TPNGImage help

As JPEG (Joint photographic experts group), Portable Network Graphics also supports image interlacing. This technique encodes the image in a way to allows the user preview the image faster as it is being transfered.

PNG's two-dimensional interlacing scheme is more complex to implement than GIF's line-wise interlacing. It also costs a little more in file size. However, it yields an initial image eight times faster than GIF (the first pass transmits only 1/64th of the pixels, compared to 1/8th for GIF). Although this initial image is coarse, it is useful in many situations. For example, if the image is a World Wide Web imagemap that the user has seen before, PNG's first pass is often enough to determine where to click. The PNG scheme also looks better than GIF's, because horizontal and vertical resolution never differ by more than a factor of two; this avoids the odd "stretched" look seen when interlaced GIFs are filled in by replicating scanlines. Preliminary results show that small text in an interlaced PNG image is typically readable about twice as fast as in an equivalent GIF, i.e., after PNG's fifth pass or 25% of the image data, instead of after GIF's third pass or 50%. This is again due to PNG's more balanced increase in resolution.