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As a general rule of thumb when is it appropriate to make a gif interlaced, a png interlaced and a jpeg progressive?
Especially when publishing the image on the web.
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With an interlaced display the picture is created by scanning every other line, and on the next scan, scanning every opposite line. Interlacing allows for a faster refresh rate by having less information during each scan at a lower cost.
Standard PNGs have a perfectly acceptable progressive rendering solution without interlacing. They render in obvious and simple fashion, from top to bottom.
Difference Between Interlaced and Progressive Scan:Progressive-scan displays all lines at once. Interlaced-scan displays half of the lines at a time.
Interlacing (also known as interleaving) is a method of encoding a bitmap image such that a person who has partially received it sees a degraded copy of the entire image.
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:
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