diff --git a/web/src/assets/variant-sample.jpg b/web/src/assets/variant-sample.jpg
new file mode 100644
index 0000000..9283c72
Binary files /dev/null and b/web/src/assets/variant-sample.jpg differ
diff --git a/web/src/components/variants/ImageCompare.vue b/web/src/components/variants/ImageCompare.vue
index 98cc027..bcf5440 100644
--- a/web/src/components/variants/ImageCompare.vue
+++ b/web/src/components/variants/ImageCompare.vue
@@ -17,6 +17,8 @@ const CHIP =
 const props = defineProps<{
 	name: string
 	comparable: boolean
+	originalSrc?: string | null
+	variantSrc?: string | null
 }>()
 
 const position = defineModel<number>({ default: 50 })
@@ -64,7 +66,13 @@ function onKeydown(event: KeyboardEvent) {
 			class="absolute inset-0 flex items-center justify-center bg-(--ui-color-secondary-200) text-muted dark:bg-(--ui-color-secondary-300)"
 			:class="PATTERN"
 		>
-			<UIcon name="i-lucide-image" class="size-8 opacity-55" />
+			<img
+				v-if="originalSrc"
+				:src="originalSrc"
+				alt=""
+				class="absolute inset-0 size-full object-cover"
+			/>
+			<UIcon v-else name="i-lucide-image" class="size-8 opacity-55" />
 		</div>
 		<template v-if="comparable">
 			<div
@@ -72,7 +80,14 @@ function onKeydown(event: KeyboardEvent) {
 				class="absolute inset-0 bg-accented"
 				:class="VARIANT_PATTERN"
 				:style="{ clipPath: `inset(0 0 0 ${position}%)` }"
-			/>
+			>
+				<img
+					v-if="variantSrc"
+					:src="variantSrc"
+					alt=""
+					class="absolute inset-0 size-full object-contain"
+				/>
+			</div>
 			<span :class="[CHIP, 'left-2']">{{ t('Original') }}</span>
 			<span :class="[CHIP, 'right-2 font-mono']">{{ shownName }}</span>
 			<input
diff --git a/web/src/components/variants/VariantPreview.vue b/web/src/components/variants/VariantPreview.vue
index 7c26be0..2b930c6 100644
--- a/web/src/components/variants/VariantPreview.vue
+++ b/web/src/components/variants/VariantPreview.vue
@@ -4,7 +4,13 @@ import { t } from '@/i18n'
 import { computed, ref } from 'vue'
 import ImageCompare from '@/components/variants/ImageCompare.vue'
 import { useCopyToClipboard } from '@/composables/useCopyToClipboard'
-import { SAMPLE_SOURCE, estimateLabel, estimateVariant } from '@/lib/variants/variantEstimate'
+import { useVariantRender } from '@/composables/useVariantRender'
+import {
+	SAMPLE_SOURCE,
+	estimateLabel,
+	estimateVariant,
+	type EstimateRequest,
+} from '@/lib/variants/variantEstimate'
 import { parseDimension, sliderValue, type VariantFormValues } from '@/lib/variants/variantForm'
 import { formatVariantSize } from '@/lib/variants/variantFormat'
 
@@ -25,22 +31,31 @@ const dimensionsAreValid = computed(
 		(props.values.height.trim() === '' || parseDimension(props.values.height) !== null),
 )
 
-const estimate = computed(() =>
+const request = computed<EstimateRequest | null>(() =>
 	dimensionsAreValid.value
-		? estimateVariant({
+		? {
 				width: parseDimension(props.values.width),
 				height: parseDimension(props.values.height),
 				fit: props.values.fit,
 				format: props.values.format,
 				quality: sliderValue(props.values.quality),
-			})
+			}
 		: null,
 )
 
