What you need to know about the JPEG format

Most users know the JPEG image format at least by name, referring here to the abbreviation itself. The fact that JPEG stands for a group of experts (Joint Photographic Experts Group) rather falls under the heading of useless knowledge and can be safely overlooked.

JPEG or JPG?

This question actually comes up for many. This can also be seen from the fact that people often search for “difference JPG JPEG”. Here is the relatively simple answer. If you know the file extension rather in the form .jpg, it is probably because you grew up with Windows. Since DOS and older Windows versions only provided three characters for the extension, it was shortened to .jpg. This tradition has survived in the Windows environment to this day. Mac users, on the other hand, are familiar with the unabridged extension. Conclusion: The terms JPEG and JPG are basically synonymous.

Special knowledge. There are even two other file extensions for the format: .jpe and .jfif. The latter goes back to the official name. That is JPEG File Interchange Format, or JFIF for short.

Why is the JPEG format so widely used?

Let’s move from the name to the most important facts. The JPEG format is, of course, the image format par excellence when it comes to the degree of distribution. No other format is so widely used to store photos and other images such as graphics or illustrations. When you send JPG files, you can be absolutely sure that the recipient will have no problems opening and processing them.

The JPEG format came into being back in 1992. But why did it become so popular? The main reason is obvious. No other format could compress images so efficiently at the time. In addition, there was and still is a high degree of individual leeway in the weighting of quality and storage size. That makes the format very flexible. In Adobe Photoshop, for example, you can choose between 12 quality levels.

JPEG options in Adobe Photoshop

Especially in the early days of the Internet age, it was important to keep files as small as possible for better portability. Of course, this is no longer true to the same extent today, but the JPEG format is still far from being obsolete.

How does JPEG compression work?

This article is primarily intended to deal with practical issues, but we cannot avoid going into a few technical basics of the format in more detail.

The JPEG format uses an intelligent combination of lossy and lossless compression to save storage space. The latter are always unproblematic, since only redundant, i.e. unnecessary, data are removed. In other words, the file becomes smaller, but there is no loss of information. Lossy compression, on the other hand, as the name implies, basically loses information. So the memory gain comes at a price with this method. However, as long as the loss of information cannot be detected even when looking closely, there is no loss of quality in practical terms. This means that data compression is successful if there is no visible reduction in quality, but storage space has still been saved. This is exactly what the JPEG format can do excellently for certain types of images.

Which image types these are, and which graphic structures are less suitable for JPEG compression, are explained in the following section.

For which types of images is the JPEG format suitable?

Surely you have already guessed that photos in particular are suitable for the JPEG format, after all most digital cameras allow storage in this format. Photography enthusiasts may prefer RAW formats or the TIF format, but for the average user the JPEG format is quite sufficient. But why is JPEG suitable for photos, but less so for uniform graphics or logos? Well, as a rule, photos have a fine texture and a wide range of colors. The more color and texture details an image has, the larger the file size. So there is lots of potential for saving with photos.

Graphics, on the other hand, are often low on detail, feature clearly delineated areas, and get by with a few shades of color. The savings potential here is correspondingly low. In addition, high-contrast edges can be a problem for JPEG compression. If the highest compression levels are selected, unsightly artifacts naturally appear in all image areas. You know this probably from meme images that are ‘broken-compressed’. Texts are often crisscrossed then and visible block boundary artifacts occur.

Interim summary

We note that the JPEG format is well suited for photos and finely structured images with high color depth. Strictly speaking, the JPEG format can store 16,777,216 different colors (24-bit). Although there are formats (e.g. TIFF) that can store images with 48-bit color depth (which corresponds to approx. 281 trillion colors), the difference is only visible to trained eyes, and only if highly professional graphics cards and monitors are available that can reproduce this color depth (commercially available devices cannot).

JPEG is unsuitable for low-detail graphics with few colors. In addition, JPEG does not support transparent pixels via alpha channel, which are needed for cropped objects or logos, for example. If you want to save an image with transparency, you will have to resort to formats such as PNG or WebP (more on this below).

Color spaces and color profiles

Here come some further remarks on the subject of colors. The JPEG format works in the RGB color space in the vast majority of cases, if only because the output on screens is always in RGB. In principle, however, JPEG can also handle the CMYK color model. So you can save JPEG images so that they are directly optimized for printing. In addition, the JPEG format enables professional color management by supporting ICC color profiles.

Color picker with color code output in Adobe Photoshop (bottom right CMYK values, left RGB)

What metadata formats are supported by JPEG?

The average user will have heard of metadata in the context of photos, but will rarely have made practical use of it. In principle, you can already view and describe the most important metadata fields, e.g. title, image description, copyright, etc., using simple on-board tools such as Windows Explorer or Photos for macOS. In this context, the standards IPTC-IIM and XMP supported by JPEG should be mentioned. These metadata formats are primarily used for content indexing. Exif was developed for the automatic documentation of technical data such as date of recording, camera manufacturer or geolocation. Exif is also fully supported by the JPEG format. This is not a matter of course. The PNG format supports embedding metadata, but not according to the mentioned standards. In this respect, JPEG has a clear advantage.

When is a conversion from JPEG to another format helpful?

JPEG is a final format for digital use. If you have a JPG file, it rarely makes sense to convert it to another format. Of course, you can convert a JPG file to TIFF or PNG, but the result would be just a larger file that has not gained any information. In other words, compression is a one-way street. Once the information is lost, it cannot be recovered by converting it to another format. That should go without saying. From professional experience, however, I can say that it is not.

Exceptions: Under very specific circumstances, a conversion from JPEG to PNG or TIFF can be useful after all. This is precisely the case when the JPEG output file is a uniform graphic with few colors. In that event, the file can be compressed even more, if necessary, without losing further quality, since the target formats mentioned also support indexed color palettes with less than 256 colors (8-bit) down to 1-bit color depth. With JPEG, on the other hand, a file is saved with 8 bits per color channel (RGB) or, in the case of grayscale, with a total of 8 bits of color depth, even if it is a line drawing of two colors such as black and white. So in short in these special cases, some memory can be saved.

In addition, it is not uncommon to convert JPG files to PDF or TIF format for print optimization. However, this is not mandatory because, as mentioned above, the JPEG format also supports the CMYK color model.

Conclusion

If you want to use an accessible file type for storing and sharing photos, and at the same time you don’t want to waste storage space, the JPEG format is a very good choice. Also keep in mind that no third-party programs are needed to open or do simple edits, as the on-board tools of common operating systems (e.g. Paint on Windows or Preview on macOS) are already designed for this. So you don’t have to buy expensive licenses for image editing programs like Adobe Photoshop, if you just want to view JPG files, crop them or change the image size (pixel dimensions). Of course, when managing a larger media pool, you cannot avoid using professional image management software.

When purchasing such software, be sure to check that all common image formats are supported and that conversion to any of them is possible.

One final thing: The JPEG format was developed 30 years ago. It is not surprising, then, that there is a successor format, WebP, which is perhaps even more suitable for universal use, since it also handles transparency while achieving even better compression performance, especially when compressing low-detail, uniform image parts. However, the disadvantage is that this format has a low dissemination rate so far and may not be opened and edited by every user. Of course, that may change in the next few years. We will be happy to keep you up to date on this in our blog.

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