Support rendering to a subregion of the terminal

[dankamongmen]

While documenting #77, the video question gave me pause. it would be pretty neat to run a (small, heh) video in the terminal below your prompt ala lsix but with video. now, imagine, you can't have this going while you've got your next program running attached to the term, but it can run until then without blowing away the screen, right?

well if there's nothing else running, there's no threat of scrolling, in which case we're actually just rendering to a subregion of the terminal. and i think that's the way to get this effect. allow the toplevel notcurses object to be restricted to a subregion (whether this ought be defined by a static geometry or a static margin is an open question). the subregion will be expressed in a fashion that allows it to be symbolically offset from the current cursor position (which we must be able to retrieve, but can be done much more portably than acquiring screen content).

this isn't high priority by any means, but something to think about.

[dankamongmen]

It would be awesome if this could be combined with #310 (at least in concept) so that we could run one notcurses program within an ncplane of another.

[dankamongmen]

So the geometry vs margin question....which of the following do we want the user to be able to specify?

• Exact size independent of resize (screen must be large enough)
• Minimum size, no restriction on how large it can get
• Maximum size, no restriction on how small it can get
• An absolute range of sizes
• Exact margins
• Minimum margins
• Maximum margins

[dankamongmen]

I'm thinking maybe minimum size + minimum margin are sufficient for now? Maybe even just minimum margin? So that would be:

• minimum horizontal margin (this can double as "x offset")
• minimum vertical margin (this can double as "y offset")

It could also be, if we want to support something other than just centering:

• minimum top, bottom, left, right margins

And then if we wanted a minimum size:

• minimum width
• minimum height

The actual size, assuming

• dimx >= minimum width + 2 * horizontal margins
• dimy >= minimum height + 2 * vertical margins

(since otherwise it can't be displayed) will be:

• dimx - 2 * horizontal margins (possibly larger than minimum with)
• dimy - 2 * vertical margins (possibly larger than minimum height)

Yeah, so 4 margins + 2 minima seem sufficient.

If we're allowing a minimum size to be specified, we ought enforce it in notcurses_init() and notcurses_resize(). What does enforcement mean? Do we exit? Stop rendering, and render some
warning message? What if we're too small for that message?

[dankamongmen]

I think maybe we want maximum absolute and minimum margin...?

[dankamongmen]

I've added margin_{tbrl} to notcurses_options, and documented it on the notcurses_init(3) man page.

[dankamongmen]

I've added a -m option to notcurses-demo, which can take either a single common margin, or all four margins explicitly supplied.

[dankamongmen]

Just ran through a full demo with a right and bottom border. Everything looks great! Need to test with left and top, and also need to test with a resize.

[dankamongmen]

Either a top or a left border gives us problems on numerous demos. We're not done yet :).

[dankamongmen]

Getting closer (-m2)

[dankamongmen]

H

[dankamongmen]

Is it just me, or is the centering off on the intro scene above (written with -m5)? I think there is. It looks like there are maybe 5 columns on the left, and 3 on the right. If so, that explains why the intro appears to be left atop later drawing (see chunli two above)...

[dankamongmen]

yep:

Term: 111x74 xterm-256color (xterm with 256 colors)
y: [5..69) x: [5..106)
y: [5..64) x: [5..101)
y: [5..64) x: [5..101)
y: [5..64) x: [5..101)
y: [5..64) x: [5..101)
y: [5..64) x: [5..101)
y: [5..64) x: [5..101)

[dankamongmen]

Fixed up the corruption we were seeing. Everything works now, except we're not drawing the last line when we have a top margin. This should be done soon.

[dankamongmen]

We're also not drawing the righmost column when we have a left margin.

[dankamongmen]

Working perfectly, and it looks AWESOME

[dankamongmen]

BOSS