+const estimate = computed(() => (request.value ? estimateVariant(request.value) : null))
+const { original, variant } = useVariantRender(request)
+
 const shownName = computed(() => props.values.name || t('name'))
-const outputLine = computed(() =>
-	estimate.value ? estimateLabel(estimate.value) : t('No output yet'),
-)
+const outputLine = computed(() => {
+	if (!estimate.value) {
+		return t('No output yet')
+	}
+	const bytes = variant.value
+		? Math.min(SAMPLE_SOURCE.bytes, variant.value.bytes)
+		: estimate.value.bytes
+	return estimateLabel({ size: estimate.value.size, bytes })
+})
 const originalLine = computed(
 	() =>
 		`${SAMPLE_SOURCE.width} × ${SAMPLE_SOURCE.height} · ${formatVariantSize(SAMPLE_SOURCE.bytes)}`,
@@ -67,7 +82,13 @@ async function onCopy() {
 				t('Sample: {0}', [SAMPLE_SOURCE.filename])
 			}}</span>
 		</div>
-		<ImageCompare v-model="position" :name="values.name" :comparable="estimate !== null" />
+		<ImageCompare
+			v-model="position"
+			:name="values.name"
+			:comparable="estimate !== null"
+			:original-src="original"
+			:variant-src="variant?.url ?? null"
+		/>
 		<div class="grid grid-cols-2 gap-3">
 			<div class="flex flex-col">
 				<span class="font-medium text-highlighted">{{ t('Original') }}</span>
diff --git a/web/src/composables/useVariantRender.ts b/web/src/composables/useVariantRender.ts
new file mode 100644
index 0000000..1064a8a
--- /dev/null
+++ b/web/src/composables/useVariantRender.ts
@@ -0,0 +1,98 @@
+import { onScopeDispose, ref, watch, type Ref } from 'vue'
+import sampleUrl from '@/assets/variant-sample.jpg'
+import { SAMPLE_SOURCE, type EstimateRequest } from '@/lib/variants/variantEstimate'
+import { outputSize } from '@/lib/variants/variantOutput'
+import { canRender, drawPlan, flattensTransparency, mimeFor } from '@/lib/variants/variantRender'
+
+const RENDER_DELAY_MS = 150
+const PERCENT = 100
+
+export interface RenderedVariant {
+	url: string
+	bytes: number
+}
+
+let sampleImage: Promise<HTMLImageElement | null> | null = null
+
+function loadSample() {
+	sampleImage ??= new Promise((resolve) => {
+		const image = new Image()
+		image.onload = () => resolve(image)
+		image.onerror = () => resolve(null)
+		image.src = sampleUrl
+	})
+	return sampleImage
+}
+
+function encode(canvas: HTMLCanvasElement, request: EstimateRequest) {
+	const mime = mimeFor(request.format)
+	const quality = mime === 'image/png' ? undefined : request.quality / PERCENT
+	return new Promise<Blob | null>((resolve) => canvas.toBlob(resolve, mime, quality))
+}
+
+async function renderVariant(image: HTMLImageElement, request: EstimateRequest) {
+	const size = outputSize(SAMPLE_SOURCE, request)
+	if (size === null || !canRender(size)) {
+		return null
+	}
+	const canvas = document.createElement('canvas')
+	canvas.width = size.width
+	canvas.height = size.height
+	const context = canvas.getContext('2d')
+	if (context === null) {
+		return null
+	}
+	if (flattensTransparency(request.format)) {
+		context.fillStyle = 'white'
+		context.fillRect(0, 0, size.width, size.height)
+	}
+	const plan = drawPlan(SAMPLE_SOURCE, size, request.fit)
+	context.drawImage(image, plan.sx, plan.sy, plan.sw, plan.sh, plan.dx, plan.dy, plan.dw, plan.dh)
+	return encode(canvas, request)
+}
+
+export function useVariantRender(request: Ref<EstimateRequest | null>) {
+	const original = ref<string | null>(null)
+	const variant = ref<RenderedVariant | null>(null)
+	let timer: ReturnType<typeof setTimeout> | undefined
+	let token = 0
+
+	function release() {
+		if (variant.value) {
+			URL.revokeObjectURL(variant.value.url)
+			variant.value = null
+		}
+	}
+
+	async function run(current: EstimateRequest | null, ticket: number) {
+		const image = await loadSample()
+		if (image === null || ticket !== token) {
+			return
+		}
+		original.value = sampleUrl
+		const blob = current === null ? null : await renderVariant(image, current)
+		if (ticket !== token) {
+			return
+		}
+		release()
+		if (blob !== null) {
+			variant.value = { url: URL.createObjectURL(blob), bytes: blob.size }
+		}
+	}
+
+	function schedule(current: EstimateRequest | null) {
+		clearTimeout(timer)
+		const ticket = ++token
+		timer = setTimeout(() => void run(current, ticket), RENDER_DELAY_MS)
+	}
+
+	watch(request, schedule, { immediate: true, deep: true })
+
+	onScopeDispose(() => {
+		clearTimeout(timer)
+		token++
+		release()
+	})
+
+	return { original, variant }
+}
diff --git a/web/src/lib/variants/__tests__/variantEstimate.spec.ts b/web/src/lib/variants/__tests__/variantEstimate.spec.ts
index 3f0f1ca..f17acf3 100644
--- a/web/src/lib/variants/__tests__/variantEstimate.spec.ts
+++ b/web/src/lib/variants/__tests__/variantEstimate.spec.ts
@@ -8,7 +8,7 @@ import {
 } from '../variantEstimate'
 import { outputSize } from '../variantOutput'
 
