kmp-liquid-glass

Imported from Yeon-sik/Yeonsik-Fitness-App (.agents/skills/kmp-liquid-glass/SKILL.md). Install upstream with npx skills add Yeon-sik/Yeonsik-Fitness-App --skill kmp-liquid-glass. Copyright stays with the author.

KMP Liquid Glass — Building Glass Components

This skill turns any Compose Multiplatform agent into an effective user of the backdrop library: iOS-style liquid glass, frosted glass, vibrancy, lens refraction, progressive blur, SDF glass textures. Works on Android, iOS, Desktop, and Web with one common API.

Mental model: two layers

Glass needs something to look through. The library makes you wire that up explicitly:

1. Backdrop layer — the content the glass samples from (an Image, gradient, video, scrolling list, etc.). Captured into a graphics layer via Modifier.layerBackdrop(backdrop).
2. Glass layer — the element that draws the glass effect by sampling from the backdrop. Built with Modifier.drawBackdrop(...) or Modifier.drawPlainBackdrop(...).

A glass element with no backdrop sampling source will show nothing useful. Every glass component depends on a LayerBackdrop in scope. Components should accept it as a parameter — never create their own internal backdrop unless they truly own the background (e.g. a self-contained widget with its own image).

Setup

Gradle (commonMain):

commonMain.dependencies {
    implementation("io.github.kashif-mehmood-km:backdrop:0.0.1-alpha02")
}

Imports you'll use most:

import com.kashif_e.backdrop.drawBackdrop
import com.kashif_e.backdrop.drawPlainBackdrop
import com.kashif_e.backdrop.Backdrop
import com.kashif_e.backdrop.backdrops.LayerBackdrop
import com.kashif_e.backdrop.backdrops.layerBackdrop
import com.kashif_e.backdrop.backdrops.rememberLayerBackdrop
import com.kashif_e.backdrop.backdrops.rememberCombinedBackdrop
import com.kashif_e.backdrop.effects.blur
import com.kashif_e.backdrop.effects.lens
import com.kashif_e.backdrop.effects.vibrancy
import com.kashif_e.backdrop.effects.colorControls
import com.kashif_e.backdrop.highlight.Highlight
import com.kashif_e.backdrop.shadow.Shadow
import com.kashif_e.backdrop.shadow.InnerShadow

The minimum viable glass

@Composable
fun GlassDemo() {
    val backdrop = rememberLayerBackdrop()

    Box(Modifier.fillMaxSize()) {
        // 1. Background captured into the backdrop
        Image(
            painter = painterResource(Res.drawable.wallpaper),
            contentDescription = null,
            contentScale = ContentScale.Crop,
            modifier = Modifier
                .layerBackdrop(backdrop)
                .fillMaxSize()
        )

        // 2. Glass element samples the backdrop
        Box(
            Modifier
                .align(Alignment.Center)
                .drawBackdrop(
                    backdrop = backdrop,
                    shape = { RoundedCornerShape(24.dp) },
                    effects = {
                        vibrancy()
                        blur(8.dp.toPx())
                        lens(12.dp.toPx(), 24.dp.toPx())
                    },
                    highlight = { Highlight.Ambient }
                )
                .size(200.dp, 120.dp)
        )
    }
}

That's it. Everything else is variation.

How to design a glass component

When asked to build any glass component, follow this structure. Always accept backdrop: Backdrop as a parameter — the caller owns the background image / wallpaper / scrolling content; the component doesn't.

