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cURL cannot render HTML by itself. It sends an HTTP request to a rendering service, which runs a browser engine and returns JPEG output (usually as a URL). For HTML and CSS you control, post the markup to HCTI’s image endpoint with Basic authentication and format=jpeg. For an existing website, post its public URL and capture settings instead.
What the conversion actually involves
The workflow has two separate jobs:
- cURL is the HTTP client. The curl project describes it as “a command line tool for doing all sorts of URL manipulations and transfers”.
- A rendering service loads HTML, applies CSS, executes the browser layout process and encodes the resulting pixels as JPEG.
Therefore, a command such as curl file.html -o image.jpg cannot convert a document into a screenshot. You need a browser-based renderer, either a hosted API or software running on your own machine.
Prerequisites and safe setup
- cURL installed and available in your shell.
- An account and API ID/key for the rendering provider you choose.
- For URL capture, a fully qualified page URL that the provider can reach publicly.
- A destination directory with permission to write the response and downloaded image.
Keep credentials in environment variables or a secret manager, not in scripts committed to source control:
export HCTI_API_ID='your_api_id'
export HCTI_API_KEY='your_api_key'The examples use --data-urlencode so characters in HTML, CSS and URLs are encoded safely as form fields.
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Convert inline HTML and CSS to JPEG with HCTI
This request sends markup and styles directly to HCTI and asks for JPEG output:
curl --fail-with-body --request POST 'https://hcti.io/v1/image'
--user "$HCTI_API_ID:$HCTI_API_KEY"
--data-urlencode 'html=<div class="card"><h1>Hello, world!</h1></div>'
--data-urlencode 'css=.card { width: 480px; padding: 40px; background: #f0fdf4; }'
--data-urlencode 'format=jpeg'--user creates the documented Basic-authentication header. --fail-with-body makes HTTP errors visible while preserving the provider’s response body for diagnosis. The service responds with JSON containing a hosted image URL rather than writing JPEG bytes directly to your local file.
Use a local HTML file
Shell command substitution lets you submit a file while retaining URL encoding:
HTML=$(cat ./card.html)
CSS=$(cat ./card.css)
curl --fail-with-body --request POST 'https://hcti.io/v1/image'
--user "$HCTI_API_ID:$HCTI_API_KEY"
--data-urlencode "html=$HTML"
--data-urlencode "css=$CSS"
--data-urlencode 'format=jpeg'
-o response.jsonInspect response.json and copy its url value. Then download the actual JPEG in a second request:
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IMAGE_URL=$(python3 -c 'import json,sys; print(json.load(open("response.json"))["url"])')
curl --fail-with-body --location "$IMAGE_URL" -o output.jpgDo not assume that adding -o output.jpg to the POST saves an image: the documented response is JSON with a hosted URL and an identifier.
Screenshot a public webpage as JPEG
When the HTML already exists at a public address, send url instead of inline markup:
curl --fail-with-body --request POST 'https://hcti.io/v1/image'
--user "$HCTI_API_ID:$HCTI_API_KEY"
--data-urlencode 'url=https://example.com'
--data-urlencode 'format=jpeg'
--data-urlencode 'viewport_width=1200'
--data-urlencode 'viewport_height=630'
-o response.jsonThe page must be reachable by the rendering service; a URL that only works inside your laptop or private network will not load. After reading the returned url, download it with the same follow-up command shown above.
Choose the capture region deliberately
- Viewport dimensions: set width and height for the social-card, thumbnail or application slot that will consume the JPEG.
- Full-page mode: use the provider’s full-page option when the entire document is required rather than only the initial viewport.
- Selector capture: capture a particular CSS-selected element when surrounding navigation should be excluded.
- Timing: wait for page content or delayed JavaScript before capture; verify the current API parameter names and constraints in HCTI’s documentation.
- Browser context: color scheme, timezone and mobile behavior can change responsive layouts and therefore the pixels you receive.
These controls are provider options, not cURL features. Confirm their current names and limits before putting them into a production request.
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Python and Node.js wrappers
The same HTTP exchange can be automated after you have established a working cURL request.
