First, base the 1px minimum line width on output line width, not pen width. If scaling up with a pen width < 1, the output width may well be more than 1px, in which case the correct output is that width, not 1px.
Second, cairo_device_to_user_distance() can provide negative values. Use the absolute value of its outputs. The negative comes from mirror-type transforms (negative scale values). Using a negative value results in no line being drawn; and if dash_count > 0, also an error from Cairo.
Third, use the maximum of widthx and widthy, because scaling is not necessarily uniform. Setting a minimum line width is only necessary because Cairo produces dashed lines when scaling up and the output width is < 1px, and we don't want that happening when scaling Y more than X.
First, base the 1px minimum line width on output line width, not pen width. If scaling up with a pen width < 1, the output width may well be more than 1px, in which case the correct output is that width, not 1px.
Second,
cairo_device_to_user_distance()can provide negative values. Use the absolute value of its outputs. The negative comes from mirror-type transforms (negative scale values). Using a negative value results in no line being drawn; and if dash_count > 0, also an error from Cairo.Third, use the maximum of
widthxandwidthy, because scaling is not necessarily uniform. Setting a minimum line width is only necessary because Cairo produces dashed lines when scaling up and the output width is < 1px, and we don't want that happening when scaling Y more than X.