# note: A losslessly compressed alpha plane carrying a predictor transform,
#   from a cwebp encode of a gradient.
# expect: ok
# exercises: Compression method 1 hands the payload to the lossless decoder.
#   A predictor transform leaves the red, blue and alpha codes non-trivial, so
#   Is8bOptimizable() says no and the plane is decoded through
#   DecodeImageData() with ExtractAlphaRows() pulling the green channel out
#   afterwards. The 21 payload bytes are cwebp -q 60 output, kept as the
#   control that a written plane is shaped like a real one.
chunks VP8X ALPH VP8
alpha 1
alph_compression 1
alph_data b90a44f43f40a46d333634ff920f4f23262099a5bb
width 16
height 16
yac_qi 30
macroblock
coeffs y2 -20
coeffs y[2] 1:5 -3