What is PNG interlacing?
Interlacing is an optional way of ordering the pixels inside a PNG so that the image loads progressively. Instead of painting the picture one row at a time from top to bottom, an interlaced PNG shows a low-resolution version of the whole image almost immediately, then fills in detail as the rest of the data arrives. On a slow connection that means you see a rough preview of the entire image instead of a sliver of fully detailed pixels at the top. It's purely about loading order — the final, fully downloaded image is identical either way. To recap how a PNG stores pixels in the first place, start there.
How does Adam7 work?
PNG's interlacing method is called Adam7, named after Adam M. Costello and its seven passes. The image is divided into a repeating 8×8 grid of pixels, and each of the seven passes transmits a different interleaved subset of those pixels. The first pass sends just one pixel out of every 64 — enough to sketch a coarse preview of the entire frame. Each later pass adds more pixels in between, doubling the effective resolution until the seventh pass fills in everything that remains. Because every pass already covers the full width and height of the image, the preview sharpens evenly across the whole picture rather than crawling down from the top.
What are the downsides?
Interlacing isn't free. PNG compression works best when neighbouring pixels are stored next to each other, because it can predict each row from the one above it. Scattering pixels across seven separate passes breaks up that locality, so interlaced PNGs are usually larger than the same image saved non-interlaced. They also cost more CPU and memory to decode, since the viewer has to reassemble seven interleaved passes instead of reading a single top-to-bottom stream. If your goal is a smaller file, see how PNG compression works for the levers that actually help.
Should you use interlacing today?
Usually no. Adam7 was designed for an era of slow dial-up links, where a fast rough preview genuinely improved the experience. On modern broadband and mobile networks most PNGs finish loading so quickly that the progressive preview never has a chance to matter — you just end up with a bigger file and slower decoding for no real benefit. That's why most tools save PNGs non-interlaced by default. It's a similar idea to progressive JPG, which also renders a blurry whole-image preview first; the difference is that progressive JPGs often compress as well as or better than baseline ones, whereas interlaced PNGs almost always get larger. If file size is what you care about, it's better to compress your PNG and leave interlacing off.
Frequently asked questions
- Why is Adam7 called Adam7?
- It's named after its creator, Adam M. Costello, and the seven interleaved passes the scheme uses to lay out the image.
- Does interlacing change how a PNG looks once loaded?
- No. Interlacing only affects the order pixels arrive in. Once fully downloaded, an interlaced PNG is pixel-for-pixel identical to a non-interlaced one — it's still lossless.
- Are interlaced PNGs bigger?
- Usually yes. Spreading pixels across seven scattered passes makes compression less effective, so interlaced PNGs are typically larger than the same image saved non-interlaced.
- Should I turn interlacing on?
- For most images, no. On today's fast connections the progressive preview rarely helps, and you pay for it with a bigger file and slower decoding. Save non-interlaced unless you have a specific reason not to.