@Composable
fun MyGlassThing(
    /* ...semantic params... */,
    backdrop: Backdrop,
    modifier: Modifier = Modifier,
) {
    Box(
        modifier
            .drawBackdrop(
                backdrop = backdrop,
                shape = { /* lambda returning Shape */ },
                effects = {
                    // Compose effects in the right order (see below)
                },
                highlight = { /* Optional Highlight or null */ },
                shadow = { /* Optional Shadow or null */ },
                innerShadow = { /* Optional InnerShadow or null */ },
                onDrawSurface = { /* Optional tint/overlay */ }
            )
            // sizing and content go after drawBackdrop
            .size(...)
    ) {
        // content composables
    }
}

Effect ordering inside effects = { ... }

Effects compose in declaration order. The canonical iOS-style liquid stack is:

effects = {
    vibrancy()                                   // 1. boost saturation first
    blur(2.dp.toPx())                            // 2. soften
    lens(12.dp.toPx(), 24.dp.toPx())             // 3. bend/refract last
}

For a frostier dialog look, swap order and bump radii:

effects = {
    colorControls(brightness = 0.2f, saturation = 1.5f)
    blur(16.dp.toPx())
    lens(24.dp.toPx(), 48.dp.toPx(), depthEffect = true)
}

Rules of thumb:

• Crisp/glossy glass (buttons, toggles, tab bars): blur(2–4dp) + lens(12–24dp) + vibrancy().
• Frosted/heavy glass (dialogs, sheets, control center): blur(8–16dp) + lens(24–48dp) ± colorControls.
• Pure blur (notification bar, scroll edge): drawPlainBackdrop + blur (or progressiveBlur).
• Glass over scrolling content (sticky list headers, search bars over feeds): blur + optional colorControls ONLY. Skip vibrancy, lens, chromaticAberration — they're per-frame shader passes that turn into jank on a constantly-invalidating backdrop. Prefer Highlight.Plain over the gradient highlights for the same reason.
• Adding chromaticAberration = true to lens gives a premium prismatic edge — great for press states, toggle thumbs, magnifiers. Avoid it on always-on or scrolling glass.

Choosing the right entry point

Effects reference (quick)

All effects are functions on BackdropEffectScope — call them inside effects = { ... }. Radii are Float pixels — use .dp.toPx(). See references/effects.md for parameter ranges and tradeoffs.

Decoration: highlights & shadows

drawBackdrop accepts three optional decorators. Each is a lambda returning Highlight?, Shadow?, InnerShadow? — return null to skip.

highlight = { Highlight.Ambient },                                     // soft glow stroke
shadow = { Shadow(radius = 8.dp, color = Color.Black.copy(0.1f)) },    // drop shadow
innerShadow = { InnerShadow(radius = 4.dp, alpha = 0.5f) },            // depth inside

Highlight presets:

• Highlight.Default — gradient-shaded stroke (default look on most glass)
• Highlight.Ambient — softer environmental glow (great for interactive elements)
• Highlight.Plain — simple solid stroke (use for dialogs, large frosted surfaces)

All three are data classes — .copy(alpha = progress) etc. is the idiomatic way to animate them.

Surface fills and overlays

Pass onDrawSurface = { ... } to draw on top of the glass after the effects but under the children. This is how you tint glass:

onDrawSurface = {
    drawRect(Color.White.copy(alpha = 0.3f))   // light frost
}

For colored buttons (LiquidButton-style):

onDrawSurface = {
    if (tint.isSpecified) {
        drawRect(tint, blendMode = BlendMode.Hue)   // hue-shift the backdrop
        drawRect(tint.copy(alpha = 0.75f))
    }
}

onDrawBehind, onDrawBackdrop, and onDrawFront are also available for advanced layering — see references/advanced.md.

Common component recipes

Each is a complete, working component. Inline simple ones; for the rest see references/components.md.

A glass card

@Composable
fun GlassCard(
    backdrop: Backdrop,
    modifier: Modifier = Modifier,
    content: @Composable BoxScope.() -> Unit
) {
    Box(
        modifier
            .drawBackdrop(
                backdrop = backdrop,
                shape = { RoundedCornerShape(20.dp) },
                effects = {
                    vibrancy()
                    blur(8.dp.toPx())
                    lens(16.dp.toPx(), 24.dp.toPx())
                },
                highlight = { Highlight.Default },
                shadow = { Shadow(radius = 12.dp, color = Color.Black.copy(0.15f)) },
                onDrawSurface = { drawRect(Color.White.copy(alpha = 0.15f)) }
            )
            .padding(20.dp),
        content = content
    )
}

