Equivalent of ImageMagick's sigmoidal contrast option?

[BrianSHenderson]

I have been using the setting:

-sigmoidal-contrast 20x100%

with ImageMagick which was very useful at optimising faint scans, making them much more readable.

Is there a way this can be achieved with Sharp? For example using linear or modulate methods?

Before:

After:

[lovell]

There are no curve-based features currently available, and I think providing the sigmoidal features of *magick would involve sharp exposing libvips' tonelut operation.

If you're dealing with faint scans of handwriting then using threshold with a high value might also be suitable, e.g. with threshold(254) I see:

The improvements to normalise discussed in #200 might also help.

[lovell]

I hope this information helped. Please feel free to re-open with more details if further assistance is required.