Correct way to implement `pixel-point-and-height-at-unseen-line'?

Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Po Lu]

I don't know how to make `pixel-point-and-height-at-unseen-line' work
correctly: if I use `line-pixel-height', it will only report the height
the first individual line inside a display string if there is one above
window start, but if I use `window-text-pixel-size', I have to determine
the region to measure, which is quite troublesome.

For example, window start is often not the beginning of a visual line in
Info buffers, so in order to measure the height of the previous line (or
object), I have to find the beginning of the visual line at window
start, and pass that as the `to' argument to window-text-pixel-size
instead.

This works okay until there is a `line-prefix' property at window start:
in this case, window start is the beginning of a visual line, but it
does not display at the beginning of the line, which means
`window-text-pixel-size' will add the height of the current line to that
of the previous line.  I will add a workaround to pixel-scroll.el, but
there must be a cleaner way to implement this.

Thanks in advance.

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Emanuel Berg via Users list for the GNU Emacs text editor]

Po Lu wrote:

> I don't know how to make
> `pixel-point-and-height-at-unseen-line' work correctly: if
> I use `line-pixel-height', it will only report the height
> the first individual line inside a display string if there
> is one above window start, but if I use
> `window-text-pixel-size', I have to determine the region to
> measure, which is quite troublesome.
>
> For example, window start is often not the beginning of
> a visual line

`window-start', `beginning-of-visual-line' ...

You can post the code instead ... that way you don't have to
remove the dashes.

> This works okay until there is a `line-prefix' property at
> window start: in this case, window start is the beginning of
> a visual line, but it does not display at the beginning of
> the line, which means `window-text-pixel-size' will add the
> height of the current line to that of the previous line.
> I will add a workaround to pixel-scroll.el, but there must
> be a cleaner way to implement this.

Sounds pretty clean to me but difficult to say since you are
not posting the actual code.

-- 
underground experts united
https://dataswamp.org/~incal

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Po Lu]

Emanuel Berg via Users list for the GNU Emacs text editor
<help-gnu-emacs@gnu.org> writes:

> Po Lu wrote:
>
>> I don't know how to make
>> `pixel-point-and-height-at-unseen-line' work correctly: if
>> I use `line-pixel-height', it will only report the height
>> the first individual line inside a display string if there
>> is one above window start, but if I use
>> `window-text-pixel-size', I have to determine the region to
>> measure, which is quite troublesome.
>>
>> For example, window start is often not the beginning of
>> a visual line
>
> `window-start', `beginning-of-visual-line' ...
>
> You can post the code instead ... that way you don't have to
> remove the dashes.
>
>> This works okay until there is a `line-prefix' property at
>> window start: in this case, window start is the beginning of
>> a visual line, but it does not display at the beginning of
>> the line, which means `window-text-pixel-size' will add the
>> height of the current line to that of the previous line.
>> I will add a workaround to pixel-scroll.el, but there must
>> be a cleaner way to implement this.
>
> Sounds pretty clean to me but difficult to say since you are
> not posting the actual code.

It's in pixel-scroll.el inside the lisp folder on master.  See
`pixel-point-and-height-at-unseen-lie'.

Thanks.

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Emanuel Berg via Users list for the GNU Emacs text editor]

Po Lu wrote:

>> Sounds pretty clean to me but difficult to say since you
>> are not posting the actual code.
>
> It's in pixel-scroll.el inside the lisp folder on master.
> See `pixel-point-and-height-at-unseen-lie'.

Don't have that ...

GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, cairo version
1.16.0) of 2021-10-04

-- 
underground experts united
https://dataswamp.org/~incal

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Po Lu]

Emanuel Berg via Users list for the GNU Emacs text editor
<help-gnu-emacs@gnu.org> writes:

> Po Lu wrote:
>
>>> Sounds pretty clean to me but difficult to say since you
>>> are not posting the actual code.
>>
>> It's in pixel-scroll.el inside the lisp folder on master.
>> See `pixel-point-and-height-at-unseen-lie'.
>
> Don't have that ...
>
> GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, cairo version
> 1.16.0) of 2021-10-04

10th Oct is a bit early.  It was introduced in late November, but the
version that is relevant to this discussion is today's revision.

