Text as mask — fill, blur, spotlight

Linear: AUT-85

Text becomes a stencil: render glyphs to an alpha-coverage texture, feed any other render-texture as the foreground, and Renderer::apply_mask_to_texture clips the foreground to the glyph shape. The story below shows the same "WISP" mask with three foregrounds — a saturated color-band fill, a blurred backdrop, and a warm spotlight.

Top: gradient fill through text. Middle: blurred circles. Bottom: warm spotlight. One mask, three foregrounds.

Composition

sequenceDiagram
    participant Pipe as TextTexturePipeline
    participant Mask as mask_rt (RGBA, alpha = glyph coverage)
    participant Fg as foreground_rt (gradient | blur | spotlight)
    participant Compose as Renderer.apply_mask_to_texture
    participant Out as output_rt

    Pipe->>Mask: text → RT with alpha = coverage
    Fg->>Compose: pass through unchanged
    Mask->>Compose: clip with alpha
    Compose->>Out: foreground × mask.a

apply_mask_to_texture is the load-bearing primitive

This is the same function M-VEC.4..6 (privacy blur / redaction / spotlight composition) and M-MASK.2..4 (clip + path mask + mask combine) call. Text joins the list of valid coverage sources alongside analytic SDFs (RoundedRect, Ellipse), vector paths, and procedural masks. The renderer doesn't care where the alpha came from.

API

#![allow(unused)]
fn main() {
use wisp::text::{TextTexturePipeline, WispText, WispTextStyle};
use wisp::{RenderTexture, Texture, Color};

// 1. Render text to a coverage texture.
let text = WispText::new("WISP").with_style(
    WispTextStyle::default().with_size(0.95).with_color(Color::WHITE),
);
let text_rt = pipeline.render(app, &text, 256, 256);

// 2. Stage the text-RT into the renderer's format (linear, +y up).
let mask_rt = RenderTexture::with_format(app, 256, 256, format);
// (render text_rt onto mask_rt via a sprite with scale.y = -1)

// 3. Apply the mask to any foreground RT.
let output = RenderTexture::with_format(app, 256, 256, format);
renderer.apply_mask_to_texture(app, &foreground_rt, &mask_rt, &output);
}

Three pre-made foregrounds

Pattern: separable foreground + universal mask

The three foregrounds in the story are independent of the mask. Swap the gradient for a screen-grab, the blur for a stock photo, the spotlight for a vignette — and the same apply_mask_to_texture call just works. Caching the mask RT across frames (text is rarely re-rendered per frame) and only regenerating the foreground keeps the GPU cost in line with a regular textured sprite.

Fill — A Graphics of horizontal color bands rendered into a plain RT. Punchy, poster-style.
Blur — Three overlapping draw_ellipse circles in saturated colors, run through BlurFilter::new(radius: 8.0). Soft-focus reveal.
Spotlight — Solid warm field + a yellow ellipse + a slight dimming layer, finished with a zero-offset DropShadowFilter to soften. Glowy halo through the glyphs.

Lavapipe / CI guard

Blur filter loses the device on lavapipe

The blur + drop-shadow paths use the multi-bind-group filter pipeline that lavapipe (Linux CI's software Vulkan) loses the device on. The story checks WISP_SKIP_GPU_FILTER_TESTS and substitutes a sharp / no-filter foreground when it's set, so the smoke + snapshot tests still pass on the Ubuntu runner. macOS runners exercise the real filter path.

Pixel tests

Inside-/outside-glyph alpha is enforced upstream by the mask-compose pipeline's own tests (crates/wisp/tests/mask_compose_*.rs). This chunk verifies that text-driven masks reach the same primitive unchanged, by way of the storybook story's story_smoke (no wgpu validation errors, visible pixels) and story_fingerprints (quadrant snapshot — text shape must be stable per frame).