feat: add show mask chip for tools

web/src/lib/components/schema/PreviewTile.svelte

@@ -15,6 +15,7 @@
 		dims?: string;
 		note?: string;
 		noteTone?: "warning" | "info";
+		actions?: Snippet;
 	}
 	let {
 		label,
@@ -26,6 +27,7 @@
 		dims = undefined,
 		note = undefined,
 		noteTone = "warning",
+		actions = undefined,
 	}: Props = $props();
 </script>
 
@@ -44,6 +46,9 @@
 			{/if}
 		</span>
 		<span class="tile-note">
+			{#if actions}
+				{@render actions()}
+			{/if}
 			{#if note}
 				<Badge tone={noteTone} label={note} />
 			{/if}

web/src/lib/components/schema/SchemaPreview.svelte

@@ -5,6 +5,7 @@
 	import SchemaSourceTile from "./SchemaSourceTile.svelte";
 	import PreviewTile from "./PreviewTile.svelte";
 	import Toggle from "$lib/components/ui/Toggle.svelte";
+	import ChipButton from "$lib/components/ui/ChipButton.svelte";
 	import { toDataUrl } from "$lib/core/io";
 	import type { PixelImage } from "$lib/core/types";
 	import type { OutputMime } from "$lib/core/io";
@@ -27,9 +28,16 @@
 		textSource?: string;
 		textResult?: string | null;
 		textVars?: Record<string, string | number>;
+		mask?: PixelImage | null;
+		maskOn?: boolean;
+		ontogglemask?: () => void;
 		running?: boolean;
 		error?: string;
 		stepResults?: (PixelImage | null)[];
+		stepMaskable?: boolean[];
+		stepMaskOn?: boolean[];
+		stepMasks?: (PixelImage | null)[];
+		ontogglestepmask?: (i: number) => void;
 		aligned?: boolean;
 		ontogglealign?: () => void;
 		format?: OutputMime;
@@ -57,9 +65,16 @@
 		textSource = "",
 		textResult = null,
 		textVars = undefined,
+		mask = null,
+		maskOn = false,
+		ontogglemask,
 		running = false,
 		error = "",
 		stepResults = [],
+		stepMaskable = [],
+		stepMaskOn = [],
+		stepMasks = [],
+		ontogglestepmask,
 		aligned = false,
 		ontogglealign,
 		format = "image/png",
@@ -132,7 +147,21 @@
 				viewMode="image"
 				dims={`${img.width} × ${img.height}`}
 			>
-				<img src={toDataUrl(img)} alt="" />
+				{#snippet actions()}
+					{#if stepMaskable[i]}
+						<ChipButton
+							label={t("resultCard.maskToggle")}
+							active={stepMaskOn[i] ?? false}
+							onclick={() => ontogglestepmask?.(i)}
+						/>
+					{/if}
+				{/snippet}
+				<img
+					src={toDataUrl(
+						stepMaskOn[i] && stepMasks[i] ? (stepMasks[i] as PixelImage) : img,
+					)}
+					alt=""
+				/>
 			</PreviewTile>
 		{/if}
 	{/each}
@@ -145,6 +174,9 @@
 		{textVars}
 		{toolId}
 		{running}
+		{mask}
+		{maskOn}
+		{ontogglemask}
 		oncopy={oncopytext}
 		{ondownloadtxt}
 	/>

web/src/lib/components/schema/SchemaResultTile.svelte

@@ -4,6 +4,7 @@
 	import { t } from "$lib/i18n/t";
 	import type { FileResult, ResultKind, ResultNote } from "$lib/registry";
 	import { Check } from "@lucide/svelte";
+	import ChipButton from "$lib/components/ui/ChipButton.svelte";
 	import SchemaTextResult from "./SchemaTextResult.svelte";
 	import PreviewTile from "./PreviewTile.svelte";
 
@@ -16,6 +17,9 @@
 		textVars?: Record<string, string | number>;
 		toolId?: string;
 		running?: boolean;
+		mask?: PixelImage | null;
+		maskOn?: boolean;
+		ontogglemask?: () => void;
 		oncopy?: () => void;
 		ondownloadtxt?: () => void;
 	}
@@ -28,11 +32,15 @@
 		textVars = undefined,
 		toolId = "",
 		running = false,
+		mask = null,
+		maskOn = false,
+		ontogglemask,
 		oncopy,
 		ondownloadtxt,
 	}: Props = $props();
 
