About

APNG (Animated Portable Network Graphics) supports full 24-bit color depth with an 8-bit alpha channel, unlike GIF which is constrained to 256 indexed colors. Choosing GIF for assets with gradients or semi-transparency introduces visible banding and hard-edged matte artifacts. This tool performs real binary-level APNG assembly: each input frame is encoded as a PNG, its compressed image data (IDAT) is extracted, and the chunks are repackaged with animation control headers (acTL, fcTL, fdAT) per the APNG specification. No server-side processing occurs. All encoding happens in your browser via ArrayBuffer manipulation.

Frame timing is specified as a rational fraction delay_numdelay_den in seconds, giving sub-millisecond precision. The tool approximates this from your millisecond input. Note: browsers that do not support APNG (older IE/Edge Legacy) will display only the first frame as a static PNG, which is the correct fallback behavior defined by the spec. Maximum practical frame count depends on available browser memory. Expect reliable results up to approximately 200 frames at 1920×1080 resolution.

Formulas

The APNG format extends PNG by inserting animation-specific chunks. The binary structure follows this sequence:

PNG Signature → IHDR → acTL → fcTL₀ → IDAT₀ → fcTL₁ → fdAT₁ → … → fcTL_N → fdAT_N → IEND

Each PNG chunk follows a fixed structure of 4 bytes length, 4 bytes type, variable data, and 4 bytes CRC32 checksum:

chunk = uint32(length) + ascii(type) + data[0..length] + CRC32(type + data)

Frame delay is expressed as a rational number in seconds:

delay = delay_numdelay_den seconds

The CRC32 checksum is computed over the chunk type and data bytes using a 256-entry lookup table with polynomial 0xEDB88320:

CRC32(buf) = NOT(n−1∏i=0 table[(crc &xor; buf[i]) ∧ 0xFF] &xor; (crc >>> 8))

Where delay_num is the numerator of the delay fraction, delay_den is the denominator (if 0, treated as 100), crc is the running checksum initialized to 0xFFFFFFFF, and buf is the byte sequence over which the CRC is computed.

Reference Data

Format	Colors	Alpha	Animation	Compression	Avg File Size (100 frames, 256×256)	Browser Support
APNG	16.7M (24-bit)	Full 8-bit	Yes	Lossless (Deflate)	~2 - 8 MB	Chrome, Firefox, Safari, Edge
GIF	256 indexed	1-bit (on/off)	Yes	Lossless (LZW)	~1 - 4 MB	Universal
WebP	16.7M	Full 8-bit	Yes	Lossy/Lossless (VP8)	~0.5 - 3 MB	Chrome, Firefox, Edge (no Safari <14)
AVIF	16.7M+	Full	Yes (AVIFS)	Lossy/Lossless (AV1)	~0.3 - 2 MB	Chrome, Firefox (partial Safari)
MNG	16.7M	Full 8-bit	Yes	Lossless (Deflate)	~3 - 10 MB	None (deprecated)
FLIF	16.7M+	Full 16-bit	Yes	Lossless (MANIAC)	~1 - 5 MB	None (experimental)
APNG Chunk Structure
Chunk Type		Purpose		Required
IHDR		Image header (width, height, bit depth)		Yes (once)
acTL		Animation control (num_frames, num_plays)		Yes (once, before IDAT)
fcTL		Frame control (dimensions, offsets, delay, dispose/blend)		Yes (per frame)
IDAT		Image data for first frame		Yes (first frame)
fdAT		Frame data for subsequent frames		Yes (frames 2 - N)
IEND		Image trailer (end marker)		Yes (once)
Common Frame Delays
Animation Speed	Delay	FPS Equivalent	delay_num		delay_den
Very Fast	33 ms	30 fps	1		30
Fast	50 ms	20 fps	1		20
Standard	100 ms	10 fps	1		10
Slow	200 ms	5 fps	1		5
Very Slow	500 ms	2 fps	1		2
Slideshow	1000 ms	1 fps	1		1

Frequently Asked Questions

Non-supporting browsers (Internet Explorer, Edge Legacy before Chromium) display only the first frame as a standard static PNG. This is by design in the APNG specification: the animation chunks (acTL, fcTL, fdAT) are classified as ancillary and are silently ignored by decoders that do not recognize them. The first frame's IDAT data renders as normal. All modern browsers (Chrome 59+, Firefox 3+, Safari 8+, Edge 79+) fully support APNG playback.

APNG uses lossless Deflate compression on full RGBA pixel data (32 bits per pixel), while GIF quantizes to a maximum 256-color palette (8 bits per pixel). For a 256×256 frame, APNG stores up to 262,144 bytes of raw pixel data per frame versus GIF's ~65,536 bytes. The tradeoff is image quality: APNG preserves gradients and semi-transparent pixels perfectly. To reduce APNG file size, resize frames to smaller dimensions or reduce the total frame count.

The dispose_op field in each fcTL chunk controls what happens to the canvas region after the frame is displayed. Value 0 (APNG_DISPOSE_OP_NONE) leaves the frame in place. Value 1 (APNG_DISPOSE_OP_BACKGROUND) clears the region to transparent black. Value 2 (APNG_DISPOSE_OP_PREVIOUS) restores the region to the state before the frame was drawn. This tool uses APNG_DISPOSE_OP_NONE by default, which works correctly for full-frame animations where each frame replaces the entire canvas.

The practical limit is browser memory. Each frame is held as an Image object and rendered to an offscreen Canvas for PNG export. A single 1920×1080 RGBA frame consumes approximately 8.3 MB of uncompressed memory. With 100 frames, that is ~830 MB. Most browsers allocate 1-4 GB to a tab. For reliable operation, stay below 200 frames at 1080p or 500 frames at 512×512. The tool will display an error if a Canvas toBlob operation fails due to memory pressure.

Yes. Each frame in the APNG specification has its own fcTL chunk with independent delay_num and delay_den values. This tool exposes per-frame delay controls. Click any frame thumbnail to edit its individual delay in milliseconds. The global delay setting applies to all frames initially, but per-frame overrides take precedence. This allows effects like a slow pause on one keyframe while the rest animate at 30 fps.

A loop count (num_plays in acTL) of 0 means infinite looping, per the APNG specification. A value of 1 means play once and stop. A value of 3 means play 3 times total. This matches the behavior of GIF's Netscape Application Extension where 0 also means infinite. The tool defaults to 0 (infinite) which is the expected behavior for most animated images used on the web.