Difference between HTML Canvas and Flash render on the same sprites

I have been working on this little project converting old Adobe Flash sprites into HTML Canvas.
So far, everything is going great, I have been able to re-assemble the sprites correctly.

I just don’t understand why there is such a big difference where the shadows normally are (white lines appear).

Here is the flash render (left) and the Canvas render (right) :

enter image description here
enter image description here

What could I try on my ctx or ctrans variable to smooth this out?
Thanks