-	const resultUrl = $derived(result ? toDataUrl(result) : null);
+	const shown = $derived(maskOn && mask ? mask : result);
+	const resultUrl = $derived(shown ? toDataUrl(shown) : null);
 	const resultDims = $derived(
 		resultKind === "image" && result
 			? `${result.width} × ${result.height}`
@@ -68,6 +76,15 @@
 	note={resultNote ? t(resultNote.key) : undefined}
 	noteTone={resultNote?.tone ?? "warning"}
 >
+	{#snippet actions()}
+		{#if ontogglemask}
+			<ChipButton
+				label={t("resultCard.maskToggle")}
+				active={maskOn}
+				onclick={ontogglemask}
+			/>
+		{/if}
+	{/snippet}
 	<div
 		class="canvas"
 		class:checker={resultKind === "image"}

web/src/lib/components/schema/SchemaToolView.svelte

@@ -3,6 +3,7 @@
 	import { resolve } from "$app/paths";
 	import StepCard from "$lib/components/display/StepCard.svelte";
 	import Button from "$lib/components/ui/Button.svelte";
+	import ChipButton from "$lib/components/ui/ChipButton.svelte";
 	import { ButtonVariantDefine } from "$lib/components/ui/define";
 	import Toggle from "$lib/components/ui/Toggle.svelte";
 	import { isFav, toggleFav } from "$lib/favorites.svelte";
@@ -114,6 +115,10 @@
 	let formatQuality = $state<Record<string, number>>({});
 	// Optional download size limit (KB); undefined = no limit.
 	let limitKb = $state<number | undefined>(undefined);
+	// Mask preview per step (keyed by step.key): the toggle lives on every
+	// step result tile, not just the final one.
+	const maskOnKeys = new SvelteSet<string>();
+	let maskResults = $state<Record<string, PixelImage | null>>({});
 	let dragFrom = $state<number | null>(null);
 
 	const lastTool = $derived(
@@ -165,6 +170,10 @@
 			: current.text;
 	});
 
+	const stepMaskable = $derived(steps.map((s) => !!getTool(s.id)?.runMask));
+	const stepMaskOn = $derived(steps.map((s) => maskOnKeys.has(s.key)));
+	const stepMasks = $derived(steps.map((s) => maskResults[s.key] ?? null));
+
 	const debouncedRun = debounce(() => run(), 200);
 
 	function touchedFor(key: string): SvelteSet<string> {
@@ -209,6 +218,8 @@
 		textResult = null;
 		verdictVars = undefined;
 		displayError = null;
+		maskOnKeys.clear();
+		maskResults = {};
 	}
 
 	function setStepValue(
@@ -517,6 +528,41 @@
 		return () => debouncedRun.cancel();
 	});
 
+	// Mask preview: recomputed on the main thread per toggled step from that
+	// step's input; a mask never enters the chain itself.
+	$effect(() => {
+		void steps;
+		void stepResults;
+		void source;
+		const next: Record<string, PixelImage | null> = {};
+		steps.forEach((step, i) => {
+			if (!maskOnKeys.has(step.key)) return;
+			const stepTool = getTool(step.id);
+			if (!stepTool?.runMask) return;
+			const input = i === 0 ? source : stepResults[i - 1];
+			if (!input) return;
+			try {
+				next[step.key] = stepTool.runMask({
+					params: step.params,
+					source: input,
+				});
+			} catch {
+				next[step.key] = null;
+			}
+		});
+		maskResults = next;
+	});
+
+	function toggleStepMask(key: string) {
+		if (maskOnKeys.has(key)) maskOnKeys.delete(key);
+		else maskOnKeys.add(key);
+	}
+
+	function toggleStepMaskByIndex(i: number) {
+		const step = steps[i];
+		if (step) toggleStepMask(step.key);
+	}
+
 	function toDisplayError(e: unknown): DisplayError {
 		if (e instanceof ToolError) {
 			return { kind: "i18n", key: e.key, vars: e.vars };
@@ -614,6 +660,16 @@
 			</StepCard>
 		{/snippet}
 
