Are there any open source sprite format?

PNG just plainly sucks. I has no support for animations, timing or delta compression and sequence names. Also, PNG is slow at random access (you have to decompress image before use). Proprietary formats (like Diablo 2 DCC) are much better, but highly specialized for their games: some have no true color, other have no directions.

If you draw a stick figure with line segments.

Line 100,100 to 100,200
Line 100,200 to 75,300
Line 100,200 to 125,300

Then, move the limbs

Line 100,100 to 100,200
Line 100,200 to 85,275
Line 100,200 to 115,275

Then, do linear interpolation for corresponding points.  You get tiny data files.  The images have to have corresponding points, though.

I don't consider myself a genius for (re)inventing this notion, but who knows what counts for patents these days.

God says...
Line: 54763
14:10 The heart knoweth his own bitterness; and a stranger doth not
intermeddle with his joy.

14:11 The house of the wicked shall be overthrown: but the tabernacle
of the upright shall flourish.

14:12 There is a way which seemeth right unto a man, but the end
thereof are the ways of death.

14:13 Even in laughter the heart is sorrowful; and the end of that
mirth is heaviness.