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PhantomJS screenshots change size because two different settings control two different rectangles. page.viewportSize sets the headless browser’s viewport and therefore the responsive layout. page.clipRect sets the rectangle that page.render() rasterizes. If you omit clipRect, PhantomJS renders the entire page, which can be much taller than the viewport. Set both values explicitly for a predictable fixed-size image; leave clipRect unset only when a full-page capture is intentional.
The two dimensions PhantomJS keeps separate
A browser viewport is the coordinate space in which a page lays itself out. The screenshot bounds are the region copied into the output file. PhantomJS exposes those concepts as separate properties:
| Setting | What it controls | Typical symptom when it is wrong |
|---|---|---|
page.viewportSize |
The headless browser viewport, in CSS pixels; it affects responsive breakpoints, line wrapping and element positions. | The page switches to a mobile or tablet layout, or text wraps differently between runs. |
page.clipRect |
The rasterized rectangle, defined by top, left, width and height. |
The output is unexpectedly tall, cropped, offset or a different pixel size. |
No clipRect |
page.render() processes the entire webpage. |
A short viewport produces a very tall image because all page content is included. |
paperSize |
PDF page dimensions and margins. | A PDF has unexpected paper dimensions even though raster settings are correct. |
The names are not interchangeable. A 1024-pixel-wide viewport can produce a 1024-pixel-wide image with a 3,000-pixel height when the page is long and no clipping rectangle is supplied. Conversely, a large viewport can be cropped to a small output rectangle. Decide first whether you need a viewport-sized image, a selected region, or the complete page.
Make a fixed-size 1024 × 768 screenshot
Set the viewport and clipping rectangle before opening the URL. The following is the complete PhantomJS pattern for a 1024 × 768 raster capture:
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var page = require('webpage').create();
page.viewportSize = { width: 1024, height: 768 };
page.clipRect = { top: 0, left: 0, width: 1024, height: 768 };
page.open('http://example.com/', function (status) {
if (status === 'success') {
page.render('capture.png');
} else {
console.error('Page open failed: ' + status);
}
phantom.exit();
});The numbers are an example, not a universal standard. Choose a viewport width that triggers the layout you want, then choose a clip rectangle that covers the exact output region. Keeping the two rectangles the same size gives a viewport-sized image, but they do not have to match. For example, a 1440 × 900 viewport can be clipped to a 1200 × 700 panel if you deliberately want only that area.
Set values before navigation
- Create the page with
require('webpage').create(). - Assign
page.viewportSize. - Assign all four
page.clipRectfields. - Call
page.open()and render only after a successful status. - Exit PhantomJS after rendering so the process does not remain alive.
Setting the viewport after the page has loaded can leave you diagnosing a layout produced at an earlier width. Establish it before page.open() whenever possible.
Choose between a viewport shot, a selected crop and a full page
Viewport-sized output
Use matching viewport and clip dimensions when you need a stable “what is visible in the browser window” image. This is appropriate for visual regression baselines, documentation thumbnails and fixed-size test fixtures. A change in viewport width can still alter the page’s responsive layout even when the output dimensions remain constant.
A specific region
Use clipRect to select a bounded region. left and top move the rectangle; width and height determine its size. An incorrect offset can look like a scaling problem when it is actually a crop that starts too far left, right, high or low. Keep the rectangle inside the page area you intend to capture and verify the result after layout changes.
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Full-page output
Leave clipRect unset when the requirement is to include the complete webpage. The resulting height follows the rendered document, not the viewport height. This is intentional for long-form archives or pages where every section must be preserved, but it is unsuitable for a fixed-pixel contract.
Diagnose which dimension is changing
| What you observe | Likely cause | Check or fix |
|---|---|---|
| Different line wrapping, breakpoints or navigation | The viewport width or height differs between runs. | Log the values and assign page.viewportSize before page.open(). |
| Image height follows the amount of page content | clipRect is unset, so the whole webpage is rendered. |
Set an explicit rectangle for bounded output, or keep it unset deliberately for full-page capture. |
| Correct dimensions but wrong content location | top or left is offset. |
Inspect all four clip fields; do not change viewport dimensions to compensate for an offset. |
| Output dimensions seem correct but the PDF page is wrong | PDF sizing is controlled separately. | Configure paperSize for the PDF’s format, dimensions, units, margins or orientation. |
| Changing PNG to JPEG appears to “fix” size | The filename extension changes encoding, not the layout rectangle. | Investigate viewport and clipping first; page.render() infers format from the extension. |
When comparing captures, record the URL, viewport values, clip values, output extension and whether the run was intended to be full-page. This prevents a full-page image from being mistaken for a viewport regression.
Raster screenshots and PDFs use different controls
For PNG, JPEG, BMP, PPM and other raster output supported by your PhantomJS build, the relevant controls are the viewport and render clipping. PhantomJS infers the output format from the filename extension; GIF support depends on the Qt build. The extension does not define the pixel rectangle.
PDF output follows a different model. Set page.paperSize to control named paper formats or explicit dimensions, units, margins and landscape orientation. Do not expect a raster clipRect to define PDF page size, or a PDF paper setting to change the browser’s responsive viewport.
