the antialiasing on different background,

[tomffff]

Check this code:

< rect width="200" height="100", x="0" y="0" fill="gray" />
< path d = "M 100 0 v 200M 60 50 h80 M 60 150 h80" stroke="#ffff00" stroke-width="20" fill="none" stroke-linecap="round" />
when I rasterized to a 50x50 bitmap and display it on a black background:

same line looks in different size!
it's the same line from top to bottom, however, the lower part looks much thicker,

I think this was caused by antialiasing, can we setup a default background to blend the color for when doing the antialiasing?