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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteSet the browser viewport, not the screenshot file, to control screen size. In PhantomJS, assign page.viewportSize = { width: W, height: H } before page.open(). In CasperJS, pass the same object as viewportSize when creating the Casper instance, or call casper.viewport(W, H) during a flow. Use clipRect only when you need to crop the captured pixels, and use paperSize for PDF page dimensions.
What “screen size” means in these tools
PhantomJS and CasperJS do not resize a physical monitor. They emulate a browser window with a viewport measured in CSS pixels. The viewport width and height determine which responsive breakpoints the page sees, how text and columns reflow, and what content is initially visible.
Three settings are easy to confuse:
- Viewport: the virtual browser area used for layout.
- Clip rectangle: the rectangle copied into an image during capture.
- Paper size: the page dimensions used when rendering a PDF.
Set the viewport explicitly whenever a test or screenshot must be reproducible. CasperJS leaves PhantomJS’s documented default viewport of 400 × 300 pixels unchanged unless you configure it.
Set the viewport in PhantomJS
Initial size before navigation
Assign an object with numeric width and height properties before opening the URL. Setting it before navigation ensures the first layout and any scripts that run during page load see the intended dimensions.
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var page = require('webpage').create();
page.viewportSize = {
width: 1024,
height: 768
};
page.open('https://example.com/', function (status) {
if (status !== 'success') {
console.log('Open failed: ' + status);
phantom.exit(1);
return;
}
page.render('example-1024x768.png');
phantom.exit();
});The filename extension selects the output format in PhantomJS. For example, a .png file produces PNG output; choose the extension that matches your pipeline.
Reusable dimensions
Keeping dimensions in variables makes it easy to run the same capture at several breakpoints:
var page = require('webpage').create();
var width = 1366;
var height = 768;
page.viewportSize = { width: width, height: height };
page.open('https://example.com/', function (status) {
if (status === 'success') {
page.render('desktop.png');
}
phantom.exit(status === 'success' ? 0 : 1);
});Use positive integer CSS-pixel values. The viewport is independent of device-pixel-ratio concepts; PhantomJS’s viewport setting alone does not turn a capture into a retina image.
Set the viewport in CasperJS
Configure it when creating Casper
Pass viewportSize to casper.create(). This is the most predictable option when every step in the run should start at the same size.
var casper = require('casper').create({
viewportSize: {
width: 1024,
height: 768
}
});
casper.start('https://example.com/');
casper.then(function () {
this.capture('page-1024x768.png');
});
casper.run(function () {
this.exit();
});Because CasperJS runs on PhantomJS, this option ultimately configures PhantomJS’s page viewport. Explicit values also avoid accidentally testing the 400 × 300 default.
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Change size during a run
Use casper.viewport(width, height) when one workflow must test multiple responsive states or when navigation has already begun.
var casper = require('casper').create();
casper.start('https://example.com/');
casper.viewport(1024, 768).then(function () {
this.capture('wide.png');
});
casper.viewport(375, 812).then(function () {
this.capture('phone.png');
});
casper.run(function () {
this.exit();
});The callback form is important when later actions depend on the new layout. CasperJS documents the asynchronous continuation so page reflow can finish before the next step:
casper.viewport(375, 812, function () {
this.echo('Viewport changed; continue after reflow.');
this.capture('mobile.png');
});Prefer the chained .then() form in a larger sequence because it keeps navigation, waits, clicks and captures in CasperJS’s step queue.
Viewport size versus screenshot crop
Changing viewportSize changes page layout; it does not automatically define the rectangle written to an image. PhantomJS’s clipRect selects a capture region separately.
var page = require('webpage').create();
page.viewportSize = { width: 1280, height: 900 };
page.clipRect = { top: 0, left: 0, width: 800, height: 600 };
page.open('https://example.com/', function (status) {
if (status === 'success') {
page.render('cropped.png');
}
phantom.exit();
});Here the page lays out at 1280 × 900, while the saved image is cropped to 800 × 600. If you want the entire viewport, make the clip rectangle match it, or omit clipRect and let the renderer use its normal capture area.
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PDF output uses paper settings
For PDF generation, set PhantomJS’s paperSize; it defines print-page dimensions rather than the browser viewport. A viewport can still affect responsive layout before printing, so configure both when you need a particular on-screen breakpoint and a particular paper format.
var page = require('webpage').create();
page.viewportSize = { width: 1024, height: 768 };
page.paperSize = {
format: 'A4',
orientation: 'portrait',
margin: '1cm'
};
page.open('https://example.com/', function (status) {
if (status === 'success') {
page.render('page.pdf');
}
phantom.exit(status === 'success' ? 0 : 1);
});See the PhantomJS paper-size API for the supported paper-size forms. Do not try to obtain an A4 PDF by changing only viewportSize.
