera make :

   Motorola

Camera model :

   XT1097

Date/Time :

   2016/05/20 17:07:04

Resolution :

   4160 x 2340

Flash used :

   No

Focal length :

   4.2mm

Digital Zoom :

   2.240x

Exposure time :

   0.0012 s (1/820)

Aperture :

   f/2.2

ISO equiv. :

   50

Whitebalance :

   Auto

Metering Mode :

   average

Exposure :

   Manual

Exposure Mode :

   Auto bracketing

GPS Latitude :

   ? ?º ' "

GPS Longitude :

   ? ?º ' "

There is also a thumbnail of the image saved in the metadata.

Note that none of the information here is required for the image to render properly in a browser or in an application. In general, your customers do not care about the exposure, use of flash, etc.

There is also the potential for location data to leak in the metadata – a potential privacy issue.

Simply removing this metadata will reduce the image size, but will not change the quality of the image to your users. Seems like an easy win, right? Many image processing tools have functions that will remove metadata automatically – so you can include this function when building any image on your servers.

Best Practice Recommendation

Video Optimizer recommends that you remove all EXIF metadata from your images before serving them to your customers. When the above image of Bern is run through a tool to remove metadata, the file becomes 6.7% smaller – from 3 MB to 2.8MB. That is a significant savings of 200 KB – without any degradation of the image!

If any % of data can be saved, the image will appear in the Image Metadata best practice:

Clicking on an image will open a popup window to show the metadata, or the image can be saved, and can be viewed in any image viewing software.