ggtag protocol

ggtag is using the same data protocol for sound and USB programming. The protocol encodes a series of commands into binary stream which is prefixed by two bytes which specify the size of the stream. The entire payload has the following format:

[SIZE (2 bytes)] [CMD1] [CMD2] [CMD3] ... [CMDn]

The SIZE is encoded as a big-endian 16-bit unsigned integer. At most SIZE bytes follow which represent the commands.

Commands

Each command has the following format:

[CMD_CODE (4 bits)] [ARG1] [ARG2] ... [ARGn]

CMD_CODE	Name	Description
0000	TEXT_CMD	Draw text
0001	RECT_CMD	Draw rectangle
0010	FILL_RECT_CMD	Draw filled rectangle
0011	CIRCLE_CMD	Draw circle
0100	FILL_CIRCLE_CMD	Draw filled circle
0101	LINE_CMD	Draw line
0110	QRCODE_CMD	Draw QR code
0111	IMAGE_CMD	Draw image
1000	ICON_CMD	Draw icon
1001	RFID_CMD	Program RFID
1010	RLE_IMAGE_CMD	Draw RLE image

The commands are executed in the order they are received. The display is cleared before the first command is executed.

TEXT_CMD

ARG#	Description	Size (bits)
1	X coordinate	9
2	Y coordinate	8
3	Font size	3
4	Text length (N)	7
5	Char1 (7bit ASCII)	7
...	...	...
4+N	CharN (7bit ASCII)	7

Example: Draw "ABC" at position (120,95) with fontSize=3

CMD_CODE	ARG1	ARG2	ARG3	ARG4	ARG5	ARG6	ARG7
0000	001111000	01011111	010	0000011	1000001	1000010	1000011

RECT_CMD

ARG#	Description	Size (bits)
1	X coordinate	9
2	Y coordinate	8
3	Width	9
4	Height	8

Example: Draw rectangle at position (50,30) with width=280 and height=170

CMD_CODE	ARG1	ARG2	ARG3	ARG4
0001	000110010	00011110	100011000	10101010

FILL_RECT_CMD

Same as RECT_CMD.

CIRCLE_CMD

ARG#	Description	Size (bits)
1	X coordinate	9
2	Y coordinate	8
3	Radius	7

Example: Draw circle at position (50,30) with radius=100

CMD_CODE	ARG1	ARG2	ARG3
0011	000110010	00011110	1100100

FILL_CIRCLE_CMD

Same as CIRCLE_CMD.

LINE_CMD

ARG#	Description	Size (bits)
1	X1 coordinate	9
2	Y1 coordinate	8
3	X2 coordinate	9
4	Y2 coordinate	8

Example: Draw line from (50,30) to (280,170)

CMD_CODE	ARG1	ARG2	ARG3	ARG4
0101	000110010	00011110	100011000	10101010

QRCODE_CMD

ARG#	Description	Size (bits)
1	X coordinate	9
2	Y coordinate	8
3	Pixel width	2
4	Text length (N)	7
5	Char1 (7bit ASCII)	7
...	...	...
4+N	CharN (7bit ASCII)	7

Example: Draw QR code at position (50,30) with pixel width=2 and text="ABC"

CMD_CODE	ARG1	ARG2	ARG3	ARG4	ARG5	ARG6	ARG7
0110	000110010	00011110	10	0000011	1000001	1000010	1000011

IMAGE_CMD

ARG#	Description	Size (bits)
1	X coordinate	9
2	Y coordinate	8
3	Image width (W)	9
4	Image height (H)	8
5	Bitmap	W*H

Consider the following 7x9 image (* = black, . = white)

*******
*.....*
*.....*
*.***.*
*.....*
*.***.*
*.....*
*.....*
*******

Example: Draw the image above at position (50,30)

CMD_CODE	ARG1	ARG2	ARG3	ARG4	ARG5
0111	000110010	00011110	000000111	00001001	111111110000011000001101110110000011011101100000110000011111111

ICON_CMD

ARG#	Description	Size (bits)
1	X coordinate	9
2	Y coordinate	8
3	Icon height	8
4	Icon codepoint	16

Example: Draw the FontAwesome icon with codepoint 0xf552 at position (50,30) with height=40

CMD_CODE	ARG1	ARG2	ARG3	ARG4
1000	000110010	00011110	00101000	1111010111010010

RFID_CMD

ARG#	Description	Size (bits)
1	Type (0=EM4102, 1=HID)	1
2	ID	45

If type is EM4102, the ID argument is interpreted in the following way:

| unused (5 bits) | manufacturer ID (8 bits) | unique ID (32 bits) |

If type is HID, the ID argument is interpreted in the following way:

| manufacturer code (20 bits) | site code (8 bits) | unique id (16 bits) | parity (1 bit) |

Example: Program RFID to be EM4102 with manufacturer ID=0x07 and unique ID=0xad30b1

CMD_CODE	ARG1	ARG2
1001	0	000000000011100000000101011010011000010110001

RLE_IMAGE_CMD

ARG#	Description	Size (bits)
1	X coordinate	9
2	Y coordinate	8
3	Image width (W)	9
4	Image height (H)	8
5	RLE bitmap	variable

The image is encoded using run-length encoding (RLE). The bitmap is interpreted as a sequence of black and white runs, left to right, top to bottom. Consider the following 7x9 image (* = black, . = white):

.......
.......
*******
****...
.......
.......
....***
*******
*******

We have 14 white pixels, 11 black pixels, 21 white pixels and 17 black pixels. We encode these numbers in base 3:

14 = 112(3)
11 = 102(3)
21 = 210(3)
17 = 122(3)

We use 2 bits to encode each digit in base 3, so 112(3) is encoded as 010110. We use 11 to separate the runs, and we always start with a white run. The image above is encoded as 010110 11 010010 11 100100 11 011010.

Example: Draw the image above at position (50,30)

CMD_CODE	ARG1	ARG2	ARG3	ARG4	ARG5
1010	000110010	00011110	000000111	00001001	010110110100101110010011011010