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XTC/XTG/XTH/XTCH Format Technical Specification

Document Information

• Version: 1.0
• Date: 2025-01-XX
• Purpose: Public technical specification for format conversion
• Target Platform: ESP32 E-Paper Display Devices

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Format Overview

Four proprietary file formats designed for ESP32 e-paper displays:

Format	Extension	Purpose	Description
XTG	.xtg	Monochrome Image	1-bit per pixel bitmap
XTH	.xth	4-Level Grayscale Image	2-bit per pixel bitmap
XTC	.xtc	Comic Container	Container format with multiple XTG pages
XTCH	.xtch	Comic Container Variant	Variant of XTC format

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Byte Order

All multi-byte values are stored in Little-Endian format.

• Little-Endian: Least significant byte at lowest address
• Example: uint16_t 0x1234 → [0x34, 0x12]
• Example: uint32_t 0x12345678 → [0x78, 0x56, 0x34, 0x12]

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XTG Format (Monochrome Image)

File Structure

XTG stores 1-bit per pixel monochrome bitmaps optimized for e-paper displays.

Header (22 bytes)

Offset	Size	Type	Field	Description	Value
0x00	4	uint32_t	mark	File identifier	0x00475458 ("XTG\0")
0x04	2	uint16_t	width	Image width (pixels)	1-65535
0x06	2	uint16_t	height	Image height (pixels)	1-65535
0x08	1	uint8_t	colorMode	Color mode	0=monochrome
0x09	1	uint8_t	compression	Compression	0=uncompressed
0x0A	4	uint32_t	dataSize	Image data size (bytes)	Calculated
0x0E	8	uint64_t	md5	MD5 checksum (first 8 bytes)	Optional

Image Data

• Location: After header (offset 22 bytes)
• Format: Bitmap data, 1 bit per pixel
• Data Size Calculation: dataSize = ((width + 7) / 8) * height
• Pixel Storage:
  • Rows stored top to bottom
  • Each row stored left to right
  • 8 pixels per byte (MSB first)
  • Bit order: bit 7 (MSB) = leftmost pixel, bit 0 (LSB) = rightmost pixel

Pixel Value Mapping

Bit Value	Display Color
0	Black
1	White

Note: When using inverted bitmap drawing, the display may invert these values (1=black, 0=white).

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XTH Format (4-Level Grayscale Image)

File Structure

XTH stores 2-bit per pixel grayscale bitmaps for 4-level grayscale e-paper displays.

Header (22 bytes)

Same structure as XTG, but with different file identifier:

Offset	Size	Type	Field	Description	Value
0x00	4	uint32_t	mark	File identifier	0x00485458 ("XTH\0")
0x04	2	uint16_t	width	Image width (pixels)	1-65535
0x06	2	uint16_t	height	Image height (pixels)	1-65535
0x08	1	uint8_t	colorMode	Color mode	0=monochrome
0x09	1	uint8_t	compression	Compression	0=uncompressed
0x0A	4	uint32_t	dataSize	Image data size (bytes)	Calculated
0x0E	8	uint64_t	md5	MD5 checksum (first 8 bytes)	Optional

Image Data

• Location: After header (offset 22 bytes)
• Format: Two bit planes, 2 bits per pixel total
• Data Size Calculation: dataSize = ((width * height + 7) / 8) * 2 (rounded up to bytes)
• Storage:
  • First bit plane: offset 22, size (width * height + 7) / 8 bytes (rounded up)
  • Second bit plane: immediately after first plane, same size
  • Each bit plane stores pixels in vertical scan order (column-major):
    • Columns scanned from right to left (x = width-1 to 0)
    • 8 vertical pixels packed per byte (MSB = topmost pixel in group)
  • See "Data Storage Details" section for complete specification

Pixel Value Mapping

IMPORTANT: The Xteink e-paper LUT has non-linear grayscale mapping with swapped middle values:

Pixel Value	Binary	Bit1	Bit2	LUT Level	Display Color
0	00	0	0	L0	White
1	01	0	1	L1	Dark Grey
2	10	1	0	L2	Light Grey
3	11	1	1	L3	Black

Note: Values 1 and 2 are swapped compared to typical linear grayscale ordering. This matches the Xteink device's e-paper LUT behavior.

