[FFmpeg-devel] proposal for a minor change in the behavior of the drawtext filter

[Nicolas George]

Francesco Carusi (12022-04-22):
> I'm working on an enhanced version of the drawtext filter and would like to
> discuss with you about a minor change in its behavior related to line
> spacing management.
> In the current implementation the space between two lines of text is set
> equal to the height of the highest glyph found in the text plus the optional
> user defined "line_spacing" parameter (which defaults to 0):
> 
> line_height = max_glyph_h + line_spacing
> 
> This has some drawbacks:
> 1) the line height depends on the text
>     See image "line_height-old.png" (the blue lines where added by me)
>     The filter is applied three times with tree slightly different texts:
> the line_spacing parameter was not specified,
>     but the actual line height changes due to the different heights of the
> glyphs
> 2) the line height is not consistent between the same text being rendered by
> the filter and by external tools
> 
> The proposed change is:
> 1) the default line height is set to the font-defined line height
> 2) the line_spacing parameter, if specified, sets the actual line height
> (line_height = line_spacing)
> 3) the default value of line_spacing is set to -1 (which means: use the
> default line height)
> 
> The image "line_height-new.png" shows the effect of the new behavior.
> 
> Which is the impact of the change?
> A) users using the line_spacing parameter would see less space between lines
> B) users not using the line_spacing parameter would see (in most cases) more
> space between lines
> 
> Can the impact be mitigated?
> Well, yes, we may add a new parameter named 'line_height' and deprecate
> 'line_spacing', in this way anyone using line_spacing explicitly would not
> see any change in the filter behavior. I don't like this solution but it may
> help reducing the impact of the change.

Hi. While your proposal shows merit in itself, it also feels like the
first towards re-inventing a layout engine. The problem with line
spacing is the tiniest of drawtext problems: look for right-to-left,
ligatures and so much more.

If you want to enhance the output of drawtext, I think it would be more
efficient to choose a good layout engine library, Libre and portable,
and to make it use it optionally.

Regards,

-- 
  Nicolas George
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