JPGExtra – Sample JPG File
To demonstrate what JPGExtra does and why it's needed, let's look inside a typical JPG file to see the types of "extras" (or "metadata") and how JPGExtra can optimize such files by cleanly removing those extras.
Sample JPG File
This is a fairly typical photographic image that one might see on any website:
- File name: BushOrchids240x320.jpe
- Image size: 240 x 320 pixels
- File size: 41,314 bytes
The photo was taken with a digital camera and resized with Adobe Photoshop. The resulting image fits on a web page without scrolling and has sharp detail and good color.
The problem is not what we CAN see but what we CAN'T see. Let's take a peek inside the file...
Revealing What's Inside
Decoding the innards of the above sample JPG file reveals all sorts of "extras" (or "metadata"):
- JFIF Header: Small bit of data, mainly to indicate that the file is in traditional JFIF format.
- Exif data: Date and time of the original photo, shutter speed, aperture and other photographic settings, even the make and model of the camera.
- Thumbnail(s): Miniature version of the photo, which is used by some software for fast previews. Much smaller than the main image (only 96 x 128 pixels) and lower quality (note the "ripples"). The image editing software, Adobe Photoshop, adds two identical thumbnails. I've seen some JPG files with three!
- Photoshop data: Various other information added by Adobe Photoshop.
- Adobe XAP data: More info from Photoshop, this time in XML format, and often containing only 1/4 data and 3/4 blank space.
- ICC profile: Color profile for more accurate color rendition on different systems (eg. screens, printers). Ignore the actual "rainbow", which I added to show that this block has something to do with color.
- Adobe data: Tiny bit of additional data added by Adobe Photoshop.
- Main image: Finally, after all of the "extras", we reach the actual image data.
This JPG file contains about 44% of "extras", the bulk of which are downloaded and then simply discarded by most web browsers. It's a real waste!
NOTE: The sizes of the "blocks" in the above image represent their relative sizes within the file, with the important sections colored with green borders and the "extras" colored with yellow borders. It's easy to see that almost half of the file is waste.
JPGExtra to the Rescue!
Here's the innards of the same JPG file after optimization with JPGExtra:
- JFIF Header: The header is still there, completely unchanged.
- Main image: The image is still there, completely unchanged.
- Extras? All of the extras (18,000 bytes!) have been cleanly removed.
On this sample, JPGExtra achieved savings of 44%, which is pretty impressive! Of course, I've ensured that the original JPG file contained a good selection of "extras", but it's a fairly realistic example.
On smaller images, even more impressive savings are common because the "extras" constitute a greater percentage of the