What is Image Interlacing?
Image interlacing stores scanlines or pixel passes in a reordered sequence so a coarse image can appear before all data arrives. Successive passes refine that preview until the complete image has been decoded.
How Image Interlacing works
Interlaced storage changes transmission order rather than the final decoded pixels. A decoder fills widely separated rows or samples first, producing a rough whole-frame impression, then replaces gaps as later passes arrive. This behavior is distinct from interlaced video fields and from ordinary top-to-bottom raster delivery. It is selected during encoding and matters mainly during incremental network display, while the completed asset retains its normal dimensions and color model.
Key facts
- PNG defines the Adam7 scheme, whose seven passes sample progressively denser positions in both axes; a non-interlaced PNG stores rows in normal order.
- GIF interlacing rearranges rows into four passes, so a streaming decoder can display the image height early without having received every intervening scanline.
- Interlacing can enlarge some files and adds pass bookkeeping; if the client waits for the complete response before decoding, its progressive-display advantage disappears.
When Image Interlacing matters
Enable interlacing when an early full-frame preview is valuable on slow transfers or uncertain connections. It adds encoding or decoding overhead and may provide little benefit when images are already small or delivered quickly.