Chrome extensions cannot pass a rectangle directly to chrome.tabs.captureVisibleTab(). The reliable approach is to capture the visible tab once, show that image in an extension page or offscreen document, let the user drag a selection rectangle, convert the displayed coordinates to bitmap pixels, and crop the image with a canvas. This avoids repeated captures, respects Chrome’s two-calls-per-second limit, and works with Manifest V3’s service-worker restrictions.
What the API captures—and what it does not
chrome.tabs.captureVisibleTab() returns an image data URL for the visible viewport of a tab. It does not accept x, y, width, or height parameters for an arbitrary page region. Therefore, a selected-area workflow has two phases:
- Capture the viewport once.
- Crop the user’s rectangle locally from the returned bitmap.
The screenshot’s pixel dimensions may differ from the displayed image’s CSS dimensions because of device scale and browser rendering. Always calculate scale from the bitmap’s actual width and height divided by the displayed image’s width and height. Do not assume that devicePixelRatio is the correct value.
See Chrome’s tabs API reference for capture behavior, permissions, and limits.
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Permissions and Manifest V3 setup
Use activeTab for a user-invoked tool
For an extension activated by a button, context menu, or keyboard command, activeTab grants temporary access to the current tab after the user invokes the extension. It generally avoids an install-time host-permission warning. You can instead request <all_urls> when your product genuinely needs persistent access to many sites.
captureVisibleTab() accepts either activeTab or <all_urls>. Chrome pages and other sensitive pages can be captured with activeTab; file URLs require the user to enable file access for the extension.
Minimal manifest
{
"manifest_version": 3,
"name": "Area Capture",
"version": "1.0.0",
"permissions": ["activeTab", "scripting"],
"action": { "default_title": "Capture area" },
"background": { "service_worker": "service-worker.js" },
"web_accessible_resources": [{
"resources": ["selector.html", "selector.js"],
"matches": ["<all_urls>"]
}]
}The tabs permission is not required merely to call the chrome.tabs namespace. It exposes sensitive tab properties such as URL and title; capture access is controlled separately by activeTab or host permissions.
Capture once, then open a selector
The service worker should perform the capture immediately after the user action. It has no DOM, so it should not attempt to create an image element or canvas. Send the data URL to an extension page, popup, or offscreen document for display and cropping.
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Service worker
chrome.action.onClicked.addListener(async (tab) => {
if (!tab.id) return;
try {
const dataUrl = await chrome.tabs.captureVisibleTab(tab.windowId, {
format: "png"
});
await chrome.storage.session.set({
areaCapture: { tabId: tab.id, dataUrl }
});
await chrome.tabs.create({
url: chrome.runtime.getURL("selector.html")
});
} catch (error) {
console.error("Visible-tab capture failed", error);
}
});If you do not want to open a tab, use an extension popup or an offscreen document. Chrome’s offscreen API provides a hidden extension document with DOM access and is available for MV3 in Chrome 109 and later.
Build the selection UI
The selector displays the captured bitmap at a constrained size, tracks pointer movement, and draws a rectangle over it. Capture is not repeated while the pointer moves; only the local overlay changes.
selector.html
<!doctype html>
<meta charset="utf-8">
<title>Select an area</title>
<style>
body { margin: 0; font: 14px system-ui; background: #222; color: #fff; }
#stage { position: relative; display: inline-block; line-height: 0; }
#shot { max-width: 100vw; max-height: 90vh; user-select: none; }
#selection { position: absolute; border: 2px solid #28a9ff; background: rgb(40 169 255 / 20%); pointer-events: none; }
#save { margin: 12px; padding: 8px 14px; }
</style>
<div id="stage"><img id="shot" alt="Captured tab"><div id="selection" hidden></div></div>
<button id="save" disabled>Save selection</button>
<script src="selector.js"></script>selector.js
const image = document.querySelector("#shot");
const box = document.querySelector("#selection");
const save = document.querySelector("#save");
let start = null;
let rect = null;
const stored = await chrome.storage.session.get("areaCapture");
if (!stored.areaCapture?.dataUrl) throw new Error("No captured image");
image.src = stored.areaCapture.dataUrl;
await image.decode();
stage.addEventListener("pointerdown", (event) => {
const r = image.getBoundingClientRect();
start = {
x: Math.max(0, Math.min(event.clientX - r.left, r.width)),
y: Math.max(0, Math.min(event.clientY - r.top, r.height))
};
rect = { x: start.x, y: start.y, w: 0, h: 0 };
box.hidden = false;
stage.setPointerCapture(event.pointerId);
});
stage.addEventListener("pointermove", (event) => {
if (!start) return;
const r = image.getBoundingClientRect();
const x = Math.max(0, Math.min(event.clientX - r.left, r.width));
const y = Math.max(0, Math.min(event.clientY - r.top, r.height));
rect = {
x: Math.min(start.x, x), y: Math.min(start.y, y),
w: Math.abs(x - start.x), h: Math.abs(y - start.y)
