When transforming the SVG file with opacity attributes into PDF file, the 
result displays the opacification well, but also with some regular grid lines 
across all the PDF canvas, the colors of those grid lines are just the "real" 
colors of thoes original objects being opacified. 
It is doubtful that this issue was caused by the inaccurate calculation when 
doing the opacification in block based and then blending the result with those 
original objects.

This issue exists in all the recent versions of Fop. The SVG file and 
transformed PDF file are attached as reference.