+		{#snippet maskChip(i: number)}
+			{#if stepMaskable[i]}
+				<ChipButton
+					label={t("resultCard.maskToggle")}
+					active={stepMaskOn[i]}
+					onclick={() => toggleStepMask(steps[i].key)}
+				/>
+			{/if}
+		{/snippet}
+
 		{#if !alignedMode}
 			<div class="workspace">
 				<section class="settings">
@@ -648,6 +704,15 @@
 						{alphaLoss}
 						{stepResults}
 						{aligned}
+						{stepMaskable}
+						{stepMaskOn}
+						{stepMasks}
+						ontogglestepmask={toggleStepMaskByIndex}
+						mask={stepMasks.at(-1) ?? null}
+						maskOn={stepMaskOn.at(-1) ?? false}
+						ontogglemask={stepMaskable.at(-1)
+							? () => toggleStepMask(steps[steps.length - 1].key)
+							: undefined}
 						ontogglealign={() => (aligned = !aligned)}
 						onformat={(v) => (format = v)}
 						onquality={setFormatQuality}
@@ -746,6 +811,11 @@
 									textVars={verdictVars}
 									toolId={lastTool.id}
 									{running}
+									mask={stepMasks[i] ?? null}
+									maskOn={stepMaskOn[i] ?? false}
+									ontogglemask={stepMaskable[i]
+										? () => toggleStepMask(steps[i].key)
+										: undefined}
 									oncopy={copyText}
 									ondownloadtxt={downloadText}
 								/>
@@ -755,8 +825,16 @@
 									viewMode="image"
 									dims={out ? `${out.width} × ${out.height}` : undefined}
 								>
+									{#snippet actions()}
+										{@render maskChip(i)}
+									{/snippet}
 									{#if out}
-										<img src={toDataUrl(out)} alt="" />
+										<img
+											src={toDataUrl(
+												stepMaskOn[i] && stepMasks[i] ? stepMasks[i]! : out,
+											)}
+											alt=""
+										/>
 									{:else}
 										<span class="empty">{t("resultCard.noResult")}</span>
 									{/if}

web/src/lib/core/effects.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { vignette } from "./effects";
+import { vignette, vignetteMask } from "./effects";
 import { makeImage } from "./test-helpers";
 
 describe("vignette", () => {
@@ -17,3 +17,19 @@ describe("vignette", () => {
 		expect(centerR).toBeGreaterThan(150);
 	});
 });
+
+describe("vignetteMask", () => {
+	it("white at corners, black at center, opaque", () => {
+		const out = vignetteMask(7, 7, 80);
+		const center = out.data[(3 * 7 + 3) * 4];
+		const corner = out.data[0];
+		expect(corner).toBeGreaterThan(center);
+		expect(corner).toBeGreaterThan(100);
+		expect(out.data[3]).toBe(255);
+	});
+
+	it("zero strength -> all black", () => {
+		const out = vignetteMask(3, 3, 0);
+		expect(Math.max(...out.data.filter((_, i) => i % 4 !== 3))).toBe(0);
+	});
+});

web/src/lib/core/effects.ts

@@ -22,3 +22,30 @@ export function vignette(img: PixelImage, strengthPercent: number): PixelImage {
 	}
 	return out;
 }
+
+/** B/w preview of the vignette falloff: white where darkening is strongest. */
+export function vignetteMask(
+	width: number,
+	height: number,
+	strengthPercent: number,
+): PixelImage {
+	const strength = Math.min(Math.max(strengthPercent, 0), 100) / 100;
+	const cx = (width - 1) / 2;
+	const cy = (height - 1) / 2;
+	const maxDist = Math.sqrt(cx * cx + cy * cy) || 1;
+	const out = createPixelImage(width, height);
+	for (let y = 0; y < height; y++) {
+		for (let x = 0; x < width; x++) {
+			const dx = x - cx;
+			const dy = y - cy;
+			const t = Math.sqrt(dx * dx + dy * dy) / maxDist;
+			const v = Math.round(strength * t * t * 255);
+			const di = (y * width + x) * 4;
+			out.data[di] = v;
+			out.data[di + 1] = v;
+			out.data[di + 2] = v;
+			out.data[di + 3] = 255;
+		}
+	}
+	return out;
+}

web/src/lib/core/masks.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 import {
+	diffMask,
 	extractByColor,
 	isGrayscaleish,
 	luma01,
@@ -74,3 +75,26 @@ describe("rarityPredicate + extractByColor", () => {
 		expect(out.data[7]).toBe(255);
 	});
 });
+
+describe("diffMask", () => {
+	const before = makeImage(2, 1, [
+		[255, 0, 0, 255],
+		[0, 255, 0, 128],
+	]);
+
+	it("white where channels differ, black where equal, always opaque", () => {
+		const after = makeImage(2, 1, [
+			[255, 0, 0, 255],
+			[0, 255, 0, 255],
+		]);
+		const out = diffMask(before, after);
+		expect([...out.data.slice(0, 4)]).toEqual([0, 0, 0, 255]);
+		expect([...out.data.slice(4, 8)]).toEqual([255, 255, 255, 255]);
+	});
+
+	it("size mismatch means everything changed", () => {
+		const smaller = makeImage(1, 1, [[255, 0, 0, 255]]);
+		const out = diffMask(before, smaller);
+		expect([...out.data.slice(0, 4)]).toEqual([255, 255, 255, 255]);
+	});
+});

