exif – Terence Eden’s Blog

Extracting Video from Motion Photos on Linux

@edent — Sun, 28 Dec 2025 12:34:22 +0000

Modern Android cameras can take "Motion Photos". They capture a few seconds of video from before and after you hit the shutter button. You can then either select the bit of the photo where no-one is blinking, or you can send the whole thing as a little movie.

Some apps (like WhatsApp) will play the motion photo when the image is selected, others will just show a static image.

So how do you extract the movie from the image using Linux?

Step one, let's take a look at the EXIF metadata in the image. Here's what running exiftool photo.MP.jpg gets:

Motion Photo                    : 1
Motion Photo Version            : 1
Motion Photo Presentation Timestamp Us: 866808
Directory Item Mime             : image/jpeg, image/jpeg, video/mp4
Directory Item Semantic         : Primary, GainMap, MotionPhoto
Directory Item Length           : 46353, 2106347
Directory Item Padding          : 0
MPF Version                     : 0100
Number Of Images                : 2
MP Image Flags                  : (none)
MP Image Format                 : JPEG
MP Image Type                   : Undefined
MP Image Length                 : 46353
MP Image Start                  : 2570425

That can be cross-referenced with the Motion Photo metadata specification.

We can confirm this is a Motion Photo, Version 1. The video portion at 866,808 microseconds (about 0.8 seconds) is where the main photo is taken from.

The file starts with the image, then a GainMap (for HDR), and then the video.

Somewhat obtusely (in my opinion) the Directory Item Length only shows "secondary media items" - in this case, the GainMap and Video.

The filesize is 4,723,125 bytes, which equals the sum of the three values; 46,353 + 2,106,347 + 2,570,425.

So, to get the MP4 video, we need to extract the last 2,106,347 bytes. This can be double-checked by taking the filesize and subtracting the MP Image Start and the MP Image Lengths (4,723,125 - 46,353 - 2,570,425 = 2,106,347).

The extraction can be done with dd but it's probably just as easy to use tail to read the last N bytes of a file:

tail -c 2106347 photo.MP.jpg > video.mp4

You can verify that the video is valid by running ffmpeg -i video.mp4 - the output will be lower resolution than the photo and will only be a few seconds long. It will play in VLC or any other standard player.

Try It Yourself

Here's one of my motion photos - it should present in your browser as a still image, but run the above code to extract the video.

Click the photo to download the full version rather than the optimised one.

Sources

For other adventures in Motion Photo exploration, take a look at:

Should you embed alt text inside image metadata?

@edent — Fri, 28 Jul 2023 11:34:13 +0000

Not everyone can see the images you post online. They may have vision problems, they may have a slow connection, or they might be using a text-only browser. How can we let them know what the image shows? The answer is alt text. In HTML we can add a snippet of text to aid accessibility.

For example

Most social networks will let users add alt text to help describe their images. Brilliant!

But... People don't always add alt text when they upload an image. They may not realise it is helpful, or they don't know how to write a good description, or they may not have time to write something suitable. This leads to a frequently asked question: "Should I embed the alt text inside the image file? That way, whenever people share the image the alt text will automatically be attached!"

Here's my attempt to answer that.

Short answer

No.

Long answer

Nooooooooooooo!

Reasoned Answer

It is complicated.

On a technical level, yes. Most modern image formats allow you to add image metadata - known as EXIF. This EXIF commonly contains GPS location, timestamps, make and model of camera, etc. Here's an example from one of my photos:

There are a pre-defined list of acceptable EXIF tags one of which is "ImageDescription", which is defined as:

A character string giving the title of the image. It may be a comment such as "1988 company picnic" or the like. Two-byte character codes cannot be used. When a 2-byte code is necessary, the Exif Private tag UserComment is to be used.

So, there you have it. Yes you can embed text inside an image. With the right software you can read it. So, technically, you can stuff alt text in there and have a website automatically add it to the alt attribute.

But this is only half the story. What an image represents is highly context dependent.

In her article about using AI to automatically provide image captions, Léonie Watson makes that point that alt text needs to be:

provided by a content author who knows exactly what's in the image, why its being used and the context its being used in.

Take this very real photo of a famous actress who I just got an AI to dream up.

What's a suitable alt text for this image?

Famous actress Claire Flumptron standing on the red carpet.
Claire Flumptron shows off her brand new arm tattoo which has strange mystic runes covering her forearm.
Designer Giovanni Tuccini's new red dress has a small strap over the shoulder.
AI image where the hand is strangely deformed and the neck looks twisted beyond human endurance.
Smiling woman with cropped blonde hair and heavy eye makeup.
Last known photo of the Mayor (right). His blurry face is smiling.
...

There's no "right" answer. It depends on what the image is being used to illustrate.

A picture may be worth a thousand words. But those thousand words depend on context.

Update Here's another good example of embedded alt text failing badly.

Terence Eden

@Edent@mastodon.social

Whenever people talk about embedding alt text into images, I remember that lots of gif search services already try to do that.

Here's BlueSky's gif service. I searched for some popular memes. Each had alt-text baked in.

Take a look and tell me if you think that the embedded text conveys the sentiment of the image? If you couldn't see the animation, would you understand what was going on from that alt?

2025-09-07, 21:11 12 boosts 22 favorites

For more information on writing good alt text, see Harvard's Digital Accessibility blog and Design102's guide.

Reducing GPS accuracy in photos

@edent — Thu, 01 Oct 2020 11:56:50 +0000

Here's a quick one-liner to reduce the precision of location stored in a photo's EXIF metadata:

exiftool -c "%.2f" -TagsFromFile @ -GPSLatitude -GPSLongitude photo.jpg

(Thanks to the EXIFtool Forum for their help.)

Why is this useful?

Modern phones automatically attach a GPS location to every photo you take. GPS resolution is around 10 metres. When you share your photos, you're often sharing your precise location.

I wanted to upload some photos to the Wikimedia Commons of an interesting junction box installed in our home. I didn't want my home location stored on the Internet forever - but I thought it would be useful to include a rough location.

The above command takes a location of 51.123456,0.987654 and returns 51.12,0.98. That's good enough to roughly show the location, without revealing it exactly.