Backdrop blur

A backdrop blur is a save layer that opens pre-filled with a Gaussian of whatever is already painted beneath it. Children draw onto that glass; restore composites glass and children as one image, so a group alpha fades them together.

use valo::{Backdrop, ClipOp, Paint, Rect};

let panel = Rect::new(48.0, 40.0, 240.0, 160.0);
builder.save();
builder.clip_rrect(panel, 18.0, ClipOp::Intersect);
builder.save_layer_backdrop(Some(panel), &Paint::default(), Backdrop::blur(14.0));
builder.restore();
builder.restore();

JavaScript 0.2.1 keeps a convenience that opens that layer and immediately closes it — a glass panel with nothing painted on it. Canvas2D context.backdropBlur is the same call.

builder.save();
builder.clipRoundedRect(x, y, w, h, new Float32Array([18]), ClipOp.Intersect);
builder.backdropBlur(x, y, w, h, 14);
builder.restore();

Open the layer yourself in Rust when content should ride on the glass. JavaScript 0.2.1 has no saveLayerBackdrop yet — backdropBlur is the open-and-restore convenience only.

Shared keys

Tiles that share a key reuse the first tile's blur — and see the scene as of that tile. Use one key only for panels over the same background.

builder.save_layer_backdrop(
    Some(a),
    &Paint::default(),
    Backdrop::blur(20.0).shared(1),
);
builder.restore();
builder.save_layer_backdrop(
    Some(b),
    &Paint::default(),
    Backdrop::blur(20.0).shared(1),
);
builder.restore();

A later tile with a different sigma does not share. Nested replays of a retained list save and restore the keyed cache, so drawing the same list twice does not reuse the first drawing's blur. Shared keys are Rust-only in valo-web 0.2.1.

Hint the layer bounds when the list will be embedded. Without a hint or clip, a backdrop layer covers everything beneath it.