softMaskAlpha

Soft-mask alpha (ISO 32000-1 §11.6.5.2), normalised to 8-bit grayscale: one byte per pixel, 0 = transparent, 255 = opaque, row-major over softMaskWidth×softMaskHeight. Null when the image carries neither a /SMask nor a stencil /Mask.

A stencil /Mask (§8.9.6) is flattened into this same plane, so both masking forms composite through one path. The plane may be larger or smaller than the image; the raster path resamples it.