A glass pill button

@Composable
fun GlassButton(
    onClick: () -> Unit,
    backdrop: Backdrop,
    modifier: Modifier = Modifier,
    tint: Color = Color.Unspecified,
    content: @Composable RowScope.() -> Unit
) {
    Row(
        modifier
            .drawBackdrop(
                backdrop = backdrop,
                shape = { RoundedCornerShape(100.dp) },   // pill
                effects = {
                    vibrancy()
                    blur(2.dp.toPx())
                    lens(12.dp.toPx(), 24.dp.toPx())
                },
                highlight = { Highlight.Ambient },
                onDrawSurface = {
                    if (tint.isSpecified) {
                        drawRect(tint, blendMode = BlendMode.Hue)
                        drawRect(tint.copy(alpha = 0.75f))
                    }
                }
            )
            .clickable(onClick = onClick, role = Role.Button)
            .height(48.dp)
            .padding(horizontal = 16.dp),
        horizontalArrangement = Arrangement.spacedBy(8.dp, Alignment.CenterHorizontally),
        verticalAlignment = Alignment.CenterVertically,
        content = content
    )
}

A frosted dialog (heavy glass)

See references/components.md — uses colorControls + heavy blur + plain highlight + tinted surface fill, with optional dim layer behind.

Bottom tab bar / navigation pill

See references/components.md for a working LiquidBottomTabs-style recipe (uses rememberCombinedBackdrop to layer the tab labels on top of the wallpaper, and animates lens on press).

Progressive blur (scroll edges)

Box(
    Modifier
        .drawPlainBackdrop(
            backdrop = backdrop,
            shape = { RectangleShape },
            effects = {
                progressiveBlur(
                    blurRadius = 8.dp.toPx(),
                    tintColor = Color.White,
                    tintIntensity = 0.6f,
                    fadeStart = 0f,    // top fully blurred
                    fadeEnd = 1f       // bottom transparent
                )
            }
        )
        .height(96.dp)
        .fillMaxWidth()
)

Animating a glass element

The library is built around lambda-based effects/highlight/shadow blocks specifically so they can read animation state and recompute per frame without re-allocating the modifier. The pattern from the catalog:

val pressProgress by animateFloatAsState(if (pressed) 1f else 0f)

Modifier.drawBackdrop(
    backdrop = backdrop,
    shape = { RoundedCornerShape(100.dp) },
    effects = {
        // read animated state inside the lambda — recomposes cheaply
        blur(8.dp.toPx() * (1f - pressProgress))
        lens(
            10.dp.toPx() * pressProgress,
            14.dp.toPx() * pressProgress,
            chromaticAberration = true
        )
    },
    highlight = {
        Highlight.Ambient.copy(alpha = pressProgress)
    },
    innerShadow = {
        InnerShadow(radius = 4.dp * pressProgress, alpha = pressProgress)
    },
    layerBlock = {
        scaleX = 1f + 0.05f * pressProgress
        scaleY = 1f + 0.05f * pressProgress
    }
)

Use layerBlock (a GraphicsLayerScope.() -> Unit) for transforms — they reuse the same underlying layer and don't trigger relayout.

Platform support & API levels

Below API 31 effects degrade silently — your component still draws, just without glass. If you need a manual fallback, gate on Build.VERSION.SDK_INT in your shared code via expect/actual.

See references/platform.md for full details.

Common pitfalls — read this before debugging

1. No backdrop set up. If glass appears empty/black/transparent: did you put Modifier.layerBackdrop(backdrop) on the background content? The backdrop must be drawn somewhere in the same composition tree.

2. Backdrop scope. A LayerBackdrop only knows about content that flows through its layerBackdrop modifier. To layer multiple sources (e.g. wallpaper + scrolling content + a moving cursor), create separate LayerBackdrop instances and combine with rememberCombinedBackdrop(a, b, c).

3. shape is a lambda, not a Shape. shape = { RoundedCornerShape(24.dp) } — easy to drop the braces by accident.

