Support SVG filters via targeted fallback rendering

[rowland]

Problem

Some assets warn with svg: <filter>: unsupported element skipped. SVG filters cover a wide range of effects, and skipping them can materially change the appearance.

What implementation would require

• Identify which filter primitives are actually present in the asset corpus instead of aiming for the whole SVG filter spec.
• Decide on a pragmatic rendering strategy. Native PDF equivalents exist for very little of this, so localized rasterization is the most likely implementation path.
• Add parser support for <filter> references and enough structure to know when a node needs fallback rendering.
• Build a renderer path that isolates the filtered subtree, rasterizes it at a controlled resolution, and places it back into the PDF.
• Add guardrails around output size and performance.

Workarounds

• Pre-rasterize filtered assets to PNG.
• Export SVG variants with filter effects expanded or removed.
• Keep pure-vector SVG only for assets that do not depend on drop shadows, blurs, or similar effects.

[rowland]

Status audit (2026-08-14): this issue remains open, with some parser and diagnostic groundwork completed in ce03ef6 and c3cff8a.

Implemented today:

• filter="url(#...)" references are retained from presentation attributes, class rules, and inline styles.
• <filter> definitions are indexed by id, including their region and filterUnits.
• A small scene-model representation exists for feFlood and feBlend (in, in2, result, and flood color).
• Unsupported primitives warn and skip; missing filter references and parsed-but-unrendered filters produce explicit element/attribute warnings.

Still missing:

• Filters are never applied to rendering; the referenced node/subtree is rendered unfiltered after the warning.
• There is no subtree isolation, rasterization backend, resolution policy, crop/filter-region handling, image placement, caching, or output-size/performance guardrail.
• Common effect primitives such as blur, offset, drop shadow, color matrix, composite, and merge are not represented.

The checked-in SVG corpus is narrower than the issue suggests: its only real filter chain is feFlood + feBlend inside the mask/blend test fixture. Native PDF mask, opacity, gradient, and blend-mode work now produces a useful rendering for that fixture, but that is not semantic SVG filter support and should not be treated as closing this issue.

Before implementing fallback rendering, the next useful step is still the issue's original first bullet: inventory the production/legacy asset corpus and rank actual primitive chains. That will tell us whether a small native subset is worthwhile or whether localized rasterization should be the primary path.

Verified current behavior with go test ./svg ./pdf.