pdf export paper_bgcolor turns black when using image magick to convert to jpg

[etpinard]

Moved here from plotly/plotly.js#2408 where @cldougl wrote

Now after exporting a pdf image and using image magick to convert to a jpg the paper_bgcolor becomes black:

and @rafael-andrade wrote

I was having the same issue using another library. I was converting it to JPEG image, the default color is black. So I changed it to PNG before export to PDF.