HG‐X ProcessImage

Encoding: HG Image

The HG image encoding used by the HG-3 and HG-2 formats (.hg3, .hg2). This encoding method is over 20 years old, and appears to be identical to the method described in this software patent.

🚧 This page is a work in progress

Methods

The encoding process is a bit lengthy, and shares some similarity to the PSVita CSX image formats (primarily delta filtering, and image slicing). The primary methods used are:

• Zlib compression
• Elias Gamma coding
• Zero-run length encoding
• Weighted table normalization
• Absolute signed values
• Delta filtering
• Image slicing (never used, HG-3 only)

When simplified, these processes can be grouped into 2-3 functions. In addition, the following are used before the encoding or after the decoding process.

• Crop transparent borders
• Vertically flip pixel buffer (like in Bitmap image format, Orthogonal transform)
• Reduced bit depth of each RGBA channel (HG-2 only)

Implementation

Weight tables

The weight tables are 4 arrays of 256 byte values each, only used during the decoding process. The encoding process has no simple lookup table equivalent, and must be done manually, in the same manor that the weight tables are initialized.

31        23        15        7       0
0000 0000 0000 0000 0000 0000 0000 0000
1122 3344 1122 3344 1122 3344 1122 3344

32:30 24:22 16:14 8:6
30:28 22:20 14:12 6:4
28:26 20:18 12:10 4:2
26:24 18:16 10: 8 2:0

Byte	Table 1	Table 2	Table 3	Table 4
0	00000000	00000000	00000000	00000000
1	00000040	00000010	00000004	00000001
2	00000080	00000020	00000008	00000002
3	000000c0	00000030	0000000c	00000003
4	00004000	00001000	00000400	00000100
5	00004040	00001010	00000404	00000101
6	00004080	00001020	00000408	00000103
7	000040c0	00001030	0000040c	00000103
8	00008000	00002000	00000800	00000200
9	00008040	00002010	00000804	00000201
...	...	...	...	...
248	c0c08000	30302000	0c0c0800	03030200
249	c0c08040	30302010	0c0c0804	03030201
250	c0c08080	30302020	0c0c0808	03030202
251	c0c080c0	30302030	0c0c080c	03030203
252	c0c0c000	30303000	0c0c0c00	03030300
253	c0c0c040	30303010	0c0c0c04	03030301
254	c0c0c080	30303020	0c0c0c08	03030302
255	c0c0c0c0	30303030	0c0c0c0c	03030303

Mask
c0c0c0c0	255
30303030	255
0c0c0c0c	255
03030303	255

Absolute tables

To describe this section, signed and unsigned values will be used interchangeably. This part of the encoding reduces the number of '1' bits for negative numbers by 'grouping' them with positive numbers.

Normal signed int8 value:

Byte	Byte bits	Abs	Abs bits
0	0000 0000	0	0000 0000
-1	1111 1111	1	0000 0001
1	0000 0001	2	0000 0010
-2	1111 1110	3	0000 0011
2	0000 0010	4	0000 0100
-3	1111 1101	5	0000 0101
...	...	...	...
126	0111 1110	252	1111 1100
-127	1000 0001	253	1111 1101
127	0111 1111	254	1111 1110
-128	1000 0000	255	1111 1111