web/src/lib/core/masks.ts

@@ -117,6 +117,30 @@ export function extractByColor(
 	return out;
 }
 
+/**
+ * Pixels that differ between before/after become white, the rest black.
+ * Size mismatch (crop/trim) means everything changed: all white.
+ */
+export function diffMask(before: PixelImage, after: PixelImage): PixelImage {
+	const out = createPixelImage(before.width, before.height);
+	const sameSize =
+		before.width === after.width && before.height === after.height;
+	for (let i = 0; i < out.data.length; i += 4) {
+		const changed =
+			!sameSize ||
+			before.data[i] !== after.data[i] ||
+			before.data[i + 1] !== after.data[i + 1] ||
+			before.data[i + 2] !== after.data[i + 2] ||
+			before.data[i + 3] !== after.data[i + 3];
+		const v = changed ? 255 : 0;
+		out.data[i] = v;
+		out.data[i + 1] = v;
+		out.data[i + 2] = v;
+		out.data[i + 3] = 255;
+	}
+	return out;
+}
+
 /**
  * Counts color frequencies and returns a predicate "occurs at most limit times".
  */

web/src/lib/core/morphology.test.ts

@@ -3,6 +3,7 @@ import {
 	buildAlphaMask,
 	closingImage,
 	contourImage,
+	contourMask,
 	dilateImage,
 	dilateMask,
 	erodeImage,
@@ -189,3 +190,24 @@ describe("contourImage", () => {
 		expect(at(1, 3)).toEqual([0, 0, 0, 0]);
 	});
 });
+
+describe("contourMask", () => {
+	it("white line on the alpha edge, black elsewhere", () => {
+		const pixels = [];
+		for (let y = 0; y < 7; y++) {
+			for (let x = 0; x < 7; x++) {
+				pixels.push(
+					x >= 2 && x <= 4 && y >= 2 && y <= 4
+						? [255, 255, 255, 255]
+						: [0, 0, 0, 0],
+				);
+			}
+		}
+		const out = contourMask(makeImage(7, 7, pixels), 1);
+		const at = (x: number, y: number) => out.data[(y * 7 + x) * 4];
+		expect(at(2, 2)).toBe(255);
+		expect(at(3, 3)).toBe(0);
+		expect(at(0, 0)).toBe(0);
+		expect(out.data[3]).toBe(255);
+	});
+});

web/src/lib/core/morphology.ts

@@ -167,6 +167,23 @@ export function contourImage(
 	return out;
 }
 
+/** B/w preview of the contour line: white where the stroke will land. */
+export function contourMask(img: PixelImage, radiusPx: number): PixelImage {
+	const r = Math.max(1, Math.trunc(radiusPx));
+	const mask = buildAlphaMask(img);
+	const inner = erodeMask(mask, img.width, img.height, r);
+	const out = createPixelImage(img.width, img.height);
+	for (let i = 0; i < mask.length; i++) {
+		const v = mask[i] === 1 && inner[i] === 0 ? 255 : 0;
+		const di = i * 4;
+		out.data[di] = v;
+		out.data[di + 1] = v;
+		out.data[di + 2] = v;
+		out.data[di + 3] = 255;
+	}
+	return out;
+}
+
 export function openingMask(
 	mask: Mask,
 	w: number,

web/src/lib/core/shapes.test.ts

@@ -1,5 +1,12 @@
 import { describe, expect, it } from "vitest";
-import { boxTest, circleTest, renderShape, starTest, wavyTest } from "./shapes";
+import {
+	boxTest,
+	circleTest,
+	renderShape,
+	renderShapeMask,
+	starTest,
+	wavyTest,
+} from "./shapes";
 import { makeImage } from "./test-helpers";
 
 describe("shape predicates", () => {
@@ -88,3 +95,25 @@ describe("renderShape", () => {
 		expect(opaqueAt(2, 1)).toBe(false);
 	});
 });
+
+describe("renderShapeMask", () => {
+	it("white inside the shape, black outside, same geometry as renderShape", () => {
+		const shape = renderShape(
+			makeImage(5, 5, new Array(25).fill([200, 100, 50, 255])),
+			circleTest(0.4),
+			0,
+			0,
+			{ x: 0.4, y: 0.4 },
+		);
+		const mask = renderShapeMask(5, 5, circleTest(0.4), 0, 0, {
+			x: 0.4,
+			y: 0.4,
+		});
+		for (let i = 0; i < 25; i++) {
+			const shaped = shape.data[i * 4 + 3] === 255;
+			const v = mask.data[i * 4];
+			expect(v, `pixel ${i}`).toBe(shaped ? 255 : 0);
+			expect(mask.data[i * 4 + 3]).toBe(255);
+		}
+	});
+});

