Change APNG Speed

APNG frame timing is controlled by two 16-bit fields in each fcTL chunk: delay_num and delay_den. The actual delay per frame equals delay_numdelay_den seconds. A denominator of 0 is treated as 100 per the APNG specification. Miscalculating these values produces animations that stutter, freeze, or race through frames. This tool parses the raw binary structure of your APNG file, recalculates every fcTL delay pair using your chosen speed multiplier, recomputes CRC32 checksums for modified chunks, and outputs a valid APNG. No server upload occurs. All processing happens in your browser.

The tool handles edge cases: zero denominators, extremely small delays that underflow to zero (clamped to 1/65535), and files with hundreds of frames. Note: this tool modifies timing metadata only. It does not re-encode pixel data, so output quality is lossless. Files exceeding 50 MB are rejected to prevent browser memory issues.

Each frame in an APNG file contains a fcTL chunk with two timing fields. The actual display duration of a single frame is computed as:

When delay_den = 0, the specification mandates treating it as 100, making the delay unit centiseconds.

To change speed by a multiplier m, the new delay becomes:

This tool normalizes all delays to a common denominator of 1000 (milliseconds), applies the multiplier, then finds the best-fit integer pair (delay_num, delay_den) within the 16-bit unsigned range (0 - 65535). The minimum effective delay is clamped to 1/65535 seconds to prevent zero-duration frames.

After modifying each fcTL chunk, the CRC32 checksum must be recalculated over the chunk type + chunk data:

Where CRC uses the polynomial 0xEDB88320 (reversed representation of the ISO 3309 / ITU-T V.42 standard polynomial).

Variable Legend: delay_num = frame delay numerator (16-bit unsigned integer). delay_den = frame delay denominator (16-bit unsigned integer). m = user-selected speed multiplier. CRC = 32-bit cyclic redundancy check value appended to each PNG chunk.