Python
import json
import os
import requests
payload = {
"html": '<div class="card"><h1>Hello, world!</h1></div>',
"css": ".card { width: 480px; padding: 40px; background: #f0fdf4; }",
"format": "jpeg",
}
r = requests.post(
"https://hcti.io/v1/image",
auth=(os.environ["HCTI_API_ID"], os.environ["HCTI_API_KEY"]),
data=payload,
timeout=90,
)
r.raise_for_status()
image_url = r.json()["url"]
image = requests.get(image_url, timeout=90)
image.raise_for_status()
open("output.jpg", "wb").write(image.content)
print(image_url)Node.js
const form = new URLSearchParams({
html: '<div class="card"><h1>Hello, world!</h1></div>',
css: '.card { width: 480px; padding: 40px; background: #f0fdf4; }',
format: 'jpeg'
});
const auth = Buffer.from(`${process.env.HCTI_API_ID}:${process.env.HCTI_API_KEY}`).toString('base64');
const response = await fetch('https://hcti.io/v1/image', {
method: 'POST',
headers: { Authorization: `Basic ${auth}`, 'Content-Type': 'application/x-www-form-urlencoded' },
body: form
});
if (!response.ok) throw new Error(`${response.status}: ${await response.text()}`);
const result = await response.json();
const jpeg = await fetch(result.url);
if (!jpeg.ok) throw new Error(`Image download failed: ${jpeg.status}`);
require('fs').writeFileSync('output.jpg', Buffer.from(await jpeg.arrayBuffer()));
console.log(result.url);How the documented cloud workflows differ
| Need | HCTI route | Aspose.HTML Cloud route |
|---|---|---|
| Inline markup | Send html and optional css in one image request. |
Documentation describes uploading a local HTML file before conversion. |
| Public webpage | Send a URL with capture settings. | The cited conversion workflow focuses on stored HTML input; do not assume URL capture from it. |
| Output retrieval | Response supplies a hosted image URL, followed by a download. | Upload, conversion and retrieval use cloud-storage steps. |
| Default sizing | Set viewport and capture options; verify current parameter details. | Documented default corresponds to A4 dimensions with zero margins. |
Neither workflow establishes comparative quality, speed, pricing or reliability here. Treat provider documentation as authoritative for changing endpoints and parameters, especially for the Aspose page, which may be older than the current API.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo API documentation for the complete parameter set. Before capture it accepts cookie and consent banners and removes more than 60 known consent platforms, newsletter popups and chat widgets; each cleanup step can be disabled. Bot checks/CAPTCHAs, blank pages, timeouts, failed loads and cache hits are not billed, and response headers identify the page verdict and billing status. Its MCP server provides take_screenshot, get_page_info and capture_pdf tools for Claude, Cursor and other MCP clients.
Every plan includes the features: full-page capture with lazy images, CSS-selector element capture, dark mode, device presets or custom viewports, retina scale, PDF controls, custom CSS and JavaScript, clicks, waits, blocking rules, headers, cookies, user agents, Authorization, timezone, geolocation, transparent backgrounds, resizing, chosen-TTL caching, signed links, asynchronous webhooks, bulk capture of up to 100 URLs per call, usage API and OpenAPI support. The free plan includes 1,000 shots per month with no card; paid plans start at $5 for 3,000 shots. Yearly billing provides two months free.
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401 or 403 authentication errors
Check that both environment variables are set, contain the correct API ID and key, and are not surrounded by accidental whitespace. The --user value must be API_ID:API_KEY.
HTML appears as text or the request breaks at punctuation
Use --data-urlencode rather than an unescaped form field. Quote shell variables and avoid embedding untrusted markup directly in command strings.
The response is JSON, not a JPEG
That is the documented HCTI flow. Parse the returned url, then issue a second GET to save the image bytes.
The page is blank or incomplete
Confirm the URL is publicly reachable, then add an appropriate wait or timing option. JavaScript-rendered content, blocked resources and responsive breakpoints can alter the result; test the same viewport and browser settings your production request uses.
Fonts, images or styles are missing
Check that assets use reachable absolute URLs and that the page does not depend on private DNS, localhost paths or an authenticated session unavailable to the renderer. Inline critical CSS when you control the HTML.
Output dimensions are wrong
Set explicit viewport width and height, and distinguish viewport capture from full-page or selector capture. Provider defaults and parameter limits can change, so validate them against current documentation.
Requests time out
Use a client timeout long enough for browser startup and page resources, reduce unnecessary third-party assets, and retry transient failures with a bounded backoff. Do not treat retries as proof that a page rendered successfully; inspect the HTTP status and returned body.
Operational and cost considerations
- Persist the response ID and hosted URL so you can audit which conversion produced a file.
- Download immediately when URLs are temporary, unless the provider documents a retention period.
- Cache identical inputs when appropriate, but invalidate after HTML, CSS, assets or viewport settings change.
- JPEG is lossy. It is useful for photographs and compact previews; choose PNG when text edges or transparency matter (if your provider supports it).
- Do not infer rendering speed, quotas or reliability from a single successful command. Those values depend on provider plan, page complexity and current service behavior.
Frequently Asked Questions
Can cURL convert a private localhost page directly?
No. A hosted renderer must be able to reach the URL. Publish it temporarily through an access-controlled tunnel or send the HTML and CSS as inline data instead.
Why does JPEG have a different appearance from my browser?
The service uses its own browser, viewport, device context, fonts and loading timing. Differences in any of those inputs can change layout or anti-aliasing.
Can I make the HCTI POST save image bytes in one step?
The documented HCTI response is JSON containing a hosted image URL, so use a second GET to download the JPEG.
Is Aspose.HTML Cloud a drop-in replacement for HCTI URL capture?
Not according to the cited workflow: Aspose documents upload, conversion and retrieval for stored HTML, while HCTI documents sending a public URL with capture settings.
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