[Proposal]: Open up Graphics backend and allow custom shapes, add support for multi-contour Path shape

[ShukantPal]

When writing @pixi-essentials/svg's <path /> implementation, I had difficulty integrating my custom Path shape b/c:

• there is no standard way of defining shape bounds (had to override GraphicsGeometry#calculateBounds)
• Shape is not an interface, rather a union. I had to force cast Shape into Path and vice versa.
• The SHAPES enum is not exported from @pixi/graphics

We should standardize these "backend" pieces so that Graphics can properly be extended.

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Also, I was thinking of bringing in @pixi-essentials/svg's Path shape into Graphics. It supports multiple contours, and a fill-rule. Stroking is a bit complicated b/c I have to call buildLine on each contour (by overriding GraphicsGeometry#processLine).

It uses libtess for tessellation and is much more robust.

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Unrelated Rant: Graphics#currentPath should actually be called Graphics#currentPoly. I literally named the path object currentPath2 in @pixi-essentials/svg haha.

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You can see the implementation here:

• https://github.com/SukantPal/pixi-essentials/blob/master/packages/svg/src/utils/Path.ts
• https://github.com/SukantPal/pixi-essentials/blob/master/packages/svg/src/utils/buildPath.ts

[ShukantPal]

@bigtimebuddy @ivanpopelyshev Thoughts?

[bigtimebuddy]

This is a little unclear what you're asking, there are a few ideas in here I don't understand the full scope of all of them. However, I don't have any problem with adjusting those backend/typing pieces you point out, like making a Shape interface, etc. Those seem pretty basic.

As for your other ideas (adding Path, etc), I ask that you make small PRs so we can consider those things separately.

[ivanpopelyshev]

Thoughts: yeah, we have to do it, but what I recommend is to make another Graphics element. More advanced, more optimized. Dont touch what we have right now.

[ivanpopelyshev]

Then swap Graphics and new implementation in major version, like we did with Mesh in v5.

[ShukantPal]

@ivanpopelyshev I disagree, because we won't be changing anything with the Graphics code. The user can draw paths using the existing drawShape API. GraphicsGeometry will process the paths contours in a loop. See my @pixi-essentials/svg/src/SVGGraphicsGeometry code.

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