web/src/lib/core/shapes.ts

@@ -58,10 +58,31 @@ export function wavyTest(
 	};
 }
 
+/**
+ * Shared geometry: shape center in pixels from offset fractions and extent.
+ * Offset is a fraction of free space per axis: +-0.5 pins the shape to the
+ * edge regardless of size. Free space derives from extent (half-size).
+ */
+function shapeOrigin(
+	width: number,
+	height: number,
+	offsetXFrac: number,
+	offsetYFrac: number,
+	extent: ShapeExtent,
+): { cx: number; cy: number; minDim: number } {
+	const minDim = Math.min(width, height);
+	const freeX = Math.max(0, width / (2 * minDim) - extent.x);
+	const freeY = Math.max(0, height / (2 * minDim) - extent.y);
+	return {
+		cx: width / 2 + offsetXFrac * 2 * freeX * minDim,
+		cy: height / 2 + offsetYFrac * 2 * freeY * minDim,
+		minDim,
+	};
+}
+
 /**
  * Cuts a shape out of the image: source pixels kept inside, alpha zeroed
- * outside. Offset is a fraction of free space per axis: +-0.5 pins the shape
- * to the edge regardless of size. Free space derives from extent (half-size).
+ * outside.
  */
 export function renderShape(
 	img: PixelImage,
@@ -71,11 +92,13 @@ export function renderShape(
 	extent: ShapeExtent = { x: 0, y: 0 },
 ): PixelImage {
 	const out = createPixelImage(img.width, img.height);
-	const minDim = Math.min(img.width, img.height);
-	const freeX = Math.max(0, img.width / (2 * minDim) - extent.x);
-	const freeY = Math.max(0, img.height / (2 * minDim) - extent.y);
-	const cx = img.width / 2 + offsetXFrac * 2 * freeX * minDim;
-	const cy = img.height / 2 + offsetYFrac * 2 * freeY * minDim;
+	const { cx, cy, minDim } = shapeOrigin(
+		img.width,
+		img.height,
+		offsetXFrac,
+		offsetYFrac,
+		extent,
+	);
 	for (let y = 0; y < img.height; y++) {
 		for (let x = 0; x < img.width; x++) {
 			const di = (y * img.width + x) * 4;
@@ -88,3 +111,34 @@ export function renderShape(
 	}
 	return out;
 }
+
+/** B/w preview of the shape itself: white inside, black outside. */
+export function renderShapeMask(
+	width: number,
+	height: number,
+	test: ShapeTest,
+	offsetXFrac = 0,
+	offsetYFrac = 0,
+	extent: ShapeExtent = { x: 0, y: 0 },
+): PixelImage {
+	const out = createPixelImage(width, height);
+	const { cx, cy, minDim } = shapeOrigin(
+		width,
+		height,
+		offsetXFrac,
+		offsetYFrac,
+		extent,
+	);
+	for (let y = 0; y < height; y++) {
+		for (let x = 0; x < width; x++) {
+			const di = (y * width + x) * 4;
+			const inside = test((x + 0.5 - cx) / minDim, (y + 0.5 - cy) / minDim);
+			const v = inside ? 255 : 0;
+			out.data[di] = v;
+			out.data[di + 1] = v;
+			out.data[di + 2] = v;
+			out.data[di + 3] = 255;
+		}
+	}
+	return out;
+}

web/src/lib/i18n/en.ts

@@ -99,6 +99,7 @@ export const en: Dict = {
 		breakChain: "✂ Break the chain",
 		noResult: "no result yet",
 		result: "Result",
+		maskToggle: "Mask",
 		previewPanel: "Preview",
 		parts: "parts",
 		clamped: "clamped to image bounds",

web/src/lib/i18n/ru.ts

@@ -100,6 +100,7 @@ export const ru: Dict = {
 		breakChain: "✂ Оборвать цепочку",
 		noResult: "результата пока нет",
 		result: "Результат",
+		maskToggle: "Маска",
 		previewPanel: "Предпросмотр",
 		parts: "частей",
 		clamped: "уменьшено до границ изображения",

web/src/lib/registry/registry.test.ts

@@ -3,6 +3,7 @@ import { PAGES, TOOLS } from ".";
 import { PREVIEW_GROUPS } from "../catalog";
 import type { PixelImage } from "../core/types";
 import type { OutputMime } from "../core/io";
+import { makeImage } from "../core/test-helpers";
 import { defaultSchemaParams, sanitizeSchemaParams } from "../registry-schema";
 import type { FileResult, ToolResult } from "./types";
 
@@ -154,6 +155,38 @@ describe("registry-new (migrated tools)", () => {
 		}
 	});
 
