PNG with a transparent background

[AntoineAugusti]

Hello,

A straightforward question: how to fill the transparent background of a PNG image with a given colour (say white)?

Current code: trying to create a 100x100 thumbnail.

local_path = "/tmp/dinamo.png"
logo_path = "/tmp/logo.png"

local_path
|> Image.thumbnail!(100)
|> Image.embed!(100, 100, background_color: :white, extend_mode: :white)
|> Image.write!(logo_path)

Thanks a lot!

[kipcole9]

Thanks for the prompt to get the :background_color option added to Image.flatten/2. Assuming your image is already a PNG with an alpha band, the best way is:

{:ok, flattened_image} = Image.flatten(image, background_color: :green)

This will return a 3-band image with the alpha band applied and the transparent areas set to :green. You'll need to upgrade to Image 0.52.0 which I release just a few minutes ago. It has the following changelog entry:

Enhancements

• Adds :background_color option to Image.flatten/2.

• Add Image.Nx to host some Nx convenience functions.

Bug Fixes

• Fix Image.delta_e/2 to return a float (not an integer)

• Fix Image.k_means/2 to work with images in color spaces other than sRGB.

[AntoineAugusti]

This works fine!

Updated code.

Mix.install([
  {:image, "~> 0.52"},
  {:nx, "~> 0.4"}
])

local_path = "/tmp/dinamo.png"
logo_path = "/tmp/logo.png"

local_path
|> Image.open!()
|> Image.flatten!(background_color: :white)
|> Image.thumbnail!(100)
|> Image.embed!(100, 100, background_color: :white, extend_mode: :white)
|> Image.write!(logo_path)

dinamo.png

This PNG has a transparent background (GitHub displays it as white)

logo.png

[kipcole9]

Looks very cool. Just one suggestion, which is to reorder some of the operations. One key thing to know about libvips is that it is very optimised to use thumbnail with a path name. It can then optimise the resizing during the file opening and reading. Additionally, since libvips is demand driven, you're only working on pixels from the reduced size image which is also more efficient. Therefore:

local_path
|> Image.thumbnail!(100)
|> Image.flatten!(background_color: :white)
|> Image.embed!(100, 100, background_color: :white, extend_mode: :white)
|> Image.write!(logo_path)

would be the canonical way to go.

[AntoineAugusti]

Thanks for the tip and for responding so quickly ⚡ Much appreciated 🙏

Regards from the tech team @ transport.data.gouv.fr

[kipcole9]

Thanks for the feedback, it really helps motivation - even though this is such a fun library to work on. I learn such a lot from how people use it and what would make it better.

[AntoineAugusti]

Thanks for your work! We use this library to allow data producers (transportation data) to add a custom logo on the French National Access Point to transport data, and we resize/thumbnail it.