screenshot

name	screenshot
description	Capture faithful PNGs of Pulp view trees / imported UIs headlessly. Covers render_to_png backends (Skia vs CoreGraphics), the image-compositing gotcha, capture_png from a live GPU host, and the --screenshot-backend validate flag. Use whenever you render a UI to a PNG to eyeball or montage it.

Screenshotting Pulp UIs

Pulp has two headless capture surfaces plus the live-host capture. Picking the wrong one wastes time on renders that look broken but aren't.

Default to capture_view / --backend auto — it picks the backend for you

Don't hand-pick a backend unless you have a reason. capture_view(root, w, h, scale) (screenshot.hpp) and the CLI default pulp-screenshot --backend auto inspect the view tree and do the right thing, and never return a silent blank:

The view tree…	…gets	Why
has a contains_native_overlay() subtree (a WebView / native NSView)	the overlay's in-process snapshot (View::capture_native_overlay_png → e.g. WKWebView takeSnapshot); refused with a reason only if no snapshot is available	A WebKit/native overlay is composited by the OS, NOT painted into the Pulp canvas — headless raster can't see it, but a backend that exposes an in-process snapshot can still be captured
has a requires_gpu_host() view (GPU content, custom SkSL)	the gpu path (render_to_png_gpu, offscreen Dawn+Skia via HeadlessSurface)	CPU raster does NOT render GPU-required views correctly — they come out blank/wrong
neither	CPU raster (skia when Skia is built, else the platform default_backend / a registered provider)	fast, faithful for vector + image widgets

capture_view returns {png, ok, used, reason}; a blank/essentially-empty frame sets ok=false. The CLI exits 3 (not a saved blank) on refuse/blank. This is the "always-capturable" contract — if a UI didn't paint, you get told, not a blank PNG.

Wrappers must opt in. A WebView is only captured/refused correctly when the owning Pulp View actually sets set_contains_native_overlay(true) and overrides capture_native_overlay_png() to forward WebViewPanel::snapshot_png(). Without that flag, auto rasters the (blank) overlay area silently — set it on any wrapper that attaches a native child via attach_native_child_view (see examples/webview-plugin WebViewEditorPane).

Build gating. The gpu path and has_gpu_capture() are compiled in only when Skia is present (PULP_VIEW_HAS_GPU_CAPTURE, gated on PULP_HAS_SKIA — not merely on the pulp-render target, whose HeadlessSurface::create() is a null stub in a no-Skia build). In a no-Skia build the auto raster fallback is default_backend (so a host-registered ScreenshotProvider handles it), not a forced skia that would return empty bytes.

When to override: force --backend gpu to capture a GPU view that isn't flagged requires_gpu_host(); use skia/coregraphics for the explicit cases below. The image-compositing rule still applies to the raster backends.

The image rule (raster backends): use Skia for anything with images

render_to_png(root, w, h, scale, backend) (core/view/include/pulp/view/screenshot.hpp) takes a ScreenshotBackend:

Backend	Composites file-backed images?	Use when
skia	Yes	Default for anything real — designs with assets (Figma/Pencil imports), icons, photos
coregraphics (macOS default of default_backend)	No	Vector-only UIs, or when you specifically want the CG path
default_backend	macOS → CoreGraphics, else provider	Avoid for asset-heavy UIs

Why it matters (the trap): ImageView::paint (core/view/src/widgets/visualizers.cpp) decodes images on-paint via the canvas's draw_image_from_file / measure_image_from_file primitive. SkiaCanvas implements it; the CoreGraphics canvas does not. On the CG path every ImageView falls back to drawing its filename as placeholder text — so an asset-rich import renders as empty boxes with *.png strings scattered across it. That looks like "missing images / broken importer," but the import is fine — it's the backend. Vector widgets (knobs, faders drawn via canvas primitives) render on both backends, which makes the CG render partially right and even more misleading.

Confirmed 2026-06-02 on the ELYSIUM Figma import: CoreGraphics → empty vessel boxes + filename text; Skia → the gradient beakers/knobs/curves all composite and the montage matches the Figma reference.

