For fun I've been poking around at the planar mode in ETC2. From this presentation:
Okay, they are intended for use on smoothly varying blocks. I'm intrigued by planar mode because the colors are stored at high precision (676) and there are no selectors like in the other modes. But what do planar blocks really look like though? This random planar block image, sorted by standard deviation, was pixel (box filter) upsampled by 400%:
Just the green channel:
This image was computed by poking random 8-bit bytes into an ETC2 block. If the block passed the planar ETC2 mode check it gets decoded and stored in a temporary image. After the temp image was full it gets sorted by standard deviation.
Hey - most of these random planar blocks are not actually smoothly varying! I'm unsure if this is actually useful in practice, but it's interesting.
Looking at how Planar blocks are actually unpacked, the H and V vectors are interpreted relative to the "origin" color, so they're actually signed and the unpacking code uses per-component [0,255] clamping. This is where the high frequency patterns come from.
// ro, go, bo - origin color, unpacked from 676
// rv, gv, bv and rh, gh, bh - vertical and horizontal colors, unpacked from 676
// unpack planar block's pixels - there are no selectors in this mode
for (int y = 0; y < 4; y++)
for (int x = 0; x < 4; x++)
(4 * ro + x * (rh - ro) + y * (rv - ro) + 2) >> 2,
(4 * go + x * (gh - go) + y * (gv - go) + 2) >> 2,
(4 * bo + x * (bh - bo) + y * (bv - bo) + 2) >> 2,