About

PNG files store metadata inside discrete binary chunks. The most common are tEXt chunks, each containing a keyword (e.g., Author, Description, Copyright) paired with a Latin-1 encoded string value, separated by a null byte (0x00). Getting metadata wrong has real consequences: incorrect copyright fields create legal ambiguity, missing Software tags break automated asset pipelines, and bloated ancillary chunks inflate file size without benefit. This tool parses the raw binary structure of your PNG, extracts every ancillary text chunk, and lets you edit, add, or delete entries before reconstructing a valid file with correct CRC32 checksums. It operates entirely in-browser. No upload occurs. The tool assumes well-formed PNG files conforming to ISO/IEC 15948 (PNG specification). Malformed or truncated files will be rejected at the signature check stage.

Formulas

Every PNG chunk follows a fixed 12-byte envelope structure plus its data payload:

Chunk = Length (4 bytes) + Type (4 bytes) + Data (Length bytes) + CRC (4 bytes)

The CRC32 checksum covers the Type and Data fields combined:

CRC = crc32(Type + Data)

A tEXt chunk stores its payload as:

Data = Keyword + 0x00 + TextString

Where Keyword is 1 - 79 bytes of Latin-1 text, 0x00 is the null separator, and TextString is the value (zero or more Latin-1 bytes). The file signature is a fixed 8-byte sequence: 137 80 78 71 13 10 26 10 (decimal), which this tool validates before any parsing occurs.

Reference Data

Chunk Type	Category	Critical	Description
IHDR	Header	Yes	Image dimensions, bit depth, color type
PLTE	Palette	Yes	Color palette for indexed-color images
IDAT	Data	Yes	Compressed image data (deflate)
IEND	Trailer	Yes	Marks end of PNG datastream
tEXt	Text	No	Latin-1 keyword - value pair, uncompressed
zTXt	Text	No	Compressed text (deflate), Latin-1 keyword
iTXt	Text	No	International text, UTF-8 encoded
tIME	Time	No	Last modification time (UTC)
gAMA	Color	No	Image gamma (100000 × gamma value)
cHRM	Color	No	Primary chromaticities and white point
sRGB	Color	No	Standard RGB color space rendering intent
iCCP	Color	No	Embedded ICC profile (compressed)
bKGD	Display	No	Default background color
pHYs	Display	No	Physical pixel dimensions (DPI)
sBIT	Display	No	Significant bits per channel
sPLT	Palette	No	Suggested palette for color reduction
hIST	Stats	No	Histogram of palette usage
tRNS	Transparency	No	Transparency info for palette/grayscale
eXIf	EXIF	No	Exchangeable image file format data

Frequently Asked Questions

ISO/IEC 15948 defines these registered keywords for tEXt chunks: Title, Author, Description, Copyright, Creation Time, Software, Disclaimer, Warning, Source, and Comment. You can also use custom keywords up to 79 bytes long, but standard tools may not display them.

No. This tool operates at the binary chunk level. It reads the raw IDAT (compressed pixel data) chunks and copies them byte-for-byte into the output file. No decompression, re-rendering, or re-encoding of pixel data occurs. The output is bit-identical to the input for all critical chunks.

A CRC32 mismatch means the stored checksum in a chunk does not match the computed value over its Type + Data fields. This typically indicates file corruption during transfer (partial download, disk error) or a non-conforming encoder. This tool will warn you but still attempt to parse the file. Edited chunks will always have correct CRC32 values in the output.

tEXt stores uncompressed Latin-1 text. zTXt stores deflate-compressed Latin-1 text (useful for large comments). iTXt stores UTF-8 international text with optional compression and language tags. This tool reads all three types but writes new/edited entries as tEXt for maximum compatibility. Original zTXt and iTXt chunks that are not edited are preserved as-is.

The PNG specification imposes no hard limit on text chunk data length. However, the chunk length field is a 4-byte unsigned integer, giving a theoretical maximum of 2,147,483,647 bytes per chunk (the spec restricts to 2³¹ − 1). In practice, keep metadata values under a few kilobytes. Extremely large text chunks slow down decoders and may be silently dropped by some software.

Typically, text metadata accounts for a negligible fraction of PNG file size (a few hundred bytes). The bulk is IDAT compressed pixel data. Removing metadata might save 0.01 - 0.1% of total size. The exception is files with embedded ICC profiles (iCCP chunks), which can be 2 - 60 KB. This tool only modifies text chunks, not iCCP.