What does 'interlaced' mean for a GIF?

An interlaced GIF stores each frame's rows out of visual order (in 4 passes: every 8th row, then the offset 8th rows, then every 4th, then every other), a technique from the days of slow modem connections that let a partially-loaded image render as a blurry-then-sharpening preview instead of appearing top-to-bottom.

Does interlacing change the image's colors or quality?

No, interlacing only changes the storage order of rows, not any pixel value. The frame looks identical once fully loaded; the only difference is how it renders while progressively downloading.

Does this affect file size?

Only marginally, and it can go either way: reordering rows changes which pixel runs are adjacent during LZW compression, which sometimes compresses slightly better or slightly worse than the original row order, but the effect is generally small.

Why write a custom encoder instead of using this category's shared one?

This category's shared `encodeGif` always writes plain top-to-bottom row order and has no interlace option, so this tool builds its own GIF89a container, reusing the shared codec's exported color table, index mapping, and LZW compression, but adding the row-reordering and interlace-bit-setting logic itself.

Non-Interlaced to Interlaced GIF Converter

Overview

Introduction

GIF's interlacing feature, largely a relic of dial-up-era web browsing, stores a frame's rows in four passes instead of top-to-bottom, so a slow-loading image renders as a rough, blocky preview that progressively sharpens rather than appearing one row at a time.

This tool converts an ordinary GIF's frames into that interlaced row order, producing a real, spec-compliant interlaced GIF you can use to study or demonstrate the format's historical rendering behavior.

What Is Non-Interlaced GIF to Interlaced GIF Converter?

A GIF-to-GIF converter that decodes every frame of an uploaded GIF, reorders each frame's pixel rows into GIF89a's standard 4-pass interlace pattern, and re-encodes the result as a real interlaced GIF, with the Image Descriptor's interlace bit correctly set.

It reuses this category's shared color table, palette-index mapping, and LZW compression functions, adding only the row-reordering and interlace-flagging logic the shared encoder doesn't support.

How Non-Interlaced GIF to Interlaced GIF Converter Works

Every frame is decoded and composited via this category's shared GIF decoder (which already handles reading interlaced input, so any starting GIF works, interlaced or not).

Each frame's pixels are mapped to the shared 216-color-plus-transparency palette, then the resulting index array is reordered: pass 1 takes rows 0, 8, 16...; pass 2 takes rows 4, 12, 20...; pass 3 takes rows 2, 6, 10...; pass 4 takes rows 1, 3, 5....

The reordered indices are LZW-compressed and written into a hand-built GIF89a container whose Image Descriptor packed byte has bit 0x40 (the interlace flag) set.

When To Use Non-Interlaced GIF to Interlaced GIF Converter

Use it to produce a genuinely interlaced GIF for testing a decoder, demonstrating the format's historical progressive-rendering behavior, or archival curiosity.

Also useful if you're building or testing your own GIF parser and need real interlaced test fixtures.

Often used alongside Interlaced GIF to Non-Interlaced GIF Converter and GIF to Sprite Sheet Converter.

Features

Advantages

Produces a real, spec-compliant interlaced GIF (correct interlace bit and correct 4-pass row order), not a simulated or approximated one.
Reuses the shared codec's proven color table, palette mapping, and LZW compression rather than reimplementing them from scratch.
Works on any valid input GIF, including one that's already interlaced (it's simply decoded and its rows reordered again).

Limitations

Interlacing has no visible effect once a GIF is fully loaded (which is instant for essentially all modern viewing), so this is primarily an educational or archival tool rather than one with practical everyday benefit.
Like every tool built on this category's shared codec, all frames are re-quantized through a fixed 216-color palette, so subtle color banding is possible if the source GIF used a finer palette.

Examples

Interlacing a simple animated GIF

Input

logo.gif (2 frames, 64x64, non-interlaced)

Output

logo-interlaced.gif (2 frames, 64x64, interlace bit set, rows reordered per frame)

Each frame's 64 rows are reordered into the 4-pass sequence (rows 0,8,16,...,56 first, then 4,12,...,60, then 2,6,...,62, then 1,3,...,63) before compression.

Best Practices & Notes

Best Practices

Use Interlaced GIF to Non-Interlaced GIF Converter afterward if you need to verify the round trip restores the original row order.
Don't expect a visual difference when viewing the output normally; the effect only matters while a file is progressively downloading over a very slow connection.
If file size matters more than interlacing, skip this tool; it offers no compression benefit of its own.

Developer Notes

The 4-pass row-reordering table ({start:0,step:8}, {start:4,step:8}, {start:2,step:4}, {start:1,step:2}) mirrors the exact pass order the shared codec's private `deinterlace` decode step already uses, just applied in the encode direction; the interlace flag is simply OR'd into the Image Descriptor's packed byte (0x40) alongside the existing logic borrowed from the shared `encodeGif`.

Non-Interlaced GIF to Interlaced GIF Converter Use Cases

Generating real interlaced GIF test fixtures for a custom GIF decoder or parser
Demonstrating GIF's historical progressive-rendering behavior for an educational article or presentation
Archiving or studying how interlacing affects a specific image's LZW compression ratio

Common Mistakes

Expecting a visible quality or file-size improvement from interlacing; it changes rendering order during download, not the final image or meaningfully the file size.
Assuming every GIF viewer or tool renders interlaced GIFs progressively; many modern viewers simply wait for the whole file and show it directly regardless of the interlace bit.

Tips

Pair this with a network throttling tool in your browser's DevTools to actually observe the progressive-rendering effect while the file loads.
If you just need a normal, non-interlaced GIF back afterward, use this category's Interlaced GIF to Non-Interlaced GIF Converter.

Non-Interlaced GIF to Interlaced GIF Converter