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Reliable capture procedure for existing scripts
- Define the contract. Write down whether the result is a fixed viewport, a selected region or a full page, and the required pixel dimensions or PDF paper size.
- Choose the layout viewport. Select a width and height that produce the intended responsive design. The width is usually the critical breakpoint value.
- Set the crop deliberately. For fixed output, assign all of
top,left,widthandheight. For full page, omitclipRectintentionally and document that choice. - Navigate after configuration. Set the viewport before
page.open(), then render only when the callback reportssuccess. - Verify the artifact. Inspect both pixel dimensions and content boundaries. A file can have the expected size while clipping a header or starting below the intended origin.
- Keep the settings stable. Store capture dimensions in one configuration object rather than scattering literals through multiple scripts.
PhantomJS uses QtWebKit and its project homepage says, “Important: PhantomJS development is suspended until further notice.” Treat these instructions as maintenance guidance for existing automation, not as advice to build a new production browser stack around an actively maintained engine. The available documentation does not establish a particular release-specific sizing defect or a currently supported release.
Common failure modes and fixes
The screenshot is much taller than expected
Confirm whether page.clipRect was ever assigned. An unset rectangle invokes full-webpage rendering. If you need 1024 × 768, assign that exact rectangle before navigation and render.
The page looks mobile even though the image is wide
The output file’s width does not prove that the page laid out at that width. Check page.viewportSize.width, set it before opening the page, and look for code that changes it later. Also ensure that separate workers or scripts are not using different configuration values.
The right size is produced, but content is cut off
Inspect clipRect.top and left first, then confirm that the rectangle’s width and height cover the intended region. A fixed crop cannot include content outside its bounds; use full-page mode when the requirement is to preserve the entire document.
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Rendering never creates an image
Check the status argument in the page.open() callback and render only on success. Log failures, verify the URL is reachable from the capture environment, and ensure that phantom.exit() is called after the callback rather than before page.render().
The PDF has the wrong paper shape
Configure paperSize; changing clipRect addresses raster cropping, not PDF page dimensions. For a PDF, specify the required format, units, margins and orientation in the paper-size object.
Different machines produce different layouts
First normalize the viewport and clip settings. If those match, compare the PhantomJS/QtWebKit environment and page response, because the documented sizing controls cannot make differing page content or engine behavior identical.
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A full-page render can create a substantially taller bitmap than a viewport capture, increasing memory use and file size. Use a bounded clip when downstream systems expect a fixed artifact, and reserve full-page mode for workflows that need every section. Fixed dimensions also simplify cache keys, visual-diff comparisons and storage quotas.
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For repeatable results, keep the URL, viewport, clip rectangle, output format and PDF settings together in version-controlled configuration. When a page redesign changes breakpoints or content height, decide whether that is an intentional baseline update or a capture regression before replacing reference images.
Or skip the browser setup
If you only need a dependable website image rather than a legacy PhantomJS runtime, ScreenshotNeo provides a single HTTP request for PNG, JPEG, WebP or PDF output. Its API can set a viewport or device preset, capture full pages with lazy images loaded, select an element by CSS selector, use retina scale, resize images, apply custom CSS or JavaScript, wait for a selector, delay or network idle, and set PDF paper size, margins, orientation and page ranges. It also supports custom headers, cookies, user agents, timezone and geolocation when a page requires them.
Example cURL request (the ScreenshotNeo documentation lists the available parameters):
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get(
"https://api.screenshotneo.com/v1/shot",
params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"},
timeout=90,
)
r.raise_for_status()
open("shot.webp", "wb").write(r.content)Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`Screenshot failed: ${res.status}`);ScreenshotNeo accepts cookie and consent banners before capture and removes more than 60 known consent platforms, newsletter popups and chat widgets; each cleanup step can be turned off. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads and cache hits cost nothing, and the response identifies the page verdict and billing status in X-Page-Verdict and X-Billed headers. An MCP server exposes take_screenshot, get_page_info and capture_pdf to Claude, Cursor and other MCP clients. The Free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000 screenshots. Sign up for the free ScreenshotNeo plan.
A practical decision checklist
- Need the browser-window view at an exact size? Set matching
viewportSizeandclipRect. - Need a component or panel only? Keep the layout viewport appropriate to the page and use
clipRectfor the region. - Need every page section? Omit
clipRectintentionally and expect a content-dependent height. - Need PDF paper dimensions? Configure
paperSize, not raster clipping. - Maintaining an old PhantomJS job? Pin and document the environment because PhantomJS development is suspended.
- Need automated cleanup, billing visibility or AI-agent access? Use the ScreenshotNeo request instead of maintaining browser setup.
Frequently Asked Questions
Can I set a clip rectangle larger than the viewport?
You can request a rectangle that extends beyond the visible viewport, but verify the resulting capture in your PhantomJS build. If the goal is simply to include all page content, full-page rendering without a clip rectangle is the clearer choice.
Does changing the output filename change screenshot dimensions?
No. The filename extension selects the rendered format; dimensions come from the viewport, clip rectangle or, for PDFs, paper-size settings.
Why does a fixed-width screenshot still change from one run to another?
A fixed pixel size does not freeze page content. Responsive breakpoints, page responses and the QtWebKit environment can still alter what is rendered inside the same rectangle.
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