4. effects is a receiver scope. Call effects (blur(...), lens(...)) inside the braces; they return Unit. You can't effects = blur(...).

5. Radii are Float pixels. Use .dp.toPx() (the scope inherits Density). Passing raw Dp won't compile.

6. Sizing comes after drawBackdrop. The drawing modifier needs to know its bounds; chain .size(...) / .fillMaxWidth() after drawBackdrop, before any .padding() for content.

7. Heavy stacks are expensive. vibrancy() + blur(16dp) + lens(48dp, depthEffect = true) over a moving backdrop on Android API 33 is a real GPU workload. For lists/scrollers, use drawPlainBackdrop + just blur where possible, and only escalate effects on press/focus.

8. Don't recreate the backdrop every recomposition. rememberLayerBackdrop() returns a remembered instance — keep it at the screen-scaffold level and pass it down. Re-creating it discards the captured layer.

9. Animation goes inside the effect lambdas, not in the modifier chain. Reading pressProgress inside effects = { ... } works perfectly; pulling effects out into a derivedStateOf<Modifier> allocates per frame.

10. Z-order in stacked glass: tappable / readable content goes LAST. When stacking glass — e.g. a tab bar with a sliding indicator pill, or a panel with a moving cursor — Box/BoxWithConstraints draws children in declaration order, and the last child is on top. If you draw a glass indicator after your tab icons, the indicator will cover the icons. Layer order: (1) empty glass background, (2) glass indicator/decoration, (3) icons + labels + clickable surfaces. See the bottom tab recipe in references/components.md.

11. Dim layers belong on the wallpaper, not on the dialog. For modal dialogs that should appear over a darkened scene, draw the dim into the wallpaper Image's modifier chain — after layerBackdrop so the dim is recorded into the captured backdrop. If you put the dim on the dialog's parent Box with drawWithContent { drawContent(); drawRect(dim) }, you'll dim the dialog itself instead of the surroundings, because the dialog is what drawContent() rendered.

12. Magnification is onDrawBackdrop, NOT layerBlock. A magnifier needs to scale the sampled backdrop (so the user sees content beneath the lens enlarged), not the glass element itself. Use onDrawBackdrop = { drawBackdrop -> withTransform({ translate(-dragOffset); scale(1.5f) }, drawBackdrop) }. If you instead pass layerBlock = { scaleX = 1.5f; scaleY = 1.5f }, you'll scale the entire glass element — the lens looks bigger but doesn't magnify what's underneath. See references/advanced.md.

When to dive into references

• references/effects.md — full parameter ranges, gotchas per effect, what each one looks like
• references/helpers.md — inspectDragGestures, InteractiveHighlight, DampedDragAnimation (catalog helper classes that drive the physics in the interactive recipes; copy them into your project)
• references/components.md — sample-faithful recipes: LiquidButton (tanh drag-deformation), LiquidToggle (track + scaled-track-backdrop), LiquidSlider (velocity-squish thumb), LiquidBottomTabs (double-row tinted-spotlight trick), frosted dialog, glass card, search field, sheet handle, sticky frosted list header, scroll container with progressive blur edges, lazy column grid, demo scaffold
• references/advanced.md — SDF shaders, magnifier (combined backdrops + onDrawBackdrop zoom), Control Center (drag-to-reveal with overshoot), Adaptive Luminance Glass, Lock Screen widget, Glass Playground (exportedBackdrop for sliders sampling their own sheet), camera/video as backdrop, custom shaders, performance tuning
• references/platform.md — exact API-level behavior, what degrades where, custom shader templates for AGSL/SkSL

When you've read SKILL.md and you're building anything beyond the inline recipes, open the matching reference. Don't guess — the parameters in this library have specific ranges and surprising defaults (e.g. Highlight.width = 0.5dp — small numbers).

For interactive recipes (button, toggle, slider, bottom tabs), the catalog look depends on the helpers in references/helpers.md. If you skip them and use plain animateFloatAsState, you get the visual glass but not the spring/squish/velocity feel — open helpers.md first.