A simple .swf file and it’s below the line representation

A simple .swf file and it’s below the line representation

 

       At some point along your journey to write your own .swf files, you are going to have to be able to read and understand the raw bits and bytes. I created a one frame flash movie that contained only a rectangle and saved it as Rectangle.swf. Let’s take a look at the way the file is represented in its .swf format. Below is the hexadecimal representation of Rectangle.swf. Let’s take a closer look…

 

000000 46 57 53 03 4F 00 00 00    78 00 05 5F 00 00 0F A0

000010 00 00 0C 01 00 43 02 FF    FF FF BF 00 23 00 00 00

000020 01 00 70 FB 49 97 0D 0C    7D 50 00 01 14 00 00 00

000030 00 01 25 C9 92 0D 21 ED    48 87 65 30 3B 6D E1 D8

000040 B4 00 00 86 06 06 01 00    01 00 00 40 00 00 00

 

   Every .swf file has two parts, the header block and a series of tagged data blocks. Let’s begin with the header. 

 

HEADER

 

       The header is always the first 21 bytes of the .swf file (true?). It describes the file version(1,2, or 3), length of the file in bytes, the frame size in TWIPS, frame rate in frames per second, and the frame count.

 

       Here is the “SWF File Reference’ description of the header:

 

Field	Type	Comment
Signature	UI8	Signature byte always ‘F’
Signature	UI8	Signature byte always ‘W’
Signature	UI8	Signature byte always ‘S’
Version	UI8	Single byte file version
File Length	UI32	Length of entire file in bytes
Frame Size	RECT	Frame size in TWIPS
Frame Rate	UI16	Frame delay in 8.8 fixed number of frames per second
Frame Count	UI16	Total number of frames in movie

 

 

The first three bytes are the standard signature for all .swf files. They are the ASCII values of the characters ‘F’, ‘W’, and ‘S’ in that order. The fourth byte indicates the version of the file. Although a file can be of version 1, 2, or 3, we will always be creating version 3 files so this value will also be fixed.

 

      0x46           ‘F’           0x57             ‘W’         0x53           ‘S’            0x03       3

 

The next 4 bytes represent an unsigned 32-bit integer indicating the file size. Here’s where it starts getting tricky and machine architecture gets involved! The next 4 bytes are 0x4F000000 so that would imply that the file length is 1325400064 bytes, a very large number which doesn’t make sense. What we failed to do is swap all the bytes.

In .swf files, bytes are swapped whenever reading words and dwords such that a 32 bit value B1B2B3B4 is written as B4B3B2B1, and a 16 bit value B1B2 is written as B2B1. Single bytes are written unchanged since there is NO bit- swapping. The reason for this is the differences in storage and retrieval between the Mac and PC processors.

 

Reversing the bytes we can read the 4 bytes correctly and see that file is 79 bytes long.

   

0 x4F000000                     0x0000004F            79

 

       The next 9 bytes represent a data structure used in the .swf format called a Rectangle. Here is the file description of a rectangle:

 

Field	Type	Comment
Nbits	nBits = UB[5]	Bits in each rect value field
Xmin	SB[nBits ]	X minimum position for rect
Xmax	SB[nBits ]	X maximum position for rect
Ymin	SB[nBits ]	Y minimum position for rect
Ymax	SB[nBits ]	Y maximum position for rect

 

To understand these bytes, we need to look at the individual bits.

 

78 00 05 5F 00 00 0F A0 00

 

 

 

0111 1 000 0000 0000 0000 0101 0101 1111 000 0 0000

0000 0000 00 00 1111 1010 0000 0 000 0000

 

       There are five fields in a rectangle structure: nBits, xmin, xmax, ymin, ymax. The unsigned nBits field occupies the first five bits of the rectangle and indicates how long the next four signed fields are. 

Here’s where we hit another subtle point about the .swf file representation. Reading and writing bits is different from reading and writing words and dwords. There is no swapping at all! This is because when Flash is reading an n-bit field, it reads a byte at a time until it has read all n bits. You don’t do any swapping inside of bytes so there is no swapping at all. So the next five bits are read in order and evaluate to 15. Although the nBit field usually varies, it appears fixed in the header so that header has a fixed size (It may just be because the movie dims are usually the same).