+	it("runMask returns an opaque source-sized mask for tools declaring it", () => {
+		// Mask preview contract: same geometry as the step input, fully opaque
+		// b/w rendering, synchronous and DOM-free.
+		const source = makeImage(4, 3, [
+			[255, 0, 0, 255],
+			[0, 255, 0, 255],
+			[0, 0, 255, 128],
+			[255, 255, 255, 0],
+			[0, 0, 0, 255],
+			[255, 255, 0, 255],
+			[0, 255, 255, 64],
+			[255, 0, 255, 255],
+			[128, 128, 128, 255],
+			[32, 64, 128, 200],
+			[255, 128, 0, 255],
+			[0, 128, 255, 255],
+		]);
+		const withMask = TOOLS.filter((tool) => tool.runMask);
+		expect(withMask.length).toBeGreaterThan(0);
+		for (const tool of withMask) {
+			const mask = tool.runMask!({
+				params: defaultSchemaParams(tool.schema),
+				source,
+			});
+			expect(mask.width, `${tool.id}: width`).toBe(source.width);
+			expect(mask.height, `${tool.id}: height`).toBe(source.height);
+			for (let a = 3; a < mask.data.length; a += 4) {
+				expect(mask.data[a], `${tool.id}: alpha @${a}`).toBe(255);
+			}
+		}
+	});
+
 	it("source-aware dimension defaults apply only to image tools", () => {
 		for (const tool of TOOLS) {
 			for (const field of Object.values(tool.schema.fields)) {

web/src/lib/registry/tools/alpha.ts

@@ -1,4 +1,5 @@
 import {
+	colorMask,
 	extractAlphaMask,
 	flattenOntoColor,
 	hardenAlpha,
@@ -7,10 +8,12 @@ import {
 	roundCorners,
 	setAlphaChannel,
 } from "../../core/alpha";
-import { removeBackground } from "../../core/background";
+import { backgroundMaskPreview, removeBackground } from "../../core/background";
+import { diffMask, renderPredicateMask } from "../../core/masks";
 import {
 	closingImage,
 	contourImage,
+	contourMask,
 	dilateImage,
 	erodeImage,
 	openingImage,
@@ -21,12 +24,23 @@ import {
 	boxTest,
 	circleTest,
 	renderShape,
+	renderShapeMask,
 	starTest,
 	wavyTest,
 } from "../../core/shapes";
+import type { PixelImage } from "../../core/types";
 import type { Offset } from "../../registry-schema";
 import { field, toolSchema } from "../../registry-schema";
-import { imgTool, type Tool } from "../types";
+import { imgTool, requireSource, type Tool, type ToolContext } from "../types";
+
+/** Mask of an alpha-morphology tool: grayscale alpha of the result. */
+function alphaOfResult<P>(
+	ctx: ToolContext<P>,
+	fn: (img: PixelImage, p: P) => PixelImage,
+): PixelImage {
+	const img = requireSource(ctx);
+	return extractAlphaMask(fn(img, ctx.params));
+}
 
 interface AddStrokeParams {
 	color: string;
@@ -95,6 +109,10 @@ const removeColor: Tool<RemoveColorParams> = {
 	schema: removeColorSchema,
 	input: "image",
 	run: imgTool((img, p) => removeColorToAlpha(img, p.targetColor, p.tolerance)),
+	runMask: (ctx) => {
+		const img = requireSource(ctx);
+		return colorMask(img, ctx.params.targetColor, ctx.params.tolerance);
+	},
 };
 
 interface CircleMaskParams {
@@ -142,6 +160,18 @@ const circleMask: Tool<CircleMaskParams> = {
 			{ x: p.size / 200, y: p.size / 200 },
 		),
 	),
+	runMask: (ctx) => {
+		const img = requireSource(ctx);
+		const p = ctx.params;
+		return renderShapeMask(
+			img.width,
+			img.height,
+			circleTest(p.size / 200),
+			p.offset.x / 100,
+			p.offset.y / 100,
+			{ x: p.size / 200, y: p.size / 200 },
+		);
+	},
 };
 
 interface SquareMaskParams {
@@ -201,6 +231,18 @@ const squareMask: Tool<SquareMaskParams> = {
 			{ x: p.widthPct / 200, y: p.heightPct / 200 },
 		),
 	),
+	runMask: (ctx) => {
+		const img = requireSource(ctx);
+		const p = ctx.params;
+		return renderShapeMask(
+			img.width,
+			img.height,
+			boxTest(p.widthPct / 200, p.heightPct / 200),
+			p.offset.x / 100,
+			p.offset.y / 100,
+			{ x: p.widthPct / 200, y: p.heightPct / 200 },
+		);
+	},
 };
 
 interface StarMaskParams {
@@ -276,6 +318,18 @@ const starMask: Tool<StarMaskParams> = {
 			{ x: p.size / 200, y: p.size / 200 },
 		),
 	),
+	runMask: (ctx) => {
+		const img = requireSource(ctx);
+		const p = ctx.params;
+		return renderShapeMask(
+			img.width,
+			img.height,
+			starTest(p.points, p.innerRadius / 100, p.size / 200, p.rotation),
+			p.offset.x / 100,
+			p.offset.y / 100,
+			{ x: p.size / 200, y: p.size / 200 },
+		);
+	},
 };
 
