Asymptote: How to draw surfaces with more than four vertices without "seams?"

Question

In Asymptote 3D, I need to draw surfaces with more than 4 vertices, but when I do, either it simply doesn't work (I can't reproduce this in a simple example, but can provide a more complicated case where this happens if it is helpful) or a seam appears, as in the example below:

settings.prc = false;
defaultpen(fontsize(10pt));
import three;
import graph3;
import patterns;
size(11cm,0);

currentprojection=perspective((10,-20,10));
currentlight = (3,-2,3);

render render = render(compression=Low, merge = true);

draw(surface((0,0,0) -- (1,0,0) -- (1,1,0) -- (0.5,1.5,0) -- (0,1,0) -- cycle), lightblue+opacity(0.5), render);

Running this command

asy -noprc -render 0 -f pdf example.asy

gives the following image:

Is there a way to get rid of the dark blue "seam?"

Answer 1

It seems that the "seam behavior" depends on the pdf viewer and the -render option. "Seam" problems appear with opacity and -render=0 option: depending on the pdf viewer the superposition of a common border of the two patches creates such a behavior.

With -noprc and -render=0, we have "seam" with Okular, acroread, gv (the image is vectorised). No "seam" with evince, mupdf, zathura. I have transparency problems with evince and zathura. mupdf is fine.

With -noprc (so the OpenGL render is used and a bitmap image is produced) there is no "seam" at all.

To be precise, all the version are up-to-date Debian Sid.

As explain in the documentation, -render=0 is very limited.