I just added a gif to a question, and on 2 different browsers, the gif stops after 2 cycles. When I have done this previously, the gif just keeps cycling round (as desired). Is this a new stack exchange thing, or is something going on my end?

  • $\begingroup$ Huh... it stops for me too... (I'm using chrome + Windows 10) $\endgroup$ – Simply Beautiful Art Sep 12 '17 at 20:08
  • $\begingroup$ @SimplyBeautifulArt wierd huh? - I'm on Firefox and Opera (mobile) $\endgroup$ – martin Sep 12 '17 at 20:10
  • $\begingroup$ This stopped for me, too (Firefox+Windows 7). My old GIFs do keep cycling, so it may be something specific to your gif? But, I am clueless about what the problem might be? IIRC all my gifs have may be 40-60 frames in a cycle, so they are a bit shorter. But why should that be relevant? $\endgroup$ – Jyrki Lahtonen Sep 12 '17 at 20:10
  • $\begingroup$ @JyrkiLahtonen I tried uploading different versions of it too - all with the same result :/ $\endgroup$ – martin Sep 12 '17 at 20:12
  • 1
    $\begingroup$ Seems to be browser related that it doesn't work as intended but there's a workaround,looking at Jyrki's cube there's the 21 FF 0B 4E 45 54 53 43 41 50 45 32 2E 30 03 01 00 00 00 (looking at the link it will be clear what I'm talking about) while looking at yours (the one also linked by Jyrki) there isn't so that's probably the problem. $\endgroup$ – kingW3 Sep 12 '17 at 20:37
  • $\begingroup$ @kingW3 Just tried uploading with ie - same result. Is that what you mean? $\endgroup$ – martin Sep 12 '17 at 20:43
  • $\begingroup$ @martin Well no, look at the link, the first answer has the solution on how to make a gif cycle indefinitely in browsers. $\endgroup$ – kingW3 Sep 12 '17 at 20:48
  • $\begingroup$ @kingW3 ah, I see - will give it a go - thanks $\endgroup$ – martin Sep 12 '17 at 20:51

A quick Google search produced this handy run-down of the GIF file format, which includes this section on looping:

Example (not in sample file): 21 FF 0B 4E 45 54 53 43 41 50 45 32 2E 30 03 01 05 00 00

The GIF89 specification allows for application-specific information to be embedded in the GIF file itself. This capability is not much used. About the only known public one is the Netscape 2.0 extension (described below) which is used to loop an animated GIF file. We'll go into more detail on looping in when we talk about animation.

The Netscape 2.0 looping block must appear immediately after the global color table of the logical screen descriptor. It is 19 bytes long.

byte  1        : 33 (hex 0x21) GIF Extension code
byte  2        : 255 (hex 0xFF) Application Extension Label
byte  3        : 11 (hex 0x0B) Length of Application Block
                 (eleven bytes of data to follow)
bytes 4 to 11  : "NETSCAPE"
bytes 12 to 14 : "2.0"
byte  15       : 3 (hex 0x03) Length of Data Sub-Block
                 (three bytes of data to follow)
byte  16       : 1 (hex 0x01)
bytes 17 to 18 : 0 to 65535, an unsigned integer in
                 little-endian byte format. This specifies the
                 number of times the loop should
                 be executed.
byte  19       : 0 (hex 0x00) a Data Sub-Block Terminator.

As with all extensions, we start with 21 which is the extension introducer. Next is the extension label which for application extensions is FF. The next value is the block size which tells you how many bytes there are before the actual application data begins. This byte value should be 0B which indicates 11 bytes. These 11 bytes hold two pieces of information. First is the application identifier which takes up the first 8 bytes. These bytes should contain ASCII character codes that identify to which application the extension belongs. In the case of the example above, the application identifier is "NETSCAPE" which is conveniently 8 characters long. The next three bytes are the application authentication code. The spec says these bytes can be used to "authenticate the application identifier." With the Netscape 2.0 extension, this value is simply a version number, "2.0", hence the extensions name. What follows is the application data broken into data sub-blocks. As with the other extensions, the block terminates when you read a sub-block that has zero bytes of data.

With this information in hand, I went over to this handy online hex dump tool and uploaded your file. Here are the first 87 bytes (the rest of the file is excluded for irrelevance and brevity):

file name: b1dKD.gif
mime type: 

0000-0010:  47 49 46 38-39 61 68 01-64 01 77 ff-00 21 ff 0b  GIF89ah. d.w..!..
0000-0020:  4e 45 54 53-43 41 50 45-32 2e 30 03-01 01 00 00  NETSCAPE 2.0.....
0000-0030:  21 fe 33 43-72 65 61 74-65 64 20 77-69 74 68 20  !.3Creat ed.with.
0000-0040:  74 68 65 20-57 6f 6c 66-72 61 6d 20-4c 61 6e 67  the.Wolf ram.Lang
0000-0050:  75 61 67 65-20 3a 20 77-77 77 2e 77-6f 6c 66 72  uage.:.w ww.wolfr
0000-0057:  61 6d 2e 63-6f 6d 00                             am.com.

This data constitutes the first few metadata records in your image. The first 13 bytes are just a bit of information about what type of image it is. Then we get to the interesting bit: the Netscape looping extension I quoted above. Per that format, the information we're interested in are the bytes at offsets 17 and 18 - the number of times the loop should be executed...

0000-0020:  4e 45 54 53-43 41 50 45-32 2e 30 03-01 01 00 00  NETSCAPE 2.0.....

There they are; in bold. Little-endian means the least-significant byte comes first, so that gives us the value 0001 in hex, which is conveniently also 1 in decimal notation.

Thus, the file you uploaded explicitly specifies that the animation should loop exactly once.

I've provided a version of the GIF without this restriction, for your convenience: https://i.stack.imgur.com/FKwNk.gif

See also: How do I make an existing animated GIF loop repeatedly?

  • $\begingroup$ thank you so much! The online hex editors were starting to give me a bit of a headache! Thanks for your help on mine - I've updated question accordingly & I'll try to get my head around how to edit them myself for future questions :) $\endgroup$ – martin Sep 12 '17 at 21:14
  • $\begingroup$ I use The GIMP, as described in this answer - open, export as, check "animate" and "loop forever", done. Beats messing around in a hex editor! $\endgroup$ – Shog9 Sep 12 '17 at 21:15
  • $\begingroup$ Great! That's a relief!! $\endgroup$ – martin Sep 12 '17 at 21:16

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