 

01111                15

 

       What if nBit has a value of sixteen? This is exactly the size of a word so do we read the following fields as words and swap bytes? No. Fields described by bit size are always read a byte at a time. No swapping, just read the next n bits in that order.

 

000000000000000                0  = xmin

010101011111000                11000 = xmax

000000000000000                0 = ymin

001111101000000                8000 = ymax

 

       For the header, the rectangle is used to store the movie dimensions with xmax corresponding to the movie width and ymax corresponding to the movie height, both in TWIPS. “What’s this TWIPS thing?” you ask. According to whatis.com :

A twip (twentieth of a point) is a measure used in laying out space or defining objects on a page or other area that is to be printed or displayed on a computer screen. A twip is 1/1440th of an inch or 1/567th of a centimeter. That is, there are 1440 twips to an inch or 567 twips to a centimeter. The twip is 1/20th of a point , a traditional measure in printing. A point is approximately 1/72nd of an inch.

For Flash a point corresponds to a pixel, so if we convert from TWIPS to pixels, we see that our movie is 550 x 400. 

       Now we have looked at all of the fields of the rectangle and evaluated them, but what about those last seven bits which are all 0’s. Well, they were just “filled.” 

 

0000000 = filled bits

 

After the end of any structure, if the structure does not completely fill up its last byte, then that last byte is filled with 0’s to keep the next item byte aligned. So if the next item is a word or dword, you can read it as such and not worry about being in the middle of a byte. In this case, only 1 bit in the last byte is used so the last 7 bits are filled with 0’s. 

       Next in the header is the frame rate, which is kind of weird. It is supposed to be stored as a 16bit integer, but the first byte (or last depending on how you look at it) is completely ignored. So the frame rate is 12 fps.

 

0x000C               0x0C00                   0x0C         12 = frame rate

 

       Next is the frame count, which is also a 16-bit integer. So the frame count is 1.

 

0x0100                 0x0001(byte swapping)           1 = frame count

 

       Now we are done with the header. After the header is a series of tagged data blocks. Here is the file description of a tag:

 

Short Tag

 

Field	Type	Comment
Tag	UB[10]	Tag id
Length	UB[6]	Length of tag

 

Long Tag

 

Field	Type	Comment
Tag	UB[10]	Tag id
Long Header Flag	UB[6]	Always 0x3F
Length	UI32	Length of tag

 

 

 

      There are 2 types of tags. They are the short and long data header. Regardless of which case you have, you begin by looking at the first word.

 

0x4302            0x0243               0000 0010 01 00 0011

 

The first 10 bits of the tag are the unsigned tag ID. The tag ID indicates what type of data is to follow in the body of the data block to follow. In this case the value of the tag ID is 9 which corresponds to a setBackgroundColor block. (APPENDIX SHOULD HAVE ALL ID MEANINGS). The last 6 unsigned bits of the tag indicate the length of the data block to follow if it is 62 bytes or less. If the length of the data block is more than 62 bytes, then this field has all 1’s and the length is indicated in the following dword. In this situation though the field does not have all 1’s so the field does indicate the actual length which is 3 bytes.

 

0000001001 = 9 = setBackgroundColor    000011 = 3 = body length

 

       Since we know that the length of the body is 3 bytes, let’s take a look at it. A setBackgroundColor block only contains the 3 byte rgb color description so we evaluate it as such. A color is its own 3 byte data type so there is no byte swapping.

 

0XFFFFFF = white

 

       The next tag is a long tag and is a defineShape tag.

 

0xBF00          0x00BF          0000 0000 10 11 1111

 

0000000010 = 3 = defineShape       111111 = body length (so we have to look at the

                                                                              next dword)

0x23000000               0x00000023           35 = body length

 

Here is the file description of defineShape:

 

Field	Type	Comment
Header	RECORDHEADER	Tag ID = 2
ShapeId	UI16	ID for this character
ShapeBounds	RECT	Bounds of the shape
Shapes	SHAPEWITHSTYLE	Shape information

 

              The body of a defineShape is composed of an unsigned 16-bit character ID, a rectangle defining the bounds for the shape, and a ShapeWithStyle structure which contains shape information.

