On 2017-05-17 11:39, Eli Zaretskii wrote: >> From: Clément Pit--Claudel >> Date: Wed, 17 May 2017 00:16:47 -0400 >> >> Could underline thickness be made configurable? It would be nice to be able to pick between regular and thick/bold underlines (the later would be obtained by doubling the usual underline thickness, I imagine). > > You need to be aware of some subtleties with underlines as currently > implemented, and we should consider all of that when we decide what > kind of configurability we want and what should it do. See below. > >>> FWIW, on Windows I see neither straight nor wavy underline thicken. >>> They both continue to have the same line width (thickness) when >>> text-scaled. >>> >>> Should they not stay the same? Should they thicken? Why? >> >> Thanks for the reply! They do scale in GNU/Linux; the code in xftfont says: >> >> font->underline_position = -ft_face->underline_position * size / upEM; >> font->underline_thickness = ft_face->underline_thickness * size / upEM; >> >> The corresponding code in w32font says: >> >> font->underline_thickness = metrics->otmsUnderscoreSize; >> font->underline_position = -metrics->otmsUnderscorePosition; >> >> which might be missing the scaling? > > Not all font back-ends support this scaling, and not with every font. > E.g., xfont.c doesn't support this at all, AFAICS. And while we could > probably add this feature to MS-Windows, it will only be available > with OTF and TTF fonts (I believe it's the same on Unix and GNU > systems). Makes sense. And, of course, the scaling is outside of Emacs' control on TTYs. > Moreover, if you mix fonts of different sizes on the same line in the > same run of consecutive underlined characters, you will see that Emacs > defines the thickness and the position of the underline at the first > character, and then reuses those values for the entire run, even if > the size of the font changes -- it doesn't recompute the values when > the font changes. We do this because anything else will look uglier > than what we have now. I saw this, indeed. > What all this means is that currently the exact visual effect of the > underline attribute is deliberately not well-defined: about the only > thing you can rely on is that you will get a horizontal line somewhere > in the lower portion of the characters. > > Implementing your suggestion would require that we define the behavior > much better, which is not easy given the different font drivers and > fonts, on which the user has almost no control. E.g., we will need to > decide whether thickness customization overrides the font-dependent > scaling, and if not, how these two play together. And if we want to > allow customization of the underline position (why not?), we will have > to decide what to do with it when the font size changes. And then we > will need to decide what to do if the font doesn't support scaling. That makes sense, but I'm not sure all of this is needed. I agree that it would be nice, but is it really necessary? In terms of code, my suggestion would translate into multiplying the `thickness' variable in xftfont by 2 when :bold t is specified in the underline's property list. > Bottom line: I think the hard part here is to describe the new > behavior, and do that in way that makes sense. Implementing that > (assuming the fonts and font backends support the requirements) should > be relatively easy, once all of these hidden issues are figured out. Thanks for the explanation. Clément.