[FFmpeg-devel] [PATCH] vf_drawtext: make x and y options parametric

[Víctor Paesa]

Hi

On Wed, Sep 21, 2011 at 10:33, Stefano Sabatini wrote:
> On date Tuesday 2011-09-20 23:49:18 +0200, Víctor Paesa encoded:
>> Hi,
>>
>> On Tue, Sep 20, 2011 at 20:30, Stefano Sabatini wrote:
>> > Address trac issue #378.
>> > ---
>> >  doc/filters.texi          |   68 ++++++++++++++++++++-
>> >  libavfilter/vf_drawtext.c |  143 ++++++++++++++++++++++++++++++++++++++++-----
>> >  2 files changed, 191 insertions(+), 20 deletions(-)
>> >
>> > diff --git a/doc/filters.texi b/doc/filters.texi
>> > index 1ab6450..f7f7865 100644
>> > --- a/doc/filters.texi
>> > +++ b/doc/filters.texi
>> > @@ -724,10 +724,13 @@ parameter @var{text}.
>> >  If both text and textfile are specified, an error is thrown.
>> >
>> >  @item x, y
>> > -The offsets where text will be drawn within the video frame.
>> > -Relative to the top/left border of the output image.
>
>> > +The expressions which express the offsets where text will be drawn
>>
>> Maybe "x, y express the offsets where text will be drawn"
>
> I wanted to stress the fact that x and y contain "expressions", I
> removed the awkward repetition changing it to:
>
> |The expressions which specify the offsets where text will be drawn

Yes, that's nice too.

>> > +within the video frame. They are relative to the top/left border of the
>> > +output image.
>> >
>> > -The default value of @var{x} and @var{y} is 0.
>> > +The default value of @var{x} and @var{y} is "0".
>> > +
>> > +See below for the list of accepted constants.
>> >
>> >  @item fontsize
>> >  The font size to be used for drawing text.
>> > @@ -795,6 +798,43 @@ The size in number of spaces to use for rendering the tab.
>> >  Default value is 4.
>> >  @end table
>> >
>> > +The parameters for @var{x} and @var{y} are expressions containing the
>> > +following constants:
>> > +
>> > + at table @option
>> > + at item E, PI, PHI
>> > +the corresponding mathematical approximated values for e
>> > +(euler number), pi (greek PI), PHI (golden ratio)
>> > +
>> > + at item w, h
>> > +the input width and heigth
>> > +
>> > + at item tw, text_w
>> > +the width of the rendered text
>> > +
>> > + at item th, text_h
>> > +the height of the rendered text
>> > +
>> > + at item lh, line_h
>> > +the height of each text line
>>
>
>> It would be nice to expose too either the ascent or the descent
>> (y_max or y_min in code).
>
> Please can you suggest an application for this? So I can add an
> illustrative example in the docs.

The height of 'g' is the same of 'd', but one goes over the baseline,
and the other goes down it.
If you draw words splitted in several commands, you want to make all
of them follow a common baseline.

For example:
Properly show "good" with initial "g" in different color at the center
of the video frame:

drawtext=fontsize=30:fontfile=FreeSerif.ttf:fontcolor=green:text='g':x=(w-text_w)/2:y=(h-text_h+ascent-line_h)/2
drawtext=fontsize=30:fontfile=FreeSerif.ttf:fontcolor=red:text='ood':x=(w-text_w)/2:y=(h-text_h+ascent-line_h)/2

Improperly show "not good" with initial "not g" in different color at
the center of the video frame:

drawtext=fontsize=30:fontfile=FreeSerif.ttf:fontcolor=green:text='not
g':x=(w-text_w)/2:y=(h-text_h-line_h)/2
drawtext=fontsize=30:fontfile=FreeSerif.ttf:fontcolor=red:text='ood':x=(w-text_w)/2:y=(h-text_h-line_h)/2

Regards,
Víctor