-const SAMPLE = { width: 4032, height: 3024 }
+const SAMPLE = { width: 1500, height: 1000 }
 
 function request(overrides: Partial<EstimateRequest>): EstimateRequest {
 	return {
@@ -28,11 +28,11 @@ function labelOf(overrides: Partial<EstimateRequest>) {
 describe('Output size, never enlarged', () => {
 	it.each([
 		[1200, 630, 'cover', '1200×630'],
-		[800, null, 'cover', '800×600'],
-		[null, 2400, 'inside', '3200×2400'],
+		[800, null, 'cover', '800×533'],
+		[null, 2400, 'inside', '1500×1000'],
 		[600, 600, 'contain', '600×600'],
-		[1000, 1000, 'inside', '1000×750'],
-		[8000, 8000, 'cover', '4032×3024'],
+		[1000, 1000, 'inside', '1000×667'],
+		[8000, 8000, 'cover', '1500×1000'],
 	] as const)('width %s height %s fit %s gives %s', (width, height, fit, expected) => {
 		const size = outputSize(SAMPLE, { width, height, fit })
 
@@ -61,10 +61,10 @@ describe('Output size, never enlarged', () => {
 
 describe('Size estimate', () => {
 	it.each([
-		['hero', 1200, 630, 'cover', 'webp', 82, '1200×630 · about 96 KB'],
-		['hero', 1200, 630, 'cover', 'webp', 70, '1200×630 · about 71 KB'],
-		['hero', 1200, 630, 'cover', 'png', 82, '1200×630 · about 310 KB'],
-		['thumb', 400, 300, 'cover', 'webp', 80, '400×300 · about 18 KB'],
+		['hero', 1200, 630, 'cover', 'webp', 82, '1200×630 · about 30 KB'],
+		['hero', 1200, 630, 'cover', 'webp', 70, '1200×630 · about 22 KB'],
+		['hero', 1200, 630, 'cover', 'png', 82, '1200×630 · about 84 KB'],
+		['thumb', 400, 300, 'cover', 'webp', 80, '400×300 · about 6 KB'],
 	] as const)(
 		'%s %s×%s %s %s q%s reads %s',
 		(_name, width, height, fit, format, quality, expected) => {
diff --git a/web/src/lib/variants/__tests__/variantRender.spec.ts b/web/src/lib/variants/__tests__/variantRender.spec.ts
new file mode 100644
index 0000000..cb0838e
--- /dev/null
+++ b/web/src/lib/variants/__tests__/variantRender.spec.ts
@@ -0,0 +1,72 @@
+import { describe, expect, it } from 'vitest'
+import { canRender, drawPlan, flattensTransparency, mimeFor } from '../variantRender'
+
+const SOURCE = { width: 1500, height: 1000 }
+
+describe('Variant render', () => {
+	it.each([
+		['png', 'image/png'],
+		['webp', 'image/webp'],
+		['jpeg', 'image/jpeg'],
