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What each dimension setting controls
CaptureKit’s Capture endpoint documentation describes the viewport dimensions as browser viewport pixels. These settings describe the browser’s visible area, not necessarily the dimensions of the final image file.
| Setting | What it controls | What to check |
|---|---|---|
viewport_width and viewport_height |
The browser viewport width and height in pixels. Documented defaults are 1280 × 1024. | Set both explicitly when you need a specific viewport; do not assume defaults match your target. |
full_page |
Whether to capture the entire page rather than only the visible viewport. | A full-page image may be taller than the configured viewport. |
scale_factor |
A separate scale factor for high-resolution screenshots. The documented default is 1. | Test it separately from viewport dimensions; the docs do not specify a universal output-size formula for all combinations. |
| Device emulation | A separate choice that emulates a device. | Record the selected preset and isolate it from viewport and scale changes. |
Device pixel density explains why a logical size and a physical pixel size can differ. Apple’s UIScreen scale documentation describes how a scale value maps logical points to device pixels: for example, a scale of 2 or 3 maps each point to two or three pixels in each dimension. Those are platform examples, not specifications for CaptureKit’s device presets or a guarantee of its output dimensions.
Check the parameter names first
There is a naming difference between CaptureKit’s endpoint reference and its screenshot playbook. The endpoint lists viewport_width and viewport_height; the playbook example uses width and height (CaptureKit screenshot playbook). The documentation does not establish whether those shorter names are accepted aliases.
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- Inspect the actual URL query string or request parameters sent by your application.
- Compare the names with the endpoint reference for the API version you call.
- Use the documented
viewport_widthandviewport_heightnames for a controlled check rather than assumingwidthandheightare interchangeable.
Run a controlled dimension check
- Keep the page URL and all unrelated request options fixed.
- Request a viewport-only screenshot with explicit
viewport_widthandviewport_height, and no device preset or full-page capture unless required. - Inspect the returned image file’s actual pixel width and height. Compare those values with the intended viewport, not just the CSS layout or the dimensions shown in a preview.
- Change only
scale_factorand capture again. Record the setting and resulting file dimensions. - If you need the whole page, enable
full_pageas a separate test. Expect page height to differ from viewport height. - If you need device emulation, test the selected preset separately and record it alongside the viewport and scale settings.
This sequence isolates variables; it is a diagnostic method based on the documented controls, not a guaranteed output formula or a claim that a particular request has been tested.
Common causes and fixes
The request uses the wrong parameter spelling
Symptom: Changing width or height appears to have no effect. Fix: inspect the outgoing request and verify the names against the endpoint reference. The playbook’s shorter names and the endpoint’s viewport_ names differ, and the available documentation does not resolve alias support.
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The viewport is being mistaken for the image dimensions
Symptom: The screenshot is larger or taller than the requested visible area. Fix: check whether full_page is enabled and whether scale_factor differs from 1. A full-page capture can extend beyond the viewport; scale is a separate setting.
A device preset changes the coordinate context
Symptom: A capture differs from a desktop or viewport-only request even though the URL is unchanged. Fix: record the emulated device, then compare against a request without device emulation while keeping other settings fixed. Logical points and physical pixels are distinct on high-density displays, but Apple platform scaling examples do not establish the exact output dimensions of CaptureKit presets.
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The expected size assumes an undocumented formula
Symptom: You expect the final file to equal viewport dimensions multiplied by a particular scale or device value. Fix: measure the returned image and verify each option independently. CaptureKit documents the viewport and scale controls but not a universal formula covering device emulation and full-page captures.
Performance and repeatability
- For comparisons, keep the URL and other request values unchanged and alter one dimension-related setting per capture.
- Record viewport width and height, scale factor, full-page state, device preset, and resulting image pixel dimensions together. This makes it possible to distinguish a configuration change from an apparent rendering difference.
- Do not treat Apple ScreenCaptureKit’s output-scaling documentation as a CaptureKit API guarantee. Apple’s material explains general output scaling concepts, not CaptureKit behavior (Apple scalesToFit documentation; Apple WWDC22 ScreenCaptureKit session).
- No frequency statistic for this issue or success rate for a particular fix is established in the cited vendor documentation.
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Frequently Asked Questions
Do CaptureKit’s default viewport values guarantee the final image size?
No. The documented defaults describe the browser viewport; full-page capture, scale, and device emulation are separate controls, and the endpoint does not specify a universal output-dimension formula.
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Are CaptureKit’s `width` and `height` parameters aliases for `viewport_width` and `viewport_height`?
The documentation cited here does not say. Verify the parameter names for the endpoint and version your integration calls.
Quick Recap
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