Bit Plane Usage:

• Bit1 (first plane): Sent via command 0x24
• Bit2 (second plane): Sent via command 0x26

Pixel Value Calculation: pixelValue = (bit1 << 1) | bit2

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XTC Format (Comic Container)

File Structure

XTC is a container format storing multiple XTG-format pages for comic/PDF reading.

Header (56 bytes)

Offset	Size	Type	Field	Description	Value
0x00	4	uint32_t	mark	File identifier	0x00435458 ("XTC\0")
0x04	2	uint16_t	version	Version number	0x0100 (v1.0)
0x06	2	uint16_t	pageCount	Total pages	1-65535
0x08	1	uint8_t	readDirection	Reading direction	0-2
0x09	1	uint8_t	hasMetadata	Has metadata	0-1
0x0A	1	uint8_t	hasThumbnails	Has thumbnails	0-1
0x0B	1	uint8_t	hasChapters	Has chapters	0-1
0x0C	4	uint32_t	currentPage	Current page (1-based)	0-65535
0x10	8	uint64_t	metadataOffset	Metadata offset	Byte offset
0x18	8	uint64_t	indexOffset	Index table offset	Byte offset
0x20	8	uint64_t	dataOffset	Data area offset	Byte offset
0x28	8	uint64_t	thumbOffset	Thumbnail offset	Byte offset
0x30	8	uint64_t	chapterOffset	Chapter data offset	Byte offset

Reading Direction

Value	Direction	Description
0	L→R	Left to right (normal)
1	R→L	Right to left (Japanese manga)
2	Top→Bottom	Top to bottom (vertical)

Metadata Structure (256 bytes, optional)

If hasMetadata == 1, stored at metadataOffset:

Offset	Size	Type	Field	Description
0x00	128	char[]	title	Title (UTF-8, null-terminated)
0x80	64	char[]	author	Author (UTF-8, null-terminated)
0xC0	32	char[]	publisher	Publisher/source (UTF-8, null-terminated)
0xE0	16	char[]	language	Language code (e.g., "zh-CN", "en-US")
0xF0	4	uint32_t	createTime	Creation time (Unix timestamp)
0xF4	2	uint16_t	coverPage	Cover page (0-based, 0xFFFF=none)
0xF6	2	uint16_t	chapterCount	Number of chapters
0xF8	8	uint64_t	reserved	Reserved (zero-filled)

Chapter Structure (96 bytes per chapter, optional)

If hasChapters == 1, stored at chapterOffset:

Offset	Size	Type	Field	Description
0x00	80	char[]	chapterName	Chapter name (UTF-8, null-terminated)
0x50	2	uint16_t	startPage	Start page (0-based)
0x52	2	uint16_t	endPage	End page (0-based, inclusive)
0x54	4	uint32_t	reserved1	Reserved 1 (zero-filled)
0x58	4	uint32_t	reserved2	Reserved 2 (zero-filled)
0x5C	4	uint32_t	reserved3	Reserved 3 (zero-filled)

Number of chapters specified by XtcMetadata.chapterCount.

Page Index Table (16 bytes per page)

Stored at indexOffset, one entry per page:

Offset	Size	Type	Field	Description
0x00	8	uint64_t	offset	XTG/XTH image offset (absolute, from file start)
0x08	4	uint32_t	size	XTG/XTH image size in bytes (including 22-byte header)
0x0C	2	uint16_t	width	Image width (pixels)
0x0E	2	uint16_t	height	Image height (pixels)

Total index table size: pageCount * 16 bytes.

Data Area

All XTG/XTH page images stored starting at dataOffset. Each page's data is stored contiguously, with position specified by the index table's offset field (absolute offset from file start).

Thumbnail Area (optional)

If hasThumbnails == 1, thumbnails stored at thumbOffset. Thumbnails are also in XTG format with the same index structure.

File Layout

[Header: 56 bytes]
[Metadata: 256 bytes] (optional, at metadataOffset)
[Chapters: N × 96 bytes] (optional, at chapterOffset)
[Page Index Table: pageCount × 16 bytes] (at indexOffset)
[Data Area: All XTG page data] (at dataOffset)
[Thumbnail Area] (optional, at thumbOffset)

Note: The actual order of sections in the file is determined by the offset fields in the header. The typical layout places metadata first, then chapters, then the page index, then page data, and finally thumbnails.

