Add grow option to Drop Shadow

[nuxttux]

The Drop Shadow effect can already double as a drop shadow (sharp & blurry) and a glow effect. Adding a grow slider to grow the drop shadow could make it triple into a Stroke effect - one that could be used on organic shapes.

Something similar to how the Alpha Operation effect has grow soft / hard, but just "grow" in this case.

[ddennedy]

I do not think it is as simple as adding an option. The implementation in Qt would need to support it too.

[nuxttux]

All right - I'm still figuring out what does what, between QT, mlt, and ffmpeg.

[ddennedy]

That’s understandable. This discussion reminds me there is this old code that does what you describe, and I can port it into MLT.

https://sourceforge.net/p/kino/code/HEAD/tree/trunk/kino/src/dvtitler/textblock.cc

[nuxttux]

I appreciate you looking into this. It would be a good addition.

[ddennedy]

I added an outline filter in a046511. It is a separate filter and based on the Kino code I mentioned, which is a technique similar to frei0r.alpha0ps ("Alpha Operation") grow. However, the that frei0r plugin's grow operation limits itself to non-transparent pixels only, which hides its weaknesses. If you have used grow, you might have noticed it does not seem to grow much. It only increases the opacity of existing translucent pixels. This outline does not have that limitation. However, both of these appear to be a rather naive approach and only know how to extend horizontally or vertically. As a result, angles are not rounded or anti-aliased.

And large thicknesses (e.g. over 5px) look ugly IMHO and start to make funny shapes.

[nuxttux]

To confirm, can the stroke to color be changed?

[ddennedy]

I was able to fix those angle artifacts as well as some weird shapes that can occur around curves by sampling in a circular pattern instead of only horizontal and vertical. It is much more computationally heavy, but it is important for quality. It does image slice threading for parallelism.

[nuxttux]

You did it. As for the computationally heavy aspect, do you think the upcoming GPU support could alleviate some of that load?
Regardless, this will be a welcome addition.