Using pango in KMSCON (terminal-emulator)

[David Herrmann <dh herrmann googlemail com>]

Hi

I have been working on KMSCON [1] for over a year now and played with
different ways to render text. As I am no expert in font-rendering, I
thought I'd just ask for some advice/review so I can improve kmscon.

kmscon is an attempt to replace the linux-console in user-space. For
the purpose of this discussion we can reduce it to an application
similar to libvte (ie., terminal-emulator). That is, it tries to
render a table of statically sized cells with mostly one glyph per
cell. There are also multi-cell glyphs that span multiple cells.
However, no glyphs ever share a cell (no kerning, ...).

I have played with different font-backends, but I'd really like to
take advantage of all the features that pango provides.

What do I need?

1) I want to feed user-specified font-configuration (name, size, ...)
into pango and then pango should select the right font(s)
2) I want pango to provide me "normal" width/height for the selected
font so I can size my cells accordingly.
3) Whenever I have to draw a cell, I ask pango to give me a buffer for
the given Unicode char (or better: Unicode string which might contain
combining-characters)
4) It's important that I get full access to the 2D buffer of the
glyph. I sometimes need to change stride/size/etc. and the
hardware-blitters have different constraints. Hence, I cannot use any
opaque "glyph"/gtk/etc. structures but instead need _direct_ buffer
access.

What do I currently do?

Font-Management:
1) I start with pango_ft2_font_map_new()
2) For each font I use I create a pango-context from this ft2 font-map.
3) I set the user-selected font-description for the pango-context
4) I render the standard ASCII character set to measure the standard
width, height and baseline
5) These standard sizes are then used as default cell-size

Glyph-Management:
1) For each glyph I create a temporary pango-layout
2) I set the utf8 text of a single cell (one unicode character with
combining marks) via pango_layout_set_text()
3) I discard everything but the first line
4) I call pango_layout_line_get_pixel_extents to trigger rendering
5) I create an FT_Bitmap of my fixed cell size (not the layout
extents!) and initialize to 0
6) I set FT_PIXEL_MODE_GREY on the bitmap
7) render into bitmap with pango_ft2_render_layout_line()
8) I discard the pango-layout and cache the FT_Bitmap in a hashtable
for future access

This works pretty nice for me. I get smooth AA fonts, I can select
font-names and sizes. The layout works correctly for multi-line
composed glyphs (like those Unicode multi-line integrals) and I am
pretty happy with it.

However, I'd appreciate if someone could tell me whether there are
ways to improve this? And I have some questions that I couldn't figure
out myself:

1) How do I disable AA fonts? How can I enable subpixel rendering?
2) How can I detect system-defaults? That is, let the user define
whether to use AA-fonts, subpixel-rendering, font-name, ... I
currently configure this in kmscon-specific configuration but I'd
prefer font-config options.
3) How am I notified about font-updates in the system? If the
currently used font is updated I'd like to clear my cache and let
pango re-render the glyphs.
4) What should I use as default render-mode? High-DPI (>300?) no AA
but subpixel? Low-DPI AA but no subpixel? I have actually no idea what
looks best... And what to do if I cannot get DPI or subpixel-layout
information for a display?
5) Does pango do glyph-substitution if used with pango_ft2_*? That is,
if a glyph isn't found in the current font, does it try to find
another font with this glyph?

My pango-module is available at [2]. I also looked at libvte code.
Despite its dependency on gtk it doesn't do that much different to
kmscon. However, it uses gtk's rendering engine instead of dealing
with pango_ft2 directly. But I haven't figured out how to get easy
access to a glyph buffer without pango_ft2.

Any comments are welcome!
Thanks
David

[1] https://github.com/dvdhrm/kmscon
[2] https://github.com/dvdhrm/kmscon/blob/master/src/font_pango.c