Validation Error on Default Scene Render

[power-unsealed]

The following example tries to render an empty scene.

use vello::{util::RenderContext, Renderer, RendererOptions, Scene, RenderParams, peniko::Color};
use wgpu::{TextureDescriptor, Extent3d, TextureDimension, TextureFormat, TextureUsages, TextureViewDescriptor};
use pollster::FutureExt as _;

fn main() {
    let mut context = RenderContext::new().unwrap();
    let device_id = context.device(None).block_on().unwrap();
    let device_handle = &mut context.devices[device_id];
    let device = &device_handle.device;
    let queue = &device_handle.queue;
    let mut renderer = Renderer::new(
        device,
        &RendererOptions { surface_format: None }
    ).unwrap();

    let scene = Scene::default();
    let (width, height) = (1024, 1024);
    let texture = device.create_texture(&TextureDescriptor {
        label: Some("target"),
        size: Extent3d { width, height, depth_or_array_layers: 1 },
        mip_level_count: 1,
        sample_count: 1,
        dimension: TextureDimension::D2,
        format: TextureFormat::Rgba8Unorm,
        usage: TextureUsages::STORAGE_BINDING | TextureUsages::COPY_SRC,
        view_formats: &[],
    });
    let texture_view = texture.create_view(&TextureViewDescriptor::default());
    let params = RenderParams { base_color: Color::BLACK, width, height };

    renderer.render_to_texture(device, queue, &scene, &texture_view, &params).unwrap();
}

The code crashes at runtime with the following output:

thread 'main' panicked at 'wgpu error: Validation Error

Caused by:
    In a ComputePass
      note: encoder = `<CommandBuffer-(0, 1, Vulkan)>`
    In a dispatch command, indirect:false
      note: compute pipeline = `pathtag_reduce`
    Buffer is bound with size 16 where the shader expects 20 in group[0] compact index 2

The same happens, when rendering an empty scene to a winit window.

[DJMcNab]

Thanks for the report.
This is because of https://www.w3.org/TR/webgpu/#minimum-buffer-binding-size

I think the correct fix might be to skip the parts of the render which fail if there are no paths, which may well be everything before fine. In theory, this case should be equivalent with clearing the given texture to the specified background colour? Can we make that "optimisation" to avoid needing to think about this.

For now, in user code, a workaround would be to always render something, e.g. a triangle of the background colour.

[power-unsealed]

Clearing the target to a default color was actually something, that I was hoping for, when first stumbling across this bug. That would seem like a reasonable solution to me. 👍