 interface WavyMaskParams {
@@ -351,6 +405,18 @@ const wavyMask: Tool<WavyMaskParams> = {
 			{ x: (p.size + p.amplitude) / 200, y: (p.size + p.amplitude) / 200 },
 		),
 	),
+	runMask: (ctx) => {
+		const img = requireSource(ctx);
+		const p = ctx.params;
+		return renderShapeMask(
+			img.width,
+			img.height,
+			wavyTest(p.size / 200, p.amplitude / 200, p.waves, p.phase),
+			p.offset.x / 100,
+			p.offset.y / 100,
+			{ x: (p.size + p.amplitude) / 200, y: (p.size + p.amplitude) / 200 },
+		);
+	},
 };
 
 interface EmptyParams {}
@@ -362,6 +428,10 @@ const removeAlphaChannel: Tool<EmptyParams> = {
 	schema: removeAlphaChannelSchema,
 	input: "image",
 	run: imgTool((img) => flattenOntoColor(img, "#ffffff")),
+	runMask: (ctx) =>
+		renderPredicateMask(requireSource(ctx), (_r, _g, _b, a) => a < 255, {
+			mode: "binary",
+		}),
 };
 
 interface SetAlphaChannelParams {
@@ -413,6 +483,7 @@ const roundCornersTool: Tool<RoundCornersParams> = {
 	schema: roundCornersSchema,
 	input: "image",
 	run: imgTool((img, p) => roundCorners(img, p.radius)),
+	runMask: (ctx) => alphaOfResult(ctx, (img, p) => roundCorners(img, p.radius)),
 };
 
 export const invertAlphaSchema = toolSchema<EmptyParams>({});
@@ -422,6 +493,7 @@ const invertAlphaTool: Tool<EmptyParams> = {
 	schema: invertAlphaSchema,
 	input: "image",
 	run: imgTool((img) => invertAlpha(img)),
+	runMask: (ctx) => alphaOfResult(ctx, invertAlpha),
 };
 
 interface RemoveBackgroundParams {
@@ -476,6 +548,15 @@ const removeBackgroundTool: Tool<RemoveBackgroundParams> = {
 			smoothPasses: p.smooth,
 		}),
 	),
+	runMask: (ctx) => {
+		const p = ctx.params;
+		return backgroundMaskPreview(requireSource(ctx), {
+			color: p.color,
+			tolerancePercent: p.tolerance,
+			outerOnly: p.outerOnly,
+			smoothPasses: p.smooth,
+		});
+	},
 };
 
 interface MakeThickerParams {
@@ -497,6 +578,7 @@ const makeThickerTool: Tool<MakeThickerParams> = {
 	schema: makeThickerSchema,
 	input: "image",
 	run: imgTool((img, p) => dilateImage(img, p.radius)),
+	runMask: (ctx) => alphaOfResult(ctx, (img, p) => dilateImage(img, p.radius)),
 };
 
 interface MakeThinnerParams {
@@ -518,6 +600,7 @@ const makeThinnerTool: Tool<MakeThinnerParams> = {
 	schema: makeThinnerSchema,
 	input: "image",
 	run: imgTool((img, p) => erodeImage(img, p.radius)),
+	runMask: (ctx) => alphaOfResult(ctx, (img, p) => erodeImage(img, p.radius)),
 };
 
 interface FeatherEdgesParams {
@@ -539,6 +622,7 @@ const featherEdgesTool: Tool<FeatherEdgesParams> = {
 	schema: featherEdgesSchema,
 	input: "image",
 	run: imgTool((img, p) => featherAlpha(img, p.radius)),
+	runMask: (ctx) => alphaOfResult(ctx, (img, p) => featherAlpha(img, p.radius)),
 };
 
 interface CleanEdgesParams {
@@ -560,6 +644,10 @@ const cleanEdgesTool: Tool<CleanEdgesParams> = {
 	schema: cleanEdgesSchema,
 	input: "image",
 	run: imgTool((img, p) => defringe(img, p.radius)),
+	runMask: (ctx) => {
+		const img = requireSource(ctx);
+		return diffMask(img, defringe(img, ctx.params.radius));
+	},
 };
 
 interface HardenAlphaParams {
@@ -581,6 +669,8 @@ const hardenAlphaTool: Tool<HardenAlphaParams> = {
 	schema: hardenAlphaSchema,
 	input: "image",
 	run: imgTool((img, p) => hardenAlpha(img, p.threshold)),
+	runMask: (ctx) =>
+		alphaOfResult(ctx, (img, p) => hardenAlpha(img, p.threshold)),
 };
 
