colorToAlpha (#39) - #52

Adds Image.colorToAlpha(color?, options?), which makes a color transparent, typically to remove a solid background from a logo, emoji or sticker.

How it works

A simple approach ("pixels of the background color, ± a tolerance, get alpha 0") leaves a fringe of the old background around every antialiased edge, which shows as a halo on a different background. colorToAlpha() works like GIMP's "Color to Alpha" instead:

• Every pixel is unmixed: for each pixel it computes the lowest opacity at which some color, drawn over the removed color, produces exactly that pixel. The pixel takes that opacity and that color.
• Exact pixels of the color become fully transparent.
• Edges that are partly the color become semi-transparent with the color unmixed, so they look clean over any background.
• Lossless round trip: compositing the result over the removed color gives back the original image exactly.

API

• color: the color to remove. Without it, the top left pixel's color is used (usually the background).
• options.transparencyThreshold (0–1, default 0): pixels at most this far from the color become fully transparent. This helps with noisy or JPEG-compressed backgrounds.
• options.opacityThreshold (0–1, default 1): pixels at least this far from the color stay fully opaque.

The color is removed everywhere in the image, not only from the connected background: removing a white background also makes white areas inside the subject transparent. Removing only the connected area around the subject (like a magic wand) would be a separate feature.

Tests

tests/regressions.js checks a red antialiased circle on white:

• the background becomes transparent and the circle stays opaque
• compositing the result over white gives the original back exactly
• the edge becomes translucent red with no white in it
• a transparency threshold makes more of the edge transparent

Closes #39