 

0x0100       0x0001        1 = shape ID

 

Now the Rect which defines the boundaries:

 

70 FB 49 97 0D 0C 7D 50

 

 

 

 

0111 0 000 1111 1011 010 0 1001 1001 0111 0 000 1101   0000 110 0 0111 1101 0101 0 000

 

01110 = 14 = nBits

00011111011010 = 2010 = xmin /20 to covert to pixels from TWIPS   100.5

01001100101110 = 4910 = xmax                                          245.5

00011010000110 = 1670 = ymin                                    83.5

00111110101010 = 4010 = ymax                                          200.5

000 = fill bits

 

       The ShapeWithStyle structure has five parts. A fill style array, a line style array, a nFillBits field, a nLineBits field, and an array of shape records. Here is the file description:

 

Field	Type	Comment
FillStyles	FILLSTYLEARRAY	Array of fill styles
LineStyles	LINESTYLEARRAY	Array of line styles
NumFillBits	nFillBits = UB[4]	Number of fill index bits
NUML ineBits	nLineBits = UB[4]	Number of line index bits
ShapeRecords	SHAPEREC[one or more]	Shape records - see below

 

       A fill style array itself has three fields. The first field is an 8 bit integer count which indicates how many fill styles are in the array. This count works similar to the tag’s length field in that if it is all 1’s, you have to look at the next 16-bits to get the actual length. Here is the file description:

 

Field	Type	Comment
FillStyleCount	count = UI8	Count of fill styles
FillStyleCountExtended	If count = 0xFF count = UI16	Extended count of fill styles. Supported only for Shape2 and Shape3.
FillStyles	FILLSTYLE[count ]	Array of fill styles

 

 

In this case, the 8 bit count is equal to 0 so there is nothing to follow it.

 

0x00 = 0 = count         This is the end of the fill style array because it has no elements

 

       A line style array is exactly the same as a fill style array except it stores line styles. Here is the file description:

 

Field	Type	Comment
LineStyleCount	count = UI8	Count of line styles
LineStyleCountExtended	If count = 0xFF count = UI16	Extended count of line styles
LineStyles	LINESTYLE[count ]	Array of line styles

0x01 = 1 = count           So there is one line style in the array.

 

       A line style has two parts, an unsigned 16-bit integer indicating the width of a line in TWIPS, and a color. Here is the file description:

 

Field	Type	Comment
Width	UI16	Width of line in twips
Color	RGB (Shape1 or Shape2) RGBA (Shape3)	Color value including alpha channel information for Shape3s

 

 

The color in this case is a 24-bit RGB, but if we were doing a defineShape3, it would be a 32-bit RGBA where alpha is the transparency of the color. 

 

0x1400          0x0014 = 20 = width = 1 pixel

0x000000 = RGB = black

 

       Back to the ShapeWithStyle, the nFillBits field is 4 bits, as is the nLineBits.

 

0x0 = 0 = nFillBits          0x1 = 1 = nLineBits

 

       Now for the array of shape records. There are three types of shape records. Here are the file descriptions:

 

Shape Record Type 0

 

Field	Type	Comment
TypeFlag	UB[1] = 0	Non-edge record flag
EndOfShape	UB[5] = 0	End of shape flag

 

 

Shape Record Type 1

 

Field	Type	Comment
TypeFlag	UB[1] = 0	Non-edge record flag
StateNewStyles	newStyles = UB[1]	New styles flag. Used by DefineShape2 and DefineShape3 only.
StateLineStyle	lineStyle = UB[1]	Line style change flag
StateFillStyle0	fillStyle0 = UB[1]	Fill style 0 change flag
StateFillStyle1	fillStyle1 = UB[1]	Fill style 1 change flag
StateMoveTo	moveTo = UB[1]	Move to flag
MoveBits	If moveTo nMoveBits = UB[5]	Move bit count
MoveDeltaX	If moveTo UB[nMoveBits ]	Delta X value
MoveDeltaY	If moveTo UB[nMoveBits ]	Delta Y value
Fill0Style	If fillStyle0 UB[nFillBits ]	Fill 0 Style
Fill1Style	If fillStyle1 UB[nFillBits ]	Fill 1 Style
LineStyle	If lineStyle UB[nLineBits ]	Line Style
FillStyles	If newStyles FILLSTYLEARRAY	Array of fill styles
LineStyles	If newStyles LINESTYLEARRAY	Array of line styles

 

 

Shape Record Type 2

 

Field	Type	Comment
TypeFlag	UB[1] = 1	This is an edge record
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