 interface DespeckleAlphaParams {
@@ -602,6 +692,7 @@ const despeckleAlphaTool: Tool<DespeckleAlphaParams> = {
 	schema: despeckleAlphaSchema,
 	input: "image",
 	run: imgTool((img, p) => openingImage(img, p.radius)),
+	runMask: (ctx) => alphaOfResult(ctx, (img, p) => openingImage(img, p.radius)),
 };
 
 interface CloseHolesParams {
@@ -623,6 +714,7 @@ const closeHolesTool: Tool<CloseHolesParams> = {
 	schema: closeHolesSchema,
 	input: "image",
 	run: imgTool((img, p) => closingImage(img, p.radius)),
+	runMask: (ctx) => alphaOfResult(ctx, (img, p) => closingImage(img, p.radius)),
 };
 
 export const alphaTools = [

web/src/lib/registry/tools/analyze.ts

@@ -13,7 +13,8 @@ import {
 } from "../../core/masks";
 import { base64ToBytes, looksLikePng, stripDataUri } from "../../core/textio";
 import { field, toolSchema } from "../../registry-schema";
-import { imgTool, textGen, type Tool } from "../types";
+import { colorMask } from "../../core/alpha";
+import { imgTool, requireSource, textGen, type Tool } from "../types";
 
 interface ExtractColorParams {
 	color: string;
@@ -36,6 +37,10 @@ const extractColor: Tool<ExtractColorParams> = {
 	schema: extractColorSchema,
 	input: "image",
 	run: imgTool((img, p) => extractByColor(img, p.color, p.tolerance)),
+	runMask: (ctx) => {
+		const img = requireSource(ctx);
+		return colorMask(img, ctx.params.color, ctx.params.tolerance);
+	},
 };
 
 interface MaskParams {

web/src/lib/registry/tools/filters.ts

@@ -1,5 +1,5 @@
 import { gaussianBlur, sharpen as sharpenImage } from "../../core/convolution";
-import { vignette } from "../../core/effects";
+import { vignette, vignetteMask } from "../../core/effects";
 import { jpegRoundtrip } from "../../core/io";
 import {
 	addNoise,
@@ -8,7 +8,7 @@ import {
 	silhouette,
 } from "../../core/pixel-fx";
 import { field, toolSchema } from "../../registry-schema";
-import { imgTool, type Tool } from "../types";
+import { imgTool, requireSource, type Tool } from "../types";
 
 interface BlurParams {
 	radius: number;
@@ -94,6 +94,10 @@ const vignetteTool: Tool<VignetteParams> = {
 	schema: vignetteSchema,
 	input: "image",
 	run: imgTool((img, p) => vignette(img, p.strength)),
+	runMask: (ctx) => {
+		const img = requireSource(ctx);
+		return vignetteMask(img.width, img.height, ctx.params.strength);
+	},
 };
 
 interface PixelateParams {

web/src/lib/registry/tools/geometry.ts

@@ -24,6 +24,7 @@ import {
 	type Anchor9,
 	type FlipAxis,
 } from "../../core/geometry";
+import { renderPredicateMask } from "../../core/masks";
 import { field, toolSchema, type Dimension } from "../../registry-schema";
 import {
 	imgTool,
@@ -547,6 +548,10 @@ const centerByAlphaTool: Tool<EmptyParams> = {
 	schema: centerByAlphaSchema,
 	input: "image",
 	run: imgTool((img) => centerByAlpha(img)),
+	runMask: (ctx) =>
+		renderPredicateMask(requireSource(ctx), (_r, _g, _b, a) => a > 0, {
+			mode: "binary",
+		}),
 };
 
 interface SkewParams {
@@ -639,6 +644,12 @@ const trimEmptySpaceTool: Tool<TrimEmptySpaceParams> = {
 	schema: trimEmptySpaceSchema,
 	input: "image",
 	run: imgTool((img, p) => trimToContent(img, p.threshold)),
+	runMask: (ctx) =>
+		renderPredicateMask(
+			requireSource(ctx),
+			(_r, _g, _b, a) => a > ctx.params.threshold,
+			{ mode: "binary" },
+		),
 };
 
 type AspectRatio = "1:1" | "4:3" | "3:4" | "3:2" | "2:3" | "16:9" | "9:16";

web/src/lib/registry/types.ts

@@ -67,6 +67,7 @@ export type Tool<P = Record<string, unknown>> = {
 	/** Needs the DOM (canvas/document); the preview executor runs it directly, not in a worker. */
 	domOnly?: boolean;
 	output?: OutputFormat;
+	runMask?: (ctx: ToolContext<P>) => PixelImage;
 	/** Note at the result tile when the outcome differs from requested (clamped, aspect-corrected, grew past canvas). */
 	resultNote?: (params: P, result: PixelImage) => ResultNote | null;
 };