Add SVG render and export

[dzil123]

Fixes: #379

Creates a PrimitiveRenderer trait to allow separation of the lyon tesselation and wgpu rendering from the general preparation and finalization of the different primitives, which allows alternative renderers to avoid duplicating existing code that normalizes the nannou primitives into simpler rendering primitives.

Implements an SVG renderer using above renderer abstractions.

Checklist:

• PrimitiveRenderer
• Migrate existing wgpu+lyon renderer
• Verify all examples still work
• SVG export
• Draw context
  • Background
  • Blend
  • Scissor
  • Topology (point_mode, line_mode)
  • Sampler
• Single color paths
  • Shapes
  • Polygons
  • Polylines
• Text
• Meshes
• Textures
  • points_textured
• points_colored
• if stroke start_cap != end_cap
• Finalized user-facing API
• Documentation

[dzil123]

This PR is at the point where it can render the draw.rs example. Here is a sketch that renders the window to svg on mouse click: https://gist.github.com/dzil123/f52f4d554c570188ddcf73483a2dc292.

@mitchmindtree, would you mind taking a look?

[fenjalien]

Hi I've tried making my own program to generate a Sierpinski triangle SVG file and it works really well! One thing I didn't realise is it only renders whats been drawn by the Draw struct in the view method.

See here: https://gist.github.com/fenjalien/bc9fa8ca765470308ea311ea68fb499a

Unfortunately I'm very new to nannou so I can't really suggest any changes 😅

[dzil123]

Thanks for checking it out!

I see what you mean about it rendering only whats drawn by the Draw that single view call. Nannou doesn't remember all the draw commands from previous frames, so fixing this isn't trivial. It just loads last frame's rendered texture and draws on top of it if draw.background() wasn't set.

I don't know if it would be practical to store previous draw commands indefinitely because it can grow without bounds if not cleared with a background. Maybe it could be opt-in? It could also be useful to redraw everything when the render texture is lost due to a window resize.

[danwilhelm]

It probably does make sense to constrain the SVG render to the commands within a single Draw instance -- without incorporating texture contents.

There are many situations that would have to be handled otherwise, e.g.:

• frame.clear() (bypassing app.draw()),
• Multiple Draw instances,
• Use of wgpu (i.e. shaders, blending, etc.), and
• Drawing on top of an existing texture as mentioned.

[mmansion]

Great work on this so far! SVG rendering will be an excellent enhancement. Looking to seeing it integrated soon.

[akonneker]

I wanted to leave a nudge here. I'm currently setting up to use nannou to generate some input files for a laser marking system. I would like to export svgs... and so this PR would be immensely helpful. I'm currently working off my own fork of this PR in the meantime.

If the underlying abstractions are still sound and this has potential to be merged with a reduced scope (e.g. ignoring meshes and stroke end cap stuff), I may be able to sponsor someone to work on it. I've not been involved in the nannou project, so I'm not sure how many active contributors there are, or if that is a possible or desirable arrangement.