Fornjot application can crash, if specific graphics features are not supported on target

[hannobraun]

As of this writing, the graphics code uses one feature that might not be available everywhere: wgpu::Features::POLYGON_MODE_LINE

The code just requests that feature. If that features isn't available on the target (where the host application is executed), the host application would either fail with an error message, or even panic. This is not optimal, especially since POLYGON_MODE_LINE is only used for an optional drawing mode (rendering the lines of the triangle mesh).

Although I haven't seen or heard of any problems regarding this, it would be better to handle these optional features in a more robust way:

1. Check which of features we want to use are actually supported.
2. Only request those features that are actually available. Store that information somewhere.
3. Don't let the user enable anything that requires features which are not available.

Ideally, there would be a nice error message in the UI, to let the user know what the problem is. But for now, it would already be an improvement, if the host application didn't crash if such a feature wasn't available.

[hannobraun]

I've marked this as https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, since it only requires some knowledge of the wgpu API, and very little knowledge of Fornjot.

The code that requests the features lives in renderer.rs:

fornjot/src/graphics/renderer.rs

Lines 55 to 61 in fa4dbc8

	// Don't just blindly assume that we can request this
	// feature. If it isn't available, that might cause a panic,
	// or an error to be returned here.
	//
	// See this issue:
	// https://github.com/hannobraun/fornjot/issues/33
	features: wgpu::Features::POLYGON_MODE_LINE,

This wgpu method can probably be used to determine which features are actually available:
https://docs.rs/wgpu/0.12.0/wgpu/struct.Adapter.html#method.features

I assume that which features are available would be stored in a field of Renderer. Probably makes sense to create a separate features module under graphics, with a Features struct to encapsulate this (I'm just musing here; don't feel bound by my ideas, if you think something else makes more sense).

Here's some drawing code that requires this feature to exist:

fornjot/src/graphics/renderer.rs

Lines 196 to 211 in fa4dbc8

	if config.draw_mesh {
	drawables.mesh.draw(
	&mut encoder,
	&color_view,
	&self.depth_view,
	&self.bind_group,
	);
	}
	if config.draw_debug {
	drawables.lines.draw(
	&mut encoder,
	&color_view,
	&self.depth_view,
	&self.bind_group,
	);
	}

This code (and potentially other code like it) would need to check the features field of Renderer to determine whether it should execute. Eventually, this information should also be fed outside the renderer, so the UI knows what's up and can display a proper error message, but that could be done as an additional improvement down the line.

[hannobraun]

This issue has been addressed in #902. Thanks again, @hekno25!

I'm closing this issue, as Fornjot can no longer crash for this reason. I've opened the more specific #907 as a follow-up.