};
Object.assign(box.style, {
left: `${rect.x}px`, top: `${rect.y}px`,
width: `${rect.w}px`, height: `${rect.h}px`
});
save.disabled = rect.w < 1 || rect.h < 1;
});
stage.addEventListener("pointerup", () => { start = null; });
save.addEventListener("click", () => {
const scaleX = image.naturalWidth / image.getBoundingClientRect().width;
const scaleY = image.naturalHeight / image.getBoundingClientRect().height;
const sx = Math.round(rect.x * scaleX);
const sy = Math.round(rect.y * scaleY);
const sw = Math.max(1, Math.round(rect.w * scaleX));
const sh = Math.max(1, Math.round(rect.h * scaleY));
const canvas = document.createElement("canvas");
canvas.width = Math.min(sw, image.naturalWidth - sx);
canvas.height = Math.min(sh, image.naturalHeight - sy);
canvas.getContext("2d").drawImage(
image, sx, sy, canvas.width, canvas.height,
0, 0, canvas.width, canvas.height
);
const link = document.createElement("a");
link.download = "selected-area.png";
link.href = canvas.toDataURL("image/png");
link.click();
});This code uses separate horizontal and vertical scale factors. That is safer than one assumed ratio when CSS sizing, borders, or layout constraints affect the displayed image.
MV3 architecture: extension page or offscreen document
Service workers cannot use DOM APIs such as document.createElement("canvas"). Keep image decoding, pointer handling, and canvas cropping in an extension page, popup, or offscreen document. The offscreen document is useful when you need hidden processing or want the worker to remain the coordinator. Chrome’s screen-capture guide explains the offscreen pattern for media capture and processing: screen capture guidance.
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When another capture API is the right choice
| Requirement | API | Why |
|---|---|---|
| One still image of the visible viewport to crop | chrome.tabs.captureVisibleTab() |
Returns an image data URL; crop it locally. |
| Ongoing tab video or audio | chrome.tabCapture |
Provides a media stream for recording or processing. |
| DOM APIs from MV3 background logic | chrome.offscreen |
Provides a hidden extension document with DOM access. |
getDisplayMedia() presents a browser chooser for a tab, window, or screen. For a known active tab and a user-initiated stream, Chrome documents tabCapture; from Chrome 116, a service worker can obtain a stream ID after invocation and pass it to an offscreen document. Those APIs solve streaming and recording, not a simple cropped still.
Performance, limits, and image quality
- Chrome documents a maximum of two
captureVisibleTab()calls per second (Chrome 92+), and describes capture as expensive. Capture once per selection operation. - PNG preserves sharp text but can produce large data URLs. JPEG is smaller for photographic pages and supports a quality option where applicable; WebP is another compact choice.
- Large full-viewport bitmaps consume memory in the image element and canvas. Release references after saving and avoid keeping multiple data URLs in storage.
- The method captures only what is visible in the viewport. It does not automatically produce a full-page screenshot or include content below the fold.
- Cross-origin content shown in the captured bitmap is already rasterized by Chrome; the crop operation does not read the page’s DOM or pixels directly.
Troubleshooting
“Cannot access contents of the page”
Confirm that the action was user initiated and that activeTab is present. For file URLs, enable “Allow access to file URLs” on the extension’s details page. Do not assume tabs permission fixes capture access.
The image is blank or the wrong tab was captured
Pass the correct windowId from the clicked tab and capture immediately. Do not rely on a later active-tab query after opening another extension page. Restricted browser UI pages may still impose capture restrictions.
The crop is offset or the wrong size
Use image.naturalWidth / image.getBoundingClientRect().width and the corresponding height ratio. Account for the image’s actual bounding rectangle, not the page viewport or assumed device-pixel ratio.
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Nothing happens in the service worker
Move all DOM and canvas code into an extension page or offscreen document. The worker can capture, store, and message; it cannot render the selection interface.
Captures fail during rapid testing
Throttle calls to two per second or less. Pointer movement should update only the overlay and crop locally; it should never trigger another capture.
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FAQ
Can I pass a CSS selector to captureVisibleTab()?
No. Capture the viewport, then crop pixels, or use script injection to measure an element before cropping.
Can I capture an element in an iframe?
You can use chrome.scripting.executeScript() to inspect a permitted frame and obtain coordinates, but the final visible-tab image still needs pixel-coordinate cropping. The scripting API requires the scripting permission plus matching host access or activeTab; see its reference.
Should I use tabCapture for a still image?
Use tabCapture when you need a stream or recording. For one cropped still, captureVisibleTab() followed by canvas cropping is simpler.
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