Making the background of PNG output transparent

[yarusome]

The following code piece produces a PNG image with a transparent background using Asymptote 2.70 but with a white background using 2.83:

settings.outformat = "png";
unitsize(5cm);
draw(unitcircle);

The interesting part is that if:

1. commenting out the first line;
2. converting the output PDF (say output.pdf) using magick output.pdf output.png;

then output.png does have a transparent background, which confirms that the background of output.pdf is not set to be white.

Is this behavioral change intended during updates somewhere between 2.70 and 2.83? If so, how to make Asymptote produce PNG output with a transparent background now? Adding settings.convertOptions = ""; is of no avail.

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ImageMagick version: 7.1.0-45 Q8 x64

[johncbowman]

If you want a transparent background, you need to use the pngalpha driver instead of the default png16m driver (the driver recommended by the ghostscript developers for normal use):

settings.pngdriver="pngalpha";