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 no /SMask.
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 no /SMask.