#69 Remove background: plain backdrops (signatures, stamps, logos) → transparent PNG

[Anonymous]

Originally created by: SeoJCalive
Originally owned by: KnifeLemon

What would you like Filee to do?

An image option and a built-in preset that make a plain background transparent:
the paper behind a signature or stamp, the backdrop of a logo or a scan.

signature.jpg → Remove background → signature_transparent.png

Why / when would you use it?

Putting a handwritten signature or a stamp into a document (HWP, Word, PDF) needs a
transparent image. Today that means opening a photo editor or uploading the photo to
an online "remove background" site, which is not ideal for a signature. Filee already
has everything needed to do it offline in one click.

Ideas for how it could work

I have implemented this and would like to send a PR (before/after screenshots below):

• Magick.NET only: no model, no network, no new dependency
• Backdrop colour from the image border; re-estimated per block on light paper, so
  shadows and uneven lighting in phone photos are removed too
• "Colour to alpha" un-mixing: anti-aliased stroke edges stay smooth with no white
  fringe; blue/red ink keeps its colour
• Options in the preset editor: Remove background, Background tolerance (0-100),
  Keep enclosed areas (keeps the white inside a logo)
• Runs before grayscale and before alpha is flattened, so a JPG target gets the
  background colour instead (it doubles as "replace background colour")
• A built-in Remove background preset (PNG) on the image donut after Half size,
  added to existing libraries by a settings schema 10 migration
• ko/en/zh-CN strings, 15 new tests

Photos of people or busy scenes are out of scope; that would need a segmentation
model, which could later be an optional download engine like FFmpeg if you are
interested.

Questions:

1. Is the image donut a good place for the preset, or should it only be in the preset list?
2. Any preference for the output name ({name}_transparent)?

[Anonymous]

• assigned_to: KnifeLemon

[Anonymous]

Ticket changed by: KnifeLemon

• status: open --> closed