About

Every JPEG file supports an invisible text payload called the COM (Comment) marker, defined by the marker code 0xFFFE in the JPEG specification (ITU-T T.81, Annex B). Most image viewers and social media platforms silently strip these markers, which means any comment you embed survives only in the raw file. This tool performs direct binary injection into the JPEG byte stream without re-encoding, so pixel data remains bit-identical to the original. The maximum payload per COM segment is 65,533 bytes (2¹⁶ − 3). Attempting to store comments in lossy re-encoding workflows destroys quality. This tool does not.

Common use cases include watermarking provenance (photographer name, license), embedding machine-readable tags for archival pipelines, or simply hiding a message for someone who knows where to look. Note: this is not encryption. Anyone with a hex editor or this tool can read the comment. If you need secrecy, encrypt your message before embedding. The tool also reads and displays any existing COM markers already present in the file, which is useful for forensic inspection of image origin.

Formulas

The COM marker segment follows the standard JPEG marker format. The binary layout of a COM segment is:

0xFF 0xFE L_high L_low D₀ D₁ … D_n

Where L is a 16-bit big-endian unsigned integer representing the segment length (including the 2 length bytes themselves but excluding the marker bytes). The relationship between comment byte length and segment length is:

L = n + 2

Where n is the number of bytes in the UTF-8 encoded comment string and L is the value written into the length field. The maximum value of L is 2¹⁶ − 1 = 65,535, therefore the maximum comment payload n = 65,535 − 2 = 65,533 bytes. The injection offset is always byte position 2 (immediately after the SOI marker 0xFFD8). The total file size increase equals n + 4 bytes (marker + length + payload).

Reference Data

JPEG Marker	Hex Code	Name	Description	Max Payload
SOI	0xFFD8	Start of Image	First 2 bytes of every valid JPEG	0 bytes
APP0	0xFFE0	JFIF Header	JFIF format identifier and thumbnail	65,533 bytes
APP1	0xFFE1	EXIF / XMP	Camera metadata, GPS, orientation	65,533 bytes
APP2	0xFFE2	ICC Profile	Color space profile embedding	65,533 bytes
APP13	0xFFED	IPTC / Photoshop	Caption, keywords, copyright (legacy)	65,533 bytes
APP14	0xFFEE	Adobe	Adobe color transform flag	65,533 bytes
COM	0xFFFE	Comment	Arbitrary UTF-8 text payload (this tool)	65,533 bytes
DQT	0xFFDB	Quantization Table	Defines compression quality matrices	65,533 bytes
SOF0	0xFFC0	Start of Frame (Baseline)	Image dimensions, bit depth, components	65,533 bytes
SOF2	0xFFC2	Start of Frame (Progressive)	Progressive scan image parameters	65,533 bytes
DHT	0xFFC4	Huffman Table	Entropy coding table definitions	65,533 bytes
SOS	0xFFDA	Start of Scan	Begins compressed image data stream	Variable
EOI	0xFFD9	End of Image	Last 2 bytes of every valid JPEG	0 bytes
DRI	0xFFDD	Restart Interval	Defines restart marker spacing	4 bytes
RST0-RST7	0xFFD0 - 0xFFD7	Restart Markers	Error recovery sync points in scan data	0 bytes

Frequently Asked Questions

No. This tool performs direct binary injection. The compressed image data (SOS through EOI) is never decoded or re-encoded. Every pixel remains bit-identical to the original. The file size increases only by the comment length plus 4 bytes for the marker and length field.

Almost never. Facebook, Instagram, Twitter/X, and most messaging apps strip all non-essential JPEG markers (including COM, EXIF, and IPTC) during their server-side re-encoding pipeline. The comment will survive in the raw file you distribute directly (email, cloud storage, USB). If preservation through social platforms is required, consider steganographic embedding into the pixel data itself, which is a different technique.

Yes. The JPEG specification places no limit on the number of COM markers. Each is an independent segment with its own length field. This tool injects one COM segment and preserves any existing COM segments already present. If you run the tool multiple times on the same file, each pass adds an additional COM segment.

The JPEG standard (ITU-T T.81) does not specify a character encoding for COM segments. This tool encodes text as UTF-8 using the TextEncoder API. Most modern readers (exiftool, ImageMagick identify, this tool) interpret COM data as UTF-8. Legacy software may display mojibake for non-ASCII characters. For maximum compatibility, restrict comments to ASCII (code points 0x20 through 0x7E).

No. EXIF data resides in the APP1 segment (marker 0xFFE1) and follows a TIFF-based structure with typed fields (camera model, GPS coordinates, timestamps). The COM segment (0xFFFE) is a flat, unstructured text blob with no schema. EXIF editors cannot read COM markers and vice versa. Tools like exiftool can read both, but they are architecturally separate.

This tool enforces the maximum payload size of 65,533 bytes per COM segment. If your UTF-8 encoded text exceeds this limit, the tool will truncate to the nearest valid UTF-8 character boundary below the limit and display a warning. For very long payloads, you would need to split across multiple COM segments, which this tool does not currently support.