Feature request: Support for faux bold and italic

[olivierbelzile]

We are trying to match text rendering from imagesharp to the display of a browser. One limitation that we are facing comes from the the lack of capability to generate bold and italic styles on fonts where only the regular style is available. On the web, font synthesis allows for various weights and oblique typefaces when using such fonts. Is there any plan to add these features to the library?

[christianrondeau]

Implementation proposed for faux italic: SixLabors/Fonts#542

[christianrondeau]

Implementation proposed for faux bold: SixLabors/Fonts#543

[JimBobSquarePants]

I've merged the PRs by @christianrondeau so there is a version of the functionality available in the nightlies, however I have made tweaks locally to ensure that we perfectly match browser behaviour as you can see below. I'll ship that with the rest of the changes coming for v3.1.

[tocsoft]

should this functionality live behind a feature flag on the TextOptions and/or some global static, as it would always be better to have the correct italic/bold version of a font loaded rather than doing it synthetically without users being aware its happening.

[christianrondeau]

I was wondering the same thing, here's what I think (personal opinions only). Bold and Italic will use the font provided feature if it's available, but if it's NOT available, and the user explicitly specified they wanted Italic or Bold, then the expected behavior (I think) would be to fallback to the synthetic italic / bold. If you are aware that a font does not have Italic, you can opt out by not specifying Italic already. The only case where a flag like this would be useful, is if you wanted someone to specify Italic on a user-provided font and make sure only the font was allowed, in which case I'd expect a crash more than silently ignoring the italic/bold request.

If there was a setting, there's then an argument to be made to have the option to crash, fallback to faux italic/bold, or just drop the italic/bold request (e.g. an enum TextMissingStyleBehavior with Ignore, Faux and Error).

This being said, I personally would be comfortable with the current behavior (same as e.g. Chrome)

[JimBobSquarePants]

I have integrated this locally with a new nullable FontWeight enum option added to TextOptions and TextRun.

For system fonts I try to match a built in font. For custom font collections we synthesise for italic or past a certain weight. Variable fonts we will try to get a weight out of first prior to synthesis. Matching algorithms use CSS spec.

This is the same approach all the OS specific APIs use.