Imported-design validation

pulp import-design --validate --reference <png> --diff <png> [--render-size WxH] renders the generated JS and compares to a reference. As of 2026-06-02 it defaults to --screenshot-backend skia (faithful). Only pass --screenshot-backend coregraphics deliberately. A CoreGraphics validate of an asset-heavy design is not a valid fidelity check.

pulp import-design --from figma-plugin --file scene.pulp.json --output ui.js \
  --validate --reference figma-ref.png --render-size 1146x746 --diff diff.png
# (skia is the default; add --screenshot-backend coregraphics only to escape-hatch)

Pixel-% is a weak gate. compare_screenshot_files similarity is exact-pixel and very sensitive to gradients, anti-aliasing, sub-pixel placement, and background differences — a visually faithful import can report a low % on a gradient-heavy design. Treat the % as a smoke signal; eyeball the montage (reference | render) for structure + assets. Build montages with PIL.

compare_screenshots / compare_screenshot_files decode PNGs with CoreGraphics on Apple and Skia on non-Apple builds when PULP_HAS_SKIA=1. In a non-Apple no-Skia build, comparisons remain unavailable (valid=false, empty diff/crop output) because there is no PNG decoder; treat that as a missing comparison backend, not proof that the rendered PNG was empty.

Capture options at a glance

• render_to_png / render_to_file (screenshot.hpp) — headless raster of a View tree, no window. macOS/iOS have native backends; Linux/Windows use the built-in Skia raster path when PULP_HAS_SKIA=1, otherwise they need a host-registered provider via set_screenshot_provider() (else empty/"unsupported", not a silent blank — #299). Probe has_screenshot_provider().
• WindowHost::capture_png() — reads the rendered frame from a live GPU host (MacGpuWindowHost). The most faithful (real paint path, GPU). Drive it from the design-tool example's --no-show-window --automation-before <png> offscreen path, or any host that exposes capture. NOTE: the design-tool's --script expects its own entry-module shape — a raw generated ui.js from import-design does not load that way (throws). Prefer --validate with the Skia backend for an imported ui.js.
• pulp::view::render_to_file in tests — headless view-tree PNGs in CI.
• pulp::view::render_to_rgba (screenshot.hpp) — raw-pixel sibling of render_to_png. Returns the decoded RGBA8 buffer (R,G,B,A byte order, premultiplied alpha, sRGB, top-to-bottom, stride *out_width*4) + the pixel dims, instead of PNG bytes — for callers that composite/upload the frame themselves (e.g. the foreign-host embed SDK's offscreen mode) and don't want a PNG encode+decode round-trip. macOS-only (forces the Skia raster path, which is endianness-independent; the non-Apple stub returns empty — the registered ScreenshotProvider is PNG-only). The internal render_to_png_skia already holds these pixels before encoding; this just exposes them. Note AssetManager::decode_png does NOT actually decode (it stores raw PNG bytes + parses IHDR), so you cannot get RGBA by round-tripping a PNG through it — use render_to_rgba for raw pixels.

Render size: use the design's true root, not the source bbox

--render-size WxH must match the imported design's root frame size, not the source tool's reported node bounding box. A Figma node's screenshot bbox includes page margin / shadow bleed around the frame (e.g. ELYSIUM: node bbox 1146×746, but the actual root "VST Style" frame is 1000×600). Render at the bbox and the design lays out at top-left with the host's dark background filling the extra pixels — looks like a "wrong window size." Read the root size from scene.pulp.json root.style.width/height (or the generated setSize('root', …)), and render at that. When in doubt, render at the root size — the result fills the canvas and matches the design's own proportions.

Headless GPU capture: use the offscreen gpu backend, NOT a live window

The live-host path (WindowHost::capture_png(), and the standalone --screenshot=PATH flag it backs) is driven per-vsync by the macOS GPU host's CVDisplayLink. That clock only ticks for an on-screen window in an interactive WindowServer session. In a headless / SSH / CI / agent context — or for an initially_hidden accessory app — no vsync is vended, so the idle pump never fires, the one-shot capture never runs, and run_event_loop() blocks forever (the process sits in [NSApplication run] → nextEventMatchingMask). Showing the window does NOT help when there's no interactive session.

