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function main(splash, args)
assert(splash:go(args.url))
assert(splash:wait(0.5))
local element = splash:select(args.css)
assert(element, "No element matched the CSS selector")
return element:png()
endSend that script to Splash’s execute endpoint as lua_source, together with url and a selector argument. The half-second wait is only an example; replace it with a readiness condition that matches the page you are capturing.
How Splash’s single-element capture works
Splash renders JavaScript pages and exposes a Lua scripting API. In the script, splash:select(css) returns a handle for the first DOM element matching the CSS selector. Calling :png() on that handle produces the element’s rendered PNG as a binary image result. Returning the PNG directly avoids taking a full-page screenshot and cropping it later.
The selector is evaluated after navigation and any wait you specify. If no element matches, the example raises an explicit error instead of returning an apparently successful but empty image. Use a stable ID, data attribute, or narrowly scoped class whenever possible; selectors based on generated class names are more likely to break when the site changes.
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Minimal Lua script
function main(splash, args)
assert(splash:go(args.url))
assert(splash:wait(0.5))
local element = splash:select(args.css)
assert(element, "No element matched the CSS selector")
return element:png()
endWhat each line does
splash:go(args.url)loads the URL supplied by the caller and stops with an error if navigation fails.splash:wait(0.5)gives scripts and layout a chance to run. It is not a universal readiness guarantee.splash:select(args.css)finds the first matching element.assert(element, ...)turns a selector miss into a clear failure.element:png()returns the selected node as a PNG image.
Choosing a reliable wait
Use a fixed delay only when the page is predictable. For an application that inserts the node asynchronously, wait for a selector or for the page’s own state to become ready before selecting it. A delay that is too short captures an empty shell; a delay that is unnecessarily long reduces throughput. Splash’s timing and JavaScript behavior can vary with the deployed version and the target page, so verify the choice against the pages you actually crawl.
Calling the execute endpoint over HTTP
For an HTTP request, put the Lua source in the lua_source argument and provide the target URL and selector as request arguments. The response is the binary PNG returned by the script. The following cURL command uses a shell variable so the multiline script remains readable:
read -r -d '' LUA <<'EOF'
function main(splash, args)
assert(splash:go(args.url))
assert(splash:wait(0.5))
local element = splash:select(args.css)
assert(element, "No element matched the CSS selector")
return element:png()
end
EOF
curl -G "http://localhost:8050/execute"
--data-urlencode "lua_source=$LUA"
--data-urlencode "url=https://example.com"
--data-urlencode "css=main h1"
-o element.pngChange the Splash host, target URL, and selector for your deployment. If your shell does not support the shown here-document form, place the Lua in a file and URL-encode it with the HTTP client or SDK you use.
Python integrations
Python requests
This standalone example submits the same script to Splash and writes the binary response:
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import requests
lua = r'''
function main(splash, args)
assert(splash:go(args.url))
assert(splash:wait(0.5))
local element = splash:select(args.css)
assert(element, "No element matched the CSS selector")
return element:png()
end
'''
params = {
"lua_source": lua,
"url": "https://example.com",
"css": "main h1",
}
response = requests.get("http://localhost:8050/execute", params=params, timeout=90)
response.raise_for_status()
with open("element.png", "wb") as image:
image.write(response.content)Use a timeout that allows the slowest pages in your crawl to finish, but enforce an upper bound so one stalled render does not occupy a worker indefinitely. Log the URL and selector with HTTP status and response headers when diagnosing failures.
Scrapy with scrapy-splash
The Scrapy integration submits Lua to the execute endpoint through SplashRequest:
import scrapy
from scrapy_splash import SplashRequest
SCRIPT = r'''
function main(splash, args)
assert(splash:go(args.url))
assert(splash:wait(0.5))
local element = splash:select(args.css)
assert(element, "No element matched the CSS selector")
return element:png()
end
'''
class ElementSpider(scrapy.Spider):
name = "element"
def start_requests(self):
yield SplashRequest(
url="https://example.com",
endpoint="execute",
args={"lua_source": SCRIPT, "css": "main h1"},
callback=self.parse_image,
)
def parse_image(self, response):
with open("element.png", "wb") as image:
image.write(response.body)
Depending on your scrapy-splash configuration and response mode, the script may return a binary body or a JSON object containing encoded data. Follow the response type configured by your integration: decode base64 only when the response is actually encoded JSON, and do not base64-decode an already binary PNG.
When to use a padded region instead
element:png() is the simplest option, but it gives you the element’s own box. Use a region screenshot when you need explicit padding, a crop assembled from DOM geometry, or a bounding box that you calculate yourself.
