Shaper Curve with Matrix

There are actually two individual options here. -as (lowercase) specifies a different shaper curve for each color channel, whereas -aS (uppercase) specifies a single shaper curve for all color channels. In either case, a matrix is used to transform from the original color space to the standard color space.

A shaper curve is a more arbitrary curve when compared to a gamma curve. In other words, a shaper curve can include more ``twists and turns'' to model the response function of RGB sensors. Consequently, if a camera has RGB sensors that are not truely logarithmic, shaper curves can produce better profiles.

That said, however, I found that a shaper-curve based profile performs worse than a gamma-curve based profile for an CanonEOS 1Ds.

You can experiment with this option if -ag or -aG does not produce a satisfactory profile. Sometimes, if -ag or -aG fails to produce a quality profile, you should consider shooting the target again. Inconsistent lighting, glare and etc. can pollute the IT8.7/2 image to the point that the captured bits are not longer representing how the camera sees