[Bf-usd] Support for Preliminary_Text

[Sybren A. Stüvel]

Hi John & fellow USD list members,

On Tue, 19 Apr 2022 at 13:51, John Harrison via Bf-usd <bf-usd at blender.org>
wrote:

> Hi, I’m a software developer (never contributed to blender) motivated
> (possibly?) to implement USD export support for one of Apple’s schema
> additions, namely Preliminary_Text:
> https://developer.apple.com/documentation/arkit/usdz_schemas_for_ar/preliminary_text
>

That looks like a nice addition to me. Would you also be able to implement
the importer side of this?

Personally I don't have any experience with extending the standard USD
schema. How would this work on a technical level? Would it require Blender
to bundle any additional schema files? Would the mere existence of such
schema files be enough, or would Blender's USD handling code also have to
change?

On Tue, 19 Apr 2022 at 19:26, John Harrison via Bf-usd <bf-usd at blender.org>
wrote:

> Great questions Charles, here are my thoughts in the same order as your
> questions (learning as I go here):
>
> - Apple specifies a default point size of 144 in their docs (
> https://developer.apple.com/documentation/arkit/usdz_schemas_for_ar/preliminary_text/pointsize).
> My guess is Apple assumes the traditional 1/72” point unit, which
> multiplies nicely to 144 and would result in Apple choosing a default size
> of 2”. Now Blender has no such notion of point size for fonts, leading to
> your question about how blender would specify this. One suggestion is to
> have Blender convert on export, such that it measures the height in
> designer units, converts to inches, then divides by 72.
>

It would be good to get a definitive answer about this.


> - No idea. The docs refer to USD’s notion of a “stage unit”, but honestly
> having wdith/height makes no sense to me - seems like the runtime should
> figure this out. The doc also states, "The runtime ignores this value if
> `wrapMode` is singleLine.” I would (without understanding more) not export
> width/height at all and see what issues it causes
>

>From what I can tell, the width/height/depth properties are expressed in
the "stage unit" indeed, which means for Blender simply Blender units (the
exporter should ensure the USD scene is correctly set up to map it to
meters on loading).

Sybren
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