# note: An uncompressed alpha plane under the gradient filter.
# expect: ok
# exercises: WebPUnfilters[3], from left plus above minus above-left.
chunks VP8X ALPH VP8
alpha 1
alph_filtering 3
width 16
height 16
yac_qi 30
macroblock
coeffs y2 -20
coeffs y[2] 1:5 -3