+		['original', 'image/jpeg'],
+	] as const)('encodes %s as %s', (format, mime) => {
+		expect(mimeFor(format)).toBe(mime)
+	})
+
+	it('crops the centre of the source to fill a wider box', () => {
+		const plan = drawPlan(SOURCE, { width: 1200, height: 400 }, 'cover')
+
+		expect(plan).toEqual({
+			sx: 0,
+			sy: 250,
+			sw: 1500,
+			sh: 500,
+			dx: 0,
+			dy: 0,
+			dw: 1200,
+			dh: 400,
+		})
+	})
+
+	it('keeps the whole source when the ratio already matches', () => {
+		const plan = drawPlan(SOURCE, { width: 750, height: 500 }, 'inside')
+
+		expect([plan.sx, plan.sy, plan.sw, plan.sh]).toEqual([0, 0, 1500, 1000])
+		expect([plan.dx, plan.dy, plan.dw, plan.dh]).toEqual([0, 0, 750, 500])
+	})
+
+	it('letterboxes the whole source inside a contain box', () => {
+		const plan = drawPlan(SOURCE, { width: 600, height: 600 }, 'contain')
+
+		expect(plan).toEqual({
+			sx: 0,
+			sy: 0,
+			sw: 1500,
+			sh: 1000,
+			dx: 0,
+			dy: 100,
+			dw: 600,
+			dh: 400,
+		})
+	})
+
+	it('never enlarges the source inside a contain box', () => {
+		const plan = drawPlan(SOURCE, { width: 3000, height: 3000 }, 'contain')
+
+		expect([plan.dx, plan.dy, plan.dw, plan.dh]).toEqual([750, 1000, 1500, 1000])
+	})
+
+	it.each([
+		['jpeg', true],
+		['original', true],
+		['png', false],
+		['webp', false],
+	] as const)('flattens transparency for %s: %s', (format, flattens) => {
+		expect(flattensTransparency(format)).toBe(flattens)
+	})
+
+	it('refuses to render huge outputs', () => {
+		expect(canRender({ width: 4000, height: 4000 })).toBe(true)
+		expect(canRender({ width: 8000, height: 8000 })).toBe(false)
+	})
+})
diff --git a/web/src/lib/variants/variantEstimate.ts b/web/src/lib/variants/variantEstimate.ts
index 4ac5d99..c4d91e6 100644
--- a/web/src/lib/variants/variantEstimate.ts
+++ b/web/src/lib/variants/variantEstimate.ts
@@ -5,23 +5,23 @@ import { outputSize, type OutputRequest, type PixelSize } from '@/lib/variants/v
 
 export type { PixelSize } from '@/lib/variants/variantOutput'
 
-const BYTES_PER_MB = 1024 * 1024
 const FULL_QUALITY = 100
+const REFERENCE_QUALITY = 82
 const PNG_BYTES_PER_PIXEL = 0.42
 const SIZE_EXPONENT = 0.885
 const QUALITY_EXPONENT = 1.9
 