So for a headless capture of a GPU-rendered view, use the offscreen surface — render_to_file(root, w, h, path, scale, ScreenshotBackend::gpu) / render_to_png_gpu (Dawn+Skia HeadlessSurface, no window, no display link). It renders the same tree through the real GPU stack and tears down cleanly, so it neither hangs nor wedges the GPU. Reserve the live --screenshot path for a real desktop session where you actually want to prove the on-screen window.

examples/PulpTempoSampler is the worked example of all three: pulp-tempo-sampler-shot OUT.png (CPU raster, default), … OUT.png --gpu (offscreen GPU, headless), and the standalone PulpTempoSampler … --screenshot=OUT.png (live host window, interactive session only).

Gotchas

• Absolute-positioned leaf views need preferred_width/preferred_height, not just dim_width. yoga_layout.cpp applies an explicit px size from FlexStyle::preferred_width/height; dim_width = {w, px} only reaches Yoga after resolve_dimensions() runs, which a bare layout_children() pass (e.g. the screenshot path's paint_root) does NOT call for absolute leaves. Labels survive on their text measure function, but a measure-less leaf like WaveformEditor collapses to 0×0 and paint() early-returns on local_bounds().is_empty() → blank. Set v.flex().preferred_width = w; v.flex().preferred_height = h; directly when placing such a child; a post-layout set_bounds() is futile because any later layout_children() re-collapses it.
• A fresh GPU-less build silently returns the CPU host on macOS (PULP_HAS_SKIA FALSE → MacWindowHost, not MacGpuWindowHost). Verify the binary contains MacGpuWindowHost before trusting a live capture (see the import-design skill's GPU-host gotcha).
• Never show the user a CoreGraphics render of an asset-rich design and call it the import result — re-render with Skia first.
• A non-empty PNG is not a passing render. ScreenshotStats::passes_content_floor (core/view/include/pulp/view/screenshot_compare.hpp) is the oracle that catches the blank/near-blank-frame bug a raw "file written" check misses: it gates on a unique-color floor, a luminance-stddev floor, and non-background + opaque coverage floors. Assert it (not just !png.empty()) when validating a GPU capture, and pump enough settled frames first — a single unsettled frame can under-count content. The design-import screenshot-parity test asserts REQUIRE_FALSE(passes_content_floor()) on a stable flat capture to prove the oracle actually rejects empties.

Vision probe: prove image input works before reading a screenshot into context

A screenshot is only useful for review if the model can actually SEE it. Some models/harnesses silently drop image input — the render succeeds, the bytes are handed over, and the model then hallucinates a description of a picture it never received. Before you read ANY capture into context for a visual judgment, run a one-shot vision probe: hand the model a tiny, known committed PNG (e.g. one of the small rendered fixtures under test/fixtures/import-fidelity/assets/, whose content you have confirmed) and ask it to return exactly VISION_OK or VISION_UNSUPPORTED based on a describable feature of that image — not "did an image arrive" but "what does the shape look like".

• On VISION_OK: proceed to read real captures for visual review.
• On VISION_UNSUPPORTED (or any answer that does not match the known image): do NOT read screenshots into context and do NOT describe them. Save the captures to disk, report only their PATHS, and disclose explicitly: "visual review skipped — image input unavailable in this model/harness." Non-visual checks (logs, passes_content_floor on the raw bytes, pulp design lint-adherence) still run and still gate.

This degrades honestly across models instead of emitting confident fiction about an unseen image.

Verdict contract: one review artifact per pass, done or needs_work

When a verification pass reviews a render (see the read-only verifier in the import-design skill), its final output is a verdict, never prose: exactly done, or needs_work: <root cause> naming the constraint/token/log defect — not the pixel symptom. A "looks good" with no verdict is a failure mode; so is a reviewer that edits while reviewing. Keep review read-only and let the main agent apply fixes, then re-run the verifier.