Skip interlaced PNG (Adam7) unless you need a rough image to appear while a file downloads over a slow connection. You pay for that with a larger file and more complex encoding, and the benefit is rarely visible on modern networks. Here is what Adam7 does, how it affects file size and loading, and when it is worth using.
What is Interlaced PNG?
PNG supports one interlacing method, called Adam7. An interlaced PNG stores the image in 7 passes. The first pass contains a very rough grid of pixels, and each subsequent pass fills in more detail. A viewer can display a coarse version of the image while it is still downloading, then sharpen it as the remaining passes arrive. This is sometimes described as a progressive PNG, because the image appears progressively rather than all at once.
A standard PNG, by contrast, loads from top to bottom. The entire top row appears first, then the next row, and so on. No full-image glimpse appears until the download is complete.
How Adam7 Interlacing Works
Adam7 divides the image into a grid of 8x8 blocks and assigns each pixel to one of 7 passes based on its row and column position. The passes are designed so that the first pass samples every 8th row and every 8th column. Later passes fill in the gaps. After just the first pass, you see a miniature version of the whole image. Each subsequent pass sharpens that version.
Why 7 Passes?
The number 7 comes from the pattern used to cover all pixels efficiently. The first pass covers a small fraction of the pixels, the second covers a larger fraction, and so on. By the final pass, every pixel has been transmitted. This pattern is built into the PNG specification and is the only interlacing method PNG supports.
Effect on File Size
Interlacing usually makes PNG files larger. Pixels that compress well together are split across different passes. PNG compression relies on filters and DEFLATE to find patterns in adjacent pixels. When those adjacent pixels are scattered among separate passes, the patterns break up, and the compressor cannot achieve the same level of compression. The result is a larger file than a standard PNG of the same image.
The size increase varies by image content. Photographs with smooth gradients often see a modest increase, while images with lots of fine detail or noise can see a larger jump. Expect an interlaced PNG to be a few percent to tens of percent larger than its standard version.
Effect on Loading and Display
On slow connections, an interlaced PNG lets a viewer see a blurry version of the whole image quickly, then watch it sharpen. Use it for large images on websites where visitors have limited bandwidth. On fast connections, the early glimpse is rarely noticeable because the entire file downloads in a fraction of a second. Decoding and displaying an interlaced PNG also takes slightly longer than a standard one, because the decoder must process 7 passes instead of one.
When the Early Glimpse Helps
The early glimpse matters most when a user might abort the download if the image is not what they expected. A photo gallery on a slow network could let a visitor skip a large image after seeing only the first pass. In practice, this scenario is less common now than in the early days of the web.
Should You Use It?
For most purposes, do not use interlaced PNG. The file size penalty and added complexity outweigh the early glimpse on modern networks. If you are creating images for your own use or for a site with fast connections, stick with a standard PNG. If you are publishing images for an audience that may have very slow connections, test both versions and decide based on the file size difference.
Convert to a Standard PNG
Most PNG optimisers can remove interlacing to save bytes. If you receive an interlaced PNG and want to reduce its size, convert it to a standard PNG. Photoshop's PNG options include an interlacing checkbox, and GIMP's export dialog also includes one. Uncheck that option to produce a standard PNG.
How to Remove Interlacing (Simple Steps)
If you have an interlaced PNG and want to make it standard, use image editing software or a dedicated PNG tool. Here is a simple procedure using common image editors:
- Open the interlaced PNG in your image editor, such as Photoshop or GIMP.
- Export or save the file. In Photoshop, look for the interlacing checkbox in the PNG options dialog and uncheck it. In GIMP, find the interlacing checkbox in the export dialog and uncheck it.
- Save the file with a new name or overwrite the original.
This will produce a standard PNG. The file size will likely be smaller, and the image will load from top to bottom. You can also use command-line tools or online services that strip interlacing, though none are named here.
Final Recommendation
For nearly all use cases, choose a standard PNG. The small early glimpse of an interlaced PNG does not justify the larger file size and slower decoding on modern hardware and networks. If you care about file size and loading speed, focus on other optimization techniques such as choosing the right color depth and using a good compressor. To reduce the size of existing interlaced PNGs, remove the interlacing as a first step. For more on PNG compression, read the other guides on compresspng.org.