-const LOSSY_BASE: Record<Exclude<VariantFormat, 'png'>, number> = {
-	webp: 0.9,
-	jpeg: 1.4,
-	original: 1.4,
+const FORMAT_WEIGHT: Record<Exclude<VariantFormat, 'png'>, number> = {
+	webp: 0.65,
+	jpeg: 1,
+	original: 1,
 }
 
 export const SAMPLE_SOURCE = {
-	filename: 'north-wall.jpg',
-	width: 4032,
-	height: 3024,
-	bytes: Math.round(3.9 * BYTES_PER_MB),
+	filename: 'variant-sample.jpg',
+	width: 1500,
+	height: 1000,
+	bytes: 86_382,
 }
 
 export interface EstimateRequest extends OutputRequest {
@@ -38,10 +38,12 @@ function rawBytes(pixels: number, request: EstimateRequest) {
 	if (request.format === 'png') {
 		return pixels * PNG_BYTES_PER_PIXEL
 	}
-	const quality = Math.min(FULL_QUALITY, Math.max(1, request.quality)) / FULL_QUALITY
+	const quality = Math.min(FULL_QUALITY, Math.max(1, request.quality)) / REFERENCE_QUALITY
+	const area = pixels / (SAMPLE_SOURCE.width * SAMPLE_SOURCE.height)
 	return (
-		LOSSY_BASE[request.format] *
-		Math.pow(pixels, SIZE_EXPONENT) *
+		SAMPLE_SOURCE.bytes *
+		FORMAT_WEIGHT[request.format] *
+		Math.pow(area, SIZE_EXPONENT) *
 		Math.pow(quality, QUALITY_EXPONENT)
 	)
 }
diff --git a/web/src/lib/variants/variantRender.ts b/web/src/lib/variants/variantRender.ts
new file mode 100644
index 0000000..a8c4d53
--- /dev/null
+++ b/web/src/lib/variants/variantRender.ts
@@ -0,0 +1,66 @@
+import type { VariantFit, VariantFormat } from '@/composables/useImageVariants'
+import type { PixelSize } from '@/lib/variants/variantOutput'
+
+export const MAX_RENDER_PIXELS = 16_000_000
+
+export interface DrawPlan {
+	sx: number
+	sy: number
+	sw: number
+	sh: number
+	dx: number
+	dy: number
+	dw: number
+	dh: number
+}
+
+export function mimeFor(format: VariantFormat) {
+	if (format === 'png') {
+		return 'image/png'
+	}
+	if (format === 'webp') {
+		return 'image/webp'
+	}
+	return 'image/jpeg'
+}
+
+export function flattensTransparency(format: VariantFormat) {
+	return mimeFor(format) === 'image/jpeg'
+}
+
+export function canRender(size: PixelSize) {
+	return size.width * size.height <= MAX_RENDER_PIXELS
+}
+
+export function drawPlan(source: PixelSize, target: PixelSize, fit: VariantFit): DrawPlan {
+	const widthScale = target.width / source.width
+	const heightScale = target.height / source.height
+	if (fit === 'contain') {
+		const scale = Math.min(1, widthScale, heightScale)
+		const dw = source.width * scale
+		const dh = source.height * scale
+		return {
+			sx: 0,
+			sy: 0,
+			sw: source.width,
+			sh: source.height,
+			dx: (target.width - dw) / 2,
+			dy: (target.height - dh) / 2,
+			dw,
+			dh,
+		}
+	}
+	const scale = Math.max(widthScale, heightScale)
+	const sw = target.width / scale
+	const sh = target.height / scale
+	return {
+		sx: (source.width - sw) / 2,
+		sy: (source.height - sh) / 2,
+		sw,
+		sh,
+		dx: 0,
+		dy: 0,
+		dw: target.width,
+		dh: target.height,
+	}
+}
diff --git a/web/src/views/__tests__/VariantsForm.spec.ts b/web/src/views/__tests__/VariantsForm.spec.ts
index 011215d..1ad0a2f 100644
--- a/web/src/views/__tests__/VariantsForm.spec.ts
+++ b/web/src/views/__tests__/VariantsForm.spec.ts
@@ -59,7 +59,7 @@ const QUALITY_ERROR = 'Enter a value from 1 to 100.'
 const DIMENSION_ERROR = 'Set a width, a height, or both.'
 const DIMENSION_HELP = 'Set a width, a height, or both. 1–8000 px. Images are never enlarged.'
 const SAVE_ERROR_TITLE = "Couldn't save the variant"
-const URL_PREFIX = '/static/uploadedImages/north-wall.jpg?v='
+const URL_PREFIX = '/static/uploadedImages/variant-sample.jpg?v='
 const WARNING_ALERT =
 	'Changing dimensions, fit, format or quality purges 244 generated files; they will be regenerated on next request.'
 
