APNG Checker

Not every PNG that claims to be animated actually is. The APNG format extends the standard PNG specification by embedding an Animation Control chunk (acTL) and Frame Control chunks (fcTL) alongside traditional IDAT data. A missing or malformed acTL chunk means the file will render as a static image in every browser, even if frame data exists. Misidentifying a static PNG as animated (or vice versa) causes rendering failures in web applications, broken CSS sprite sheets, and inflated file sizes with no visual payoff.

This tool performs byte-level binary inspection of the uploaded file. It validates the 8-byte PNG signature, iterates every chunk in the file, and checks for the presence and structural validity of acTL, fcTL, and fdAT chunks per the APNG specification. It reports frame count, loop count, per-frame delay (delay_numdelay_den), dispose and blend operations, and flags any structural anomalies. No data leaves your browser. The file is parsed entirely client-side using the ArrayBuffer API.

A valid PNG file starts with an 8-byte magic signature that must match exactly:

Each chunk in a PNG file follows a fixed binary structure:

where Length is a big-endian unsigned 32-bit integer representing only the data field size (excluding type and CRC). The CRC is computed over Type + Data using the CRC-32 polynomial:

APNG detection requires locating an acTL chunk (Animation Control) positioned before the first IDAT chunk. The acTL contains:

where num_plays = 0 means infinite looping. Each frame's timing is defined in its fcTL chunk:

where if delay_den = 0, it is treated as 100 (i.e., delay is in hundredths of a second). The dispose_op field controls how the frame buffer is cleared: 0 = none, 1 = background, 2 = previous. The blend_op controls compositing: 0 = source, 1 = over.