Overview

Netscape Looping Application Extension is the most popular Application Extension Block that tells browser or other GIF viewer to loop the entire animated GIF file.

This block must appear immediately after the global color table of the logical screen descriptor.

This extension block was first supported in Navigator 2.0 Beta 4.

The block is 19 bytes long. First 14 bytes belongs to general Application Extension format, syntax is described in GIF89a Specification, section "26. Application Extension".

Syntax

    +---------------+
 0  |     0x21      |  Extension Label
    +---------------+
 1  |     0xFF      |  Application Extension Label
    +---------------+
 2  |     0x0B      |  Block Size
    +---------------+
 3  |               | 
    +-             -+
 4  |               | 
    +-             -+
 5  |               | 
    +-             -+
 6  |               | 
    +-  NETSCAPE   -+  Application Identifier (8 bytes)
 7  |               | 
    +-             -+
 8  |               | 
    +-             -+
 9  |               | 
    +-             -+
10  |               | 
    +---------------+
11  |               | 
    +-             -+
12  |      2.0      |  Application Authentication Code (3 bytes)
    +-             -+
13  |               | 
    +===============+                      --+
14  |     0x03      |  Sub-block Data Size   |
    +---------------+                        |
15  |     0x01      |  Sub-block ID          |
    +---------------+                        | Application Data Sub-block
16  |               |                        |
    +-             -+  Loop Count (2 bytes)  |
17  |               |                        |
    +===============+                      --+
18  |     0x00      |  Block Terminator
    +---------------+

Comments

• Extension Label - Defines this block as an extension. This field contains the fixed value 0x21 (33).

• Application Extension Label - Identifies the block as an Application Extension. This field contains the fixed value 0xFF (255).

• Block Size - Number of bytes in this extension block, following the Block Size field, up to but not including the beginning of the Application Data. This field contains the fixed value 0x0B (11).

• Application Identifier - Sequence of eight printable ASCII characters used to identify the application owning the Application Extension. This field contains the fixed value "NETSCAPE".

• Application Authentication Code - Sequence of three bytes used to authenticate the Application Identifier. An Application program may use an algorithm to compute a binary code that uniquely identifies it as the application owning the Application Extension. This field contains the fixed value "2.0". Sometimes Application Identifier and Application Authentication Code fields are referred as one "NETSCAPE2.0" field.
  

• Sub-block Data Size - Indicates the number of data bytes to follow. The size of the block does not account for the size byte itself. This field contains the fixed value 0x03 (3).

• Sub-block ID - Identifies the Netscape Looping Extension. This field contains the fixed value 0x01 (1).

• Loop Count - Indicates the number of iterations the animated GIF should be executed. This field is an unsigned 2-byte integer in little-endian (least significant byte first) byte order. 0x00 (0) means infinite loop.

• Block Terminator - This zero-length data block marks the end of the Application Extension. This field contains the fixed value 0x00 (0).

Example

byte#  hexadecimal               text         comments
(hex)

30D:   21                                     # Extension
30E:   FF                                     # Application Extension
30F    0B                                     # Block Size
310:   4E 45 54 53 43 41 50 45   NETSCAPE     # Application Identifier
318:   32 2E 30                  2.0          # Application Authentication Code
31B:   03                                     # Sub-block data size
31C:   01                                     # Sub-block ID
31D:   00 00                                  # Loop forever
31F:   00                                     # Block Terminator

See also