WIP: Add remove_transparency pre-processing function

[wmanley]

Often stb UIs consist of a transparent overlay over live TV or some other
changable image. The transparency can cause OCR and match failures due to
the background bleeding through. This function replaces these transparent
background with solid colour which can be used as a pre-processing step
before performing a match or ocr.

color is an HTML-style hex colour string of the form #rrggbbaa.

It assumes that an overlay image was alpha-blended according to:

frame = color[rgb] * color[alpha] + underlay * (1 - color[alpha])

where color specifies overlay and alpha.

See https://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending

Usage:

remove_transparency('#bf000079', frame=...)

TODO:

• Tests
• Release notes.
• Should it be called "remove translucency" instead of "transparency"?
• Expose it in stbt.
• Should we create a new stbt.preprocessing module or just keep jamming stuff into stbt? This doesn't have any dependencies except numpy (not even other stbt functions).

[drothlis]

Should it be called "remove translucency" instead of "transparency"?

Should we create a new stbt.preprocessing module or just keep jamming stuff into stbt? This doesn't have any dependencies except numpy (not even other stbt functions).