UVLC File Specification

Picture Header

	Like in H.263 UVLC start with a PSC (Picture start code) which is 22 bits long
		0000 0000 0000 0000 1000 00
	A PSC is always byte aligned
	The second information is the picture's format which can be one of following : CIF or VGA (2 bits)
		00  : forbidden
		01  : CIF
		10  : VGA
	Then comes the picture resolution which is used in combination with the picture's format (3 bits)
		000 : forbidden
		001 : for CIF it means sub-QCIF
		010 : for CIF it means QCIF
		011 : for CIF it means CIF
		100 : for CIF it means 4-CIF
		101 : for CIF it means 16-CIF
	Then comes the picture's type (3 bits)
		000 : INTRA picture
		001 : INTER picture
	Then comes the quantizer's reference for the picture that ranges from 1 to 31 (5 bits)
	Then comes the frame number (32 bits)
	
	Picture header is at least 67 bits long

Group of Blocks Header

	Each GOB starts with a GOBSC (Group of Blocks start code) wich is 22 bits long
		0000 0000 0000 0000 1xxx xx
	A GOBSC is always byte aligned. The least significant bytes represent the blockline's number. We can see that PSC
	means first gob too. So for the first GOB, GOB's header is always omitted.
	
	Then comes the quantizer's reference for the GOB that ranges from 1 to 31 (5 bits)
	
	Group of Blocks Header is at least 27 bits long
	
MacroBlock Header

	Coded Macroblock bit (1 bit)
		bit 0 : 1 means there's a macoblock, 0 means macroblock is all zero

	Macrobock description code (7 bits)
		bit 0 : 1 means there's non dc coefficients for block y0
		bit 1 : 1 means there's non dc coefficients for block y1
		bit 2 : 1 means there's non dc coefficients for block y2
		bit 3 : 1 means there's non dc coefficients for block y3
		bit 4 : 1 means there's non dc coefficients for block cb
		bit 5 : 1 means there's non dc coefficients for block cr
		bit 6 : 1 means there's a quantization value following this code
		bit 7 : Always 1 to avoid a zero byte
		
	Then comes a differential value for the quantization (2 bits)
		00 : -1
		01 : -2
		10 :  1
		11 :  2
	

Block layer

	Codes used for uvlc encoding

 1/ Run encoding
 
  coarse | additional | size |   value of run      | length of run
       1 |            |   1  |                  0  |       1
      01 |            |   2  |                  1  |       1
     001 | x          |   4  |     x +  2   (2:3)  |       2
    0001 | xx         |   6  |    xx +  4   (4:7)  |       3
   00001 | xxx        |   8  |   xxx +  8  (8:15)  |       4
  000001 | xxxx       |  10  |  xxxx + 16 (16:31)  |       5
 0000001 | xxxxx      |  12  | xxxxx + 32 (32:63)  |       6

 2/ Level encoding

   coarse | additional | size | value of level | length of level
        1 | s          |   2  |              1 |       1
       01 |            |   2  |            EOB |
      001 | xs         |   5  |         x +  2 |       2
     0001 | xxs        |   7  |        xx +  4 |       3
    00001 | xxxs       |   9  |       xxx +  8 |       4
   000001 | xxxxs      |  11  |      xxxx + 16 |       5
  0000001 | xxxxxs     |  13  |     xxxxx + 32 |       6
 00000001 | xxxxxxs    |  15  |    xxxxxx + 64 |       7

 (s stands for sign)

End of Picture header

	A picture ends alwways with a code 0000 0000 0000 0000 1111 11 that is byte aligned and 22 bytes long