[CLI/PNG] Exporting to PNG always fills the background with white color

[dahooz]

When using the CLI to export to PNG, the background is always filled with Color::WHITE:

typst/crates/typst-cli/src/compile.rs

Line 279 in 72dd792

let pixmap = typst_render::render(frame, command.ppi / 72.0, Color::WHITE);

I'd expect the background to be filled with the #page(fill: "...") color. Is there a reason for the white background, or does it make sense to open a PR?

[LaurenzV]

The reason is probably because PDFs by default are also white. However, if you do something like #set page(fill: red) then the page should also be red in the PNG export. It will basically draw red on top of the white background.

[dahooz]

I'm pretty sure that the white background in PDFs is rendering, not an actual background:
Due to this hardcoded color, I currently use imagemagick's convert to convert from PDF to PNG. This leads to a transparent background in the png output.

Maybe I should make my usecase more clear: I'm creating "heading" PNGs with fancy #text settings, that I can use in other documents (read: usual office software).

[LaurenzV]

I mean I don't see why this shouldn't be configurable. We could add some kind of CLI argument that sets the background color for PNG outputs.

[laurmaedje]

In addition to what @LaurenzV said, the page.fill property can also take various other kinds of fills (patterns, gradients), so we couldn't fully handle it there anyway.

[dahooz]

With @laurmaedje 's argument in mind, I would argue that it doesn't actually make sense to allow inputting background colors via CLI argument: I can imagine someone expecting to be able to put sth. like rgb("#abcdef").darken(80%).desaturate(20%)... in that flag, requiring to parse typst code from this flag...

I'd rather just add a --png-fill-transparent flag, that changes the Color::WHITE to #rgb("#00000000") (sorry, didn't find how to write the equivalent in Rust). Then it's just the page.fill property handling the background.