Version 1 (modified by Sam Hocevar, 15 years ago) (diff)

bit allocation discussion

A few notes and thoughts about compressing images to 140 characters for use on Twitter.

The first I read about this "competition" was ​here.

Bit allocation discussion

Twitter allows for 140 characters in a message. UTF-8 is allowed.

UTF-8 is restricted to the formal Unicode definition by RFC 3629. It means that the only legal UTF-8 characters range from U+0000 to U+10FFFF. The following restrictions must also be added:

  • The high and low surrogates, used for UTF-16 encoding, restricting the Unicode range to U+0000..U+D7FF and U+E000..U+10FFFF.
  • The 66 non-characters.

The final size of this set is:

$(2^20 + 2^16) - 2^11 - 66 = 1111998$

The number of bits that can be encoded using 140 such characters is computed as follows:

$n_{bits} = floor(\dfrac{140 \log(1111998)}{\log(2)}) = 2811$

In theory, 2811 bits is therefore the maximum we can stuff into a Twitter message. However, a lot of these characters are undefined, not yet allocated or are control characters. As of Unicode 5.1 there are 100507 graphic characters, reducing the number of expressed bits to:

$n_{bits} = floor(\dfrac{140 \log(100507)}{\log(2)}) = 2326$

We'll go on with this value of 2326 encodable bits.

Attachments (12)

Download all attachments as: .zip