@@ -438,7 +438,7 @@ describe('Rule: Form states', () => {
 			'i-lucide-circle-dot',
 		)
 		expect(previewText()).toContain('hero')
-		expect(previewText()).toContain('1200×630 · about 96 KB')
+		expect(previewText()).toContain('1200×630 · about 30 KB')
 	})
 
 	it.each([
@@ -853,14 +853,14 @@ describe('Rule: Saving an edit', () => {
 	it('Edit: dirty', async () => {
 		await mountPage()
 		await openEditForm('hero')
-		expect(previewText()).toContain('1200×630 · about 96 KB')
+		expect(previewText()).toContain('1200×630 · about 30 KB')
 
 		await typeInto(qualityInput(), '70')
 
 		expect(saveButton('Save changes').disabled).toBe(false)
 		expect(footerStatus()).toBe('Unsaved changes')
 		expect(qualityInput()!.value).toBe('70')
-		expect(previewText()).toContain('1200×630 · about 71 KB')
+		expect(previewText()).toContain('1200×630 · about 22 KB')
 		expect(form().querySelector('[data-testid="edit-warning"]')).not.toBeNull()
 	})
 
@@ -954,13 +954,13 @@ describe('Rule: Preview and ImageCompare', () => {
 
 		const panel = form()
 		expect(panel.querySelector('h3')?.textContent).toBe('Preview')
-		expect(panel.textContent).toContain('Sample: north-wall.jpg')
+		expect(panel.textContent).toContain('Sample: variant-sample.jpg')
 		const box = panel.querySelector('[data-testid="image-compare"]')!
 		expect(box.className).toContain('aspect-3/2')
 		expect(box.className).toContain('border')
 		expect(box.className).toContain('overflow-hidden')
 		const labels = box.nextElementSibling!
-		expect(textOf(labels)).toBe('Original 4032 × 3024 · 3.9 MB hero 1200×630 · about 96 KB')
+		expect(textOf(labels)).toBe('Original 1500 × 1000 · 84 KB hero 1200×630 · about 30 KB')
 		expect(panel.textContent).toContain(
 			'Drag the divider, or focus it and use the arrow keys. The estimate comes from a sample; output is never larger than the source.',
 		)
@@ -1078,7 +1078,7 @@ describe('Rule: Preview and ImageCompare', () => {
 		expect(overlay.className).toContain('bottom-3')
 		expect(overlay.className).toContain('left-3')
 		expect(textOf(box.nextElementSibling!)).toBe(
-			'Original 4032 × 3024 · 3.9 MB name No output yet',
+			'Original 1500 × 1000 · 84 KB name No output yet',
 		)
 	})
 
@@ -1089,7 +1089,7 @@ describe('Rule: Preview and ImageCompare', () => {
 		await typeInto(field('width'), '800')
 
 		expect(compareInput().getAttribute('aria-label')).toBe('Compare original and name')
-		expect(previewText()).toContain('name 800×600 · about')
+		expect(previewText()).toContain('name 800×533 · about')
 		expect(
 			Array.from(form().querySelectorAll('span.absolute.top-2')).map(
 				(chip) => chip.textContent,
@@ -1107,7 +1107,7 @@ describe('Rule: Preview and ImageCompare', () => {
 
 		expect(labels.map(Number)).toEqual([...labels.map(Number)].sort((a, b) => a - b))
 		await setFormat('png')
-		expect(previewText()).toContain('1200×630 · about 310 KB')
+		expect(previewText()).toContain('1200×630 · about 84 KB')
 	})
 })