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XTCH Format (Comic Container Variant)

File Structure

XTCH is identical to XTC in all aspects except the file identifier.

Header (56 bytes)

Same structure as XTC, only mark field differs:

Offset	Size	Type	Field	Description	Value
0x00	4	uint32_t	mark	File identifier	0x48435458 ("XTCH")

File Identifier Comparison

Format	Identifier (Little-Endian)	ASCII
XTC	0x00435458	"XTC\0"
XTCH	0x48435458	"XTCH"

All other structures, fields, and data formats are identical to XTC.

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String Encoding

• All string fields use UTF-8 encoding
• Strings are null-terminated (\0)
• Remaining space in fixed-size string fields is zero-filled

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Data Storage Details

Bitmap Storage Rules

XTG (1 bit/pixel)

• Rows: top to bottom
• Pixels per row: left to right
• 8 pixels per byte
• Bit order: MSB (bit 7) = leftmost pixel, LSB (bit 0) = rightmost pixel

Example: 10-pixel wide image

• 2 bytes per row (rounded up)
• Byte 1: pixels 0-7 (bit 7 = pixel 0, bit 0 = pixel 7)
• Byte 2: pixels 8-9 (bit 7 = pixel 8, bit 6 = pixel 9, unused bits = 0)

XTH (2 bits/pixel)

• Two bit planes stored sequentially
• First plane: All pixels' Bit1 (pixel value bit 1), packed 8 pixels per byte
• Second plane: All pixels' Bit2 (pixel value bit 0), packed 8 pixels per byte
• IMPORTANT: Uses vertical scan order (column-major), NOT row-major:
  • Columns are scanned from right to left (x = width-1 down to 0)
  • Within each column, 8 vertical pixels are packed per byte (top to bottom)
  • Each byte contains 8 vertically-adjacent pixels, MSB (bit 7) = topmost pixel in group
• This scan order matches the Xteink e-paper display refresh pattern

Pixel Value Calculation: pixelValue = (bit1 << 1) | bit2

Example: 480×800 pixel image

• Total pixels: 384000
• Each column: 800 pixels = 100 bytes (800/8 = 100 vertical groups)
• Total columns: 480 (scanned right to left)
• Each bit plane: 480 × 100 = 48000 bytes
• First plane: offset 22, size 48000 bytes (contains Bit1 for all pixels)
• Second plane: offset 48022, size 48000 bytes (contains Bit2 for all pixels)
• Total data size: 96000 bytes

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Conversion Guidelines

From Other Formats to XTG/XTH

1. Prepare Source Image:

   • Convert to grayscale
   • Resize to target resolution
   • For XTG: Apply threshold/binarization
   • For XTH: Map grayscale to 4 levels (0-3)

2. Generate Bitmap Data:

   • XTG: Pack 1 bit per pixel, calculate dataSize = ((width + 7) / 8) * height
   • XTH: Generate two bit planes, calculate dataSize = ((width * height + 7) / 8) * 2

3. Write File:

   • Write header with correct file identifier
   • Write bitmap data
   • Ensure Little-Endian byte order for all multi-byte values

From XTC/XTCH to Other Formats

1. Read header to get page count and offsets
2. Read page index table to locate each page
3. Extract XTG data for each page from data area
4. Convert each XTG page using XTG conversion process

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File Identifier Reference

Format	Little-Endian uint32_t	Byte Sequence	ASCII
XTG	0x00475458	[0x58, 0x54, 0x47, 0x00]	"XTG\0"
XTH	0x00485458	[0x58, 0x54, 0x48, 0x00]	"XTH\0"
XTC	0x00435458	[0x58, 0x54, 0x43, 0x00]	"XTC\0"
XTCH	0x48435458	[0x58, 0x54, 0x43, 0x48]	"XTCH"

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Notes

• All offsets are byte offsets from the start of the file
• Page numbers in XTC/XTCH are 1-based for display, but 0-based in internal structures
• Reserved fields should be zero-filled
• MD5 checksum field is optional and may be zero
• Compression is currently not implemented (compression field = 0)