When to interlace an image?
Solution 1:
JPEG: YES — use progressive scan. It makes files smaller (each pass gets its own Huffman table), and partial rendering looks quite good.
GIF: NO — it's unlikely to make the file smaller, partial rendering is poor, and it's pointless for animGIFs. It's best not to use GIF at all (yes, even for anims).
PNG: NO — it hurts compression (as data from each pass is statistically quite different). If the image is large, use high-quality JPEG or lossy PNG if possible, as these may load quicker than a pixelated preview of a large lossless PNG.
ImageOptim will automatically change progressive/interlaced formats when it makes files smaller.
Disclaimers for nitpickers:
- In case of small and medium-sized images the progressive preview of each image is not going to be visible long enough for the user to appreciate it. Some browsers don't even bother rendering anything until the whole file is downloaded, so it's better to focus on saving bandwidth to get the whole page loaded ASAP.
- Non-progressive JPEG is a bit more efficient when the files are tiny (small thumbnails), but then the savings are tiny, too.
- iOS Safari has a higher maximum allowed image size for baseline JPEG than progressive, but the right solution there is to serve images at sizes reasonable for mobile in the first place.
Solution 2:
My general rule of thumb: don't ever use interlacing. Interlaced formats typically occupy more space, have (slightly) more complexity and less support in decoders, and the alleged advantages for the user experience are at least debatable. Some arguments for PNG, and in general.
Some people like interlaced or "progressive" images, which load gradually. The theory behind these formats is that the user can at least look at a fuzzy full-size proxy for the image while all the bits are loading. In practice, the user is forced to look at a fuzzy full-size proxy for the image while all the bits are loading. Is it done? Well, it looks kind of fuzzy. Oh wait, the top of the image seems to be getting a little more detail. Maybe it is done now. It is still kind of fuzzy, though. Maybe the photographer wasn't using a tripod. Oh wait, it seems to be clearing up now ...