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function pad(r, amount)
return {r[1] - amount, r[2] - amount,
r[3] + amount, r[4] + amount}
end
function main(splash, args)
local get_bbox = splash:jsfunc([[
function(css) {
var el = document.querySelector(css);
if (!el) return null;
var r = el.getBoundingClientRect();
return [r.left, r.top, r.right, r.bottom];
}
]])
assert(splash:go(args.url))
assert(splash:wait(0.5))
splash:set_viewport_full()
local bbox = get_bbox(args.css)
assert(bbox, "No element matched the CSS selector")
return splash:png{region=pad(bbox, args.pad or 0)}
endRegion rules that matter
- The region order is
{left, top, right, bottom}, not x, y, width, height. getBoundingClientRect()returns coordinates relative to the current scroll position.- Call
splash:set_viewport_full()before rendering when the target could otherwise be clipped by the viewport. - The documented region method cannot capture content outside the current viewport unless the viewport is adjusted appropriately; test long or dynamically scrolling pages rather than assuming a full-page crop.
- Padding can produce negative left or top values. Clamp or validate them if your target pages place the element at the viewport edge.
Viewport, full-page, and scaling choices
Element capture does not require render_all=1. That option extends the viewport to the whole page, is intended for full-page rendering, and requires a non-zero wait. Enabling it by default adds work without improving a target that already fits in the viewport. For a region workflow, splash:set_viewport_full() is the documented way to reduce viewport clipping.
Splash’s PNG rendering also supports scale_method values raster and vector. Vector scaling can be faster and sharper in some cases, but the documentation warns that it can cause rendering issues. Treat it as a per-site rendering choice: compare output on representative pages before making it your default.
Selectors, dynamic content, and difficult pages
Selector misses
Check the selector in the page’s DOM, not just the visual source. A class may be added only after JavaScript runs, and a selector that matches several nodes returns the first match. Narrow it with an ancestor, ID, or data attribute when the page contains repeated cards.
Late layout changes
Images, fonts, ads, and client-side data can change the box after the first paint. Wait for a page-specific marker or for the target selector to exist, then allow any known layout transition to settle. If the element is present but the image is blank, the problem is readiness rather than selection.
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Shadow DOM and iframes
A normal document selector does not automatically cross every encapsulation boundary. The reviewed Splash workflow does not establish guarantees for cross-origin iframe targets. If the element lives in an iframe or shadow tree, verify what the deployed Splash version can access and consider capturing the frame’s own URL when it is available.
Troubleshooting checklist
| Symptom | Likely cause | Fix |
|---|---|---|
| “No element matched” assertion | Wrong selector, page not ready, or element inside another document | Inspect the rendered DOM, wait for the insertion point, narrow or correct the selector, and check iframe or shadow boundaries. |
| PNG is blank or shows a skeleton | Asynchronous data or images have not finished | Replace the illustrative delay with a page-specific readiness wait and verify that required resources are reachable. |
| Only part of the target appears | Viewport clipping or a region outside the visible area | Use splash:set_viewport_full() for region captures and check scroll-relative coordinates. |
| HTTP request returns an error page | Lua runtime error, navigation failure, or malformed arguments | Test the script in Splash directly, keep assert messages, URL-encode lua_source, and log the returned status and body. |
| Scrapy callback cannot decode output | Response mode mismatch | Inspect whether the endpoint returned binary PNG or JSON/base64, then handle exactly that format; do not decode binary data as base64. |
| Output differs between runs | Animation, changing content, fonts, or network timing | Disable or wait through animations where possible, use deterministic test URLs, and capture after the same readiness condition. |
Operational and cost considerations
Element screenshots are smaller and usually cheaper to store and transfer than full-page images, but Splash still has to load and render the page. Reuse a Splash service rather than starting a browser for every request, cap concurrency to the resources available, and set request timeouts. Cache captures when the source and selector are unchanged; invalidate that cache when content or CSS changes. Keep the selector and wait policy alongside the capture code so a visual regression can be reproduced.
There is no universal wait duration, rendering speed, or cross-site success rate established for this workflow. Measure on your own Splash version, network, viewport, and page mix. Treat bot checks, login walls, consent dialogs, and network failures as page-specific conditions that may require headers, cookies, or a different capture path.
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Here is the equivalent cURL request (see the ScreenshotNeo documentation for all options):
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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.webpPython
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)Node.js
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
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Frequently Asked Questions
Does Splash capture the first matching element or every match?
splash:select(css) selects the first matching element. Use a more specific selector or write a script that iterates over matches when you need multiple images.
Can I return JPEG or WebP from element:png()?
The documented element helper returns PNG data. Converting the result is a separate image-processing step; do not assume the Lua helper changes formats.
Should I use render.png instead of execute?
render.png is suited to whole-page captures. A custom Lua script sent to execute is the direct workflow for selecting one element or calculating a padded region.
Where are Splash region coordinates measured from?
They are ordered left, top, right, bottom and are relative to the current scroll position.
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