.SWF File Format

Introduction

This is the Flash movie of the first five versions of the format: SWF 1, from FutureSplash Animator and Macromedia Flash 1 in 1996, to SWF 5, from Flash 5 in August 2000. It holds vector shapes, bitmaps, sounds, buttons, text and, from Flash 3, scripts, laid out frame by frame for the player. Adobe documented it in the SWF File Format Specification, whose last edition, version 19 of January 2013, still marks what each version added. Today these movies play in Ruffle.

Two things set these movies apart from later ones, and make them a separate format. Their text, from frame labels and text fields to every string in the scripts, is in the code page of the computer playing the movie, ANSI or Shift-JIS on Japanese systems, and nothing in the file says which. And the event flags of a movie clip’s handlers are 16 bits. SWF 6 changed both, to UTF-8 and 32 bits. There is no compression of the whole file, no video and no ActionScript 3: the scripts are AVM1 actions, the stack actions of Flash 4 and then the objects, functions and constant pool of Flash 5’s ActionScript.

A .swf file dropped on any page opens in the explorer, which draws the frames, lists the tags and the characters, decodes the bitmaps and sounds, and disassembles the AVM1 actions. It reads the text as Windows-1252.

Structure

A movie is a header followed by a run of tags up to an End tag. All numbers are little-endian, and many fields are packed into bits.

00000000  46 57 53 04 A3 11 00 00 78 00 05 5F 00 00 0F A0
00000010  00 00 1E 02 00 43 02 FF FF FF 3F 03 3E 03 00 00
00000020  96 08 00 00 78 61 6E 67 6C 65 00

Reference pages

• Header – the signature, version, file length, frame size, frame rate and frame count
• Tags and data types – the record header, the tag run, numbers, bit fields, code-page strings, colours, RECT, MATRIX and CXFORM
• Tag codes – every tag code, the version that brought it and where it is described
• Control tags – End, SetBackgroundColor, Protect, ExportAssets, ImportAssets and EnableDebugger
• Display list and frames – PlaceObject, PlaceObject2, RemoveObject, ShowFrame, sprites, frame labels and 16-bit clip actions
• Shapes – DefineShape 1 to 3, morph shapes, fill and line styles, gradients and shape records
• Bitmaps – JPEG, lossless and alpha bitmaps
• Sounds – event and streaming sounds, uncompressed, ADPCM and MP3
• Fonts and text – DefineFont, DefineFont2, font information, static text and text fields
• Buttons – DefineButton and DefineButton2, button records, condition actions and sounds
• AVM1 actions – the actions of Flash 3, 4 and 5: every action, its operands and what it does

Telling it apart

The explorer’s parser for this format checks that the file starts with FWS (46 57 53) and that the version byte at offset 3 is from 1 to 5. It then reads the header and walks the tag headers to the End tag; a tag whose length runs past the end of the file turns the file down.

An FWS file with version 6 or more is the later uncompressed format, and a file starting CWS or ZWS is a compressed movie; each is a separate format. Nearly all the files of this format examined for this site are SWF 4 and SWF 5 movies; SWF 1, 2 and 3 movies are rare.

Variants

• Flash movie, SWF 1 to 5 (FWS) – the first SWF layout: an uncompressed movie whose text is in the player’s system code page
• Flash movie, SWF 6 and later (FWS) – text in UTF-8 and 32-bit clip event flags, uncompressed; the form ActionScript 3 movies also take
• Compressed Flash movie, zlib (CWS) – everything after the first 8 bytes is a zlib stream; inside is the same movie as an SWF 6 file
• Compressed Flash movie, LZMA (ZWS) – an LZMA-compressed movie: a compressed length and LZMA properties, then LZMA data

Links