SWF header (SWF 1 to 5)

Every movie starts with a header of six fields. The first three, 8 bytes, are always the same size; the frame size is a bit-packed rectangle whose size depends on the numbers in it, so the frame rate, the frame count and the first tag move with it. With the usual 15-bit values the header is 21 bytes and the first tag starts at offset 0x15.

00000000  46 57 53 04 A3 11 00 00 78 00 05 5F 00 00 0F A0
00000010  00 00 1E 02 00

The header of a Flash 4 movie: FWS, version 4, 4,515 bytes, a 550 × 400 pixel stage, 30 frames a second, two frames

Fields

Offset	Size	Type	Name	Meaning
0	3	UI8[3]	`Signature`	`FWS` (`46 57 53`)
3	1	UI8	`Version`	The SWF version, 1 to 5
4	4	UI32	`FileLength`	The length of the whole file in bytes, these 8 included
8	varies	RECT	`FrameSize`	The stage, in twips; 9 bytes with 15-bit values
after	2	UI16 (8.8)	`FrameRate`	Frames a second, 8.8 fixed point
after	2	UI16	`FrameCount`	The number of frames on the main timeline

The tags follow directly.

Signature

Three bytes, FWS (46 57 53). Every movie of SWF 1 to 5 is uncompressed and starts this way.

Version

One byte: the SWF version, the Flash Player the movie was made for, from 1 to 5.

Version	Player	Released	What it brought
1	FutureSplash Player, Flash Player 1	1996	Shapes, bitmaps, fonts, static text, sounds, simple buttons
2	Flash Player 2	June 1997	Lossless bitmaps, JPEG with its own tables, more shape styles, button sounds and colour, Protect
3	Flash Player 3	May 1998	Frame actions (DoAction), sprites, PlaceObject2, alpha, morph shapes, DefineFont2, DefineButton2
4	Flash Player 4	June 1999	Stack actions and variables, text fields (DefineEditText), MP3 sound
5	Flash Player 5	August 2000	ActionScript with objects and functions, clip actions, ExportAssets and ImportAssets, the debugger

The versions table lists every version, with the authoring tools.

File length

A UI32: the length of the whole movie in bytes, the header included. It equals the size of the file. In the example, A3 11 00 00 is 0x11A3, 4,515 bytes.

Frame size

A RECT: a 5-bit count Nbits, then Xmin, Xmax, Ymin and Ymax, each a signed Nbits-bit number, padded to a whole byte. The values are in twips, 20 to a pixel. Xmin and Ymin are normally 0, so Xmax and Ymax are the stage’s width and height.

The example’s nine bytes 78 00 05 5F 00 00 0F A0 00, as bits:

01111 000000000000000 010101011111000 000000000000000 001111101000000 0000000
Nbits Xmin            Xmax            Ymin            Ymax            padding
15    0               11000           0               8000

Xmax 11,000 twips is 550 pixels; Ymax 8,000 twips is 400 pixels. 5 + 4 × 15 = 65 bits, padded to 72

Frame rate

Frames a second, as an unsigned 8.8 fixed-point number stored little-endian: the first byte is the fraction in 256ths, the second the whole number. In the example 00 1E is 30.0; 00 0C would be 12.0. The player tries to show frames at this rate.

Frame count

A UI16: the number of frames on the main timeline, which is the number of ShowFrame tags in the movie’s own tag run, not counting those inside sprites. In the example 02 00 is two frames.

.SWF File Format – Header