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A practical matrix names each viewport and output so failures are attributable to a breakpoint, not an overwritten file:
| Purpose | Width × height (CSS px) | Typical check |
|---|---|---|
| Small phone | 375 × 812 | Navigation menu, wrapping, fixed controls |
| Tablet portrait | 768 × 1024 | Two-column transitions and touch targets |
| Laptop | 1366 × 768 | Primary desktop layout and above-the-fold content |
| Large desktop | 1920 × 1080 | Maximum-width containers and excess whitespace |
These are test choices, not PhantomJS requirements. Select dimensions that match the breakpoints your CSS and users actually need. Keep the height explicit when comparing screenshots, because a taller viewport can reveal lazy content or change sticky-element behavior.
Common problems and fixes
The screenshot is only 400 × 300
Cause: no viewport was configured, and CasperJS did not override PhantomJS’s default. Fix: set viewportSize in casper.create(), or assign page.viewportSize before page.open().
The page still looks like the old breakpoint
Cause: the viewport was changed after a layout-dependent action, or the next step ran before reflow completed. Fix: put casper.viewport() in the step flow and use its callback or a chained continuation before clicking, querying dimensions or capturing.
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The image dimensions do not match the viewport
Cause: clipRect is selecting a different output rectangle. Fix: inspect or remove page.clipRect; set its width and height to the desired crop independently of the viewport.
A full-page capture is cut off
Cause: the viewport controls the visible browser area, but it does not automatically stitch an arbitrarily tall page. Fix: measure the document, set an appropriate capture rectangle or capture in sections, and account for lazy-loaded content before rendering. Verify the result at the exact viewport used by your test.
Navigation fails or renders blank content
Check the callback status, network access, redirects, TLS compatibility and page scripts. Log status and exit with a nonzero code on failure, as in the examples. PhantomJS and CasperJS are legacy projects; their documentation may not reflect modern browser behavior, so verify that your site, JavaScript and runtime versions are compatible before adopting them for new automation.
Reliability and performance practices
- Set one explicit viewport at the start of every run; do not rely on process defaults.
- Use deterministic filenames containing the width and height when generating a matrix.
- Wait for the application state you need, not merely for a fixed delay. A selector-based condition is preferable when your CasperJS flow can express it.
- Keep viewport changes together with the capture or interaction that depends on them, and wait for reflow.
- Separate layout testing from crop testing so a failing image has one clear cause.
- Record the PhantomJS/CasperJS versions and the operating environment. Legacy rendering engines can differ from current Chrome, Firefox and Safari.
Or skip the browser setup
If you only need a dependable website image or PDF, ScreenshotNeo accepts the URL and capture settings through one API call. Its cleanup step accepts cookie-consent banners and removes more than 60 known consent platforms, newsletter popups and chat widgets before capture; each cleanup action can be disabled. Bot checks, CAPTCHAs, blank pages, timeouts, failed loads and cache hits are not billed, and the response identifies the result with X-Page-Verdict and X-Billed headers.
For a direct request, see the ScreenshotNeo API documentation:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpThe same call in Python:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)And Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
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FAQ
Can I use decimal viewport dimensions?
Use positive integer CSS-pixel dimensions. Integer values make breakpoints and screenshot comparisons predictable.
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Does viewport width change the PDF paper size?
No. Width and height control web layout; paperSize controls the PDF’s print page.
Should I resize before or after opening a URL in CasperJS?
Configure the initial size in casper.create() when possible. If resizing after navigation, wait on the asynchronous viewport step before layout-dependent work.
Frequently Asked Questions
Is PhantomJS’s 400 × 300 size a screen resolution?
No. It is the documented default browser viewport used when no explicit size is configured.
What controls the pixels saved from a PhantomJS page?
The viewport controls layout, while clipRect independently controls the captured rectangle.
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Are PhantomJS and CasperJS suitable for a new browser-compatibility suite?
They are legacy tools, so verify runtime and site compatibility; current-browser automation may be more appropriate for new projects.
The Bottom Line
Set page.viewportSize before PhantomJS navigation or configure CasperJS’s viewportSize; use an asynchronous casper.viewport() step for runtime changes. Keep viewport, crop and PDF paper settings separate so your captures and responsive tests match the dimensions you intended.
Quick Recap
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