Why was it that, back in the early days of the WWW, some browsers would load a low-res image which would then become sharper, while others loaded the image bit-by-bit, top to bottom?

Question

Why was it that, back in the early days of the WWW, some browsers would load a low-res image which would then become sharper, while others loaded the image bit-by-bit, top to bottom?

In: Technology

Answer 1

That’s a feature of jpeg. Somewhat simplified jpeg encodes images in blocks of 8×8 pixels. It computes a discrete cosine transform of those blocks and quantizes the resulting coefficients. An interesting side effect is that one of those transformed values happens to be the average value of the 8×8 block. So by putting all of those values at the front of the file you can get a low res approximation before having downloaded the entire file.