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Date: Mon, 13 Dec 2021 09:38:30 +0800
> 
> I don't know how to make `pixel-point-and-height-at-unseen-line' work
> correctly: if I use `line-pixel-height', it will only report the height
> the first individual line inside a display string if there is one above
> window start, but if I use `window-text-pixel-size', I have to determine
> the region to measure, which is quite troublesome.
> 
> For example, window start is often not the beginning of a visual line in
> Info buffers, so in order to measure the height of the previous line (or
> object), I have to find the beginning of the visual line at window
> start, and pass that as the `to' argument to window-text-pixel-size
> instead.
> 
> This works okay until there is a `line-prefix' property at window start:
> in this case, window start is the beginning of a visual line, but it
> does not display at the beginning of the line, which means
> `window-text-pixel-size' will add the height of the current line to that
> of the previous line.  I will add a workaround to pixel-scroll.el, but
> there must be a cleaner way to implement this.

The correct way is to use window-text-pixel-size.  Whatever problems
that gives you in some corner use cases, we'd need to teach
window-text-pixel-size to handle those at least optionally.

And you didn't describe why do you need
pixel-point-and-height-at-unseen-line's data in the first place, so
maybe there's a different solution to whatever you are trying to do
that doesn't involve such a function.

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Emanuel Berg via Users list for the GNU Emacs text editor]

Po Lu wrote:

> 10th Oct is a bit early. It was introduced in late November,
> but the version that is relevant to this discussion is
> today's revision.

Okay, what's your suggestion then?

-- 
underground experts united
https://dataswamp.org/~incal

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Po Lu]

Eli Zaretskii <eliz@gnu.org> writes:

> And you didn't describe why do you need
> pixel-point-and-height-at-unseen-line's data in the first place, so
> maybe there's a different solution to whatever you are trying to do
> that doesn't involve such a function.

It's used to find a suitable new start position that's N pixels above
window start, so vscroll can then be set to compensate for the
difference.  For instance, scrolling up by a delta of 1 pixel if vscroll
is already zero will move to such a start position, and set vscroll to
the returned height - 1.

Thanks.

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Po Lu]

Emanuel Berg via Users list for the GNU Emacs text editor
<help-gnu-emacs@gnu.org> writes:

> Po Lu wrote:
>
>> 10th Oct is a bit early. It was introduced in late November,
>> but the version that is relevant to this discussion is
>> today's revision.
>
> Okay, what's your suggestion then?

To look at today's master then?

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: help-gnu-emacs@gnu.org
> Date: Tue, 14 Dec 2021 08:41:06 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > And you didn't describe why do you need
> > pixel-point-and-height-at-unseen-line's data in the first place, so
> > maybe there's a different solution to whatever you are trying to do
> > that doesn't involve such a function.
> 
> It's used to find a suitable new start position that's N pixels above
> window start, so vscroll can then be set to compensate for the
> difference.

Did you try something like this:

  (save-excursion
    (goto-char (window-start))
    (vertical-motion (- (ceiling N (frame-char-height))))
    (point))

where N is your number of pixels?

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Po Lu]

Eli Zaretskii <eliz@gnu.org> writes:

> Did you try something like this:
>
>   (save-excursion
>     (goto-char (window-start))
>     (vertical-motion (- (ceiling N (frame-char-height))))
>     (point))
>
> where N is your number of pixels?

That won't always work correctly with lines of different height: for
example, if the line above start is smaller than the default character
height of the frame, the number of lines by which to move upwards could
be too small.

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: help-gnu-emacs@gnu.org
> Date: Tue, 14 Dec 2021 21:48:35 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Did you try something like this:
> >
> >   (save-excursion
> >     (goto-char (window-start))
> >     (vertical-motion (- (ceiling N (frame-char-height))))
> >     (point))
> >
> > where N is your number of pixels?
> 
> That won't always work correctly with lines of different height: for
> example, if the line above start is smaller than the default character
> height of the frame, the number of lines by which to move upwards could
> be too small.

But if you always need the line before window-start, i.e. use N = 1,
then why is that a problem?

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Po Lu]

Eli Zaretskii <eliz@gnu.org> writes:

> But if you always need the line before window-start, i.e. use N = 1,
> then why is that a problem?

Ah, I think we're misunderstanding things:
`pixel-point-and-height-at-unseen-line' is part of a much larger system
that tries to find the line (and pixel distance) for N = foo, by
iterating through lines where N = 1.

Re: Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: help-gnu-emacs@gnu.org
> Date: Wed, 15 Dec 2021 08:40:37 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > But if you always need the line before window-start, i.e. use N = 1,
> > then why is that a problem?
> 
> Ah, I think we're misunderstanding things:
> `pixel-point-and-height-at-unseen-line' is part of a much larger system
> that tries to find the line (and pixel distance) for N = foo, by
> iterating through lines where N = 1.

Well, I did ask you to describe what you need to do, and responded to
the only description you gave ;-)