Font reuse and image resolution in WASM previews
WASM 预览的字体复用与图像分辨率
Office conversion repeats font metadata parsing across documents and font matching within one document. Image-heavy documents also spend substantial PDF export time resampling images above preview resolution. These costs need separate controls because font reuse does not reduce image decoding or resampling.
English
Problem
Office conversion repeats font metadata parsing across documents and font matching within one document. Image-heavy documents also spend substantial PDF export time resampling images above preview resolution. These costs need separate controls because font reuse does not reduce image decoding or resampling.
Decision
The WASM provider builds a font metadata snapshot once during initialization. Each conversion Worker receives a structured clone. The Host retains names, face attributes, paths, sizes, and modification times; glyph coverage, request caches, and imported bytes belong to the Worker. Reloading the provider refreshes the snapshot. Workers validate indexed files when reading them, and an already imported font remains usable within that conversion.
Each Worker memoizes complete requests: family, style, weight, italic, width, pitch, language, and ordered code points. The cache stores MEMFS paths and missing-family observations. Hits replay those observations because an initialization request can recur after document font collection starts. Worker termination releases both the cache and engine memory.
Runtime PDF filter options reduce raster images to a configurable maxImageResolution, defaulting to 192 DPI. The shared PDF canvas renders at 96 CSS DPI times the device pixel ratio; the default covers ratio 2. Text and vector graphics remain scalable. Explicit bookmark export preserves LibreOfficeKit's default when JSON filter options replace its implicit filter data.
The local Office preview decision continues to own conversion lifetime, authorization, missing-font presentation, and PDF transport.
Alternatives considered
Index fonts inside every conversion Worker. This keeps discovery off the Host event loop and sees newly installed fonts immediately, but repeats full-file metadata parsing across previews. A provider snapshot removes that repeated work at the cost of synchronous initialization and explicit reload after font changes.
Cache only the family name. Style, language, pitch, and missing characters can select different files. A complete request key retains those distinctions and still captures repeated layout requests.
Keep 300 DPI for every preview. Higher raster resolution retains detail for high pixel ratios and magnification but increases image export work. The configurable 192-DPI default follows the common display target without rasterizing text or rebuilding the engine.
Consequences
Cold provider startup includes indexing and remains separate from conversion deadlines. Shared metadata retains no original font buffers, while each Worker still reads selected fonts and lazily decodes coverage. Installing or replacing fonts requires provider reload; a file changed since indexing can fail a later conversion.
Reducing image resolution trades raster detail at high magnification for less export work. It is not an engine memory cap: LibreOffice can still decode full-resolution source images, load large font collections, or exhaust its build-time WASM memory maximum. No persistent font cache, filesystem watcher, or cross-document engine instance is introduced.
Font tests cover complete request keys, repeated missing-family observations, snapshot isolation, and failed imports. Provider tests cover snapshot reuse and reload. The real-engine tests inspect exported image dimensions with default and overridden DPI settings alongside selectable text and page count; ABI forwarding alone cannot establish filter behavior.
中文
问题
Office 转换在不同文档间重复解析字体元数据，并在同一文档内重复匹配字体。图片较多的文档还会在 PDF 导出时花费大量时间，重采样高于预览显示分辨率的图像。这些成本需要分别控制，因为字体复用不会减少图像解码或重采样。
决策
WASM 提供方在初始化时建立一次字体元数据快照。每个转换 Worker 接收结构化克隆。Host 保留名称、字体面属性、路径、大小和修改时间；字形覆盖、请求缓存及导入字节属于 Worker。重新加载提供方会刷新快照。Worker 读取已索引文件时校验文件，已经导入的字体在该次转换内仍可使用。
每个 Worker 记忆化完整请求：字体族、样式、字重、斜体、字宽、字距类型、语言及有序码点。缓存保存 MEMFS 路径和缺失字体族观察值。命中时重放这些观察值，因为初始化请求可能在文档字体收集开始后再次出现。Worker 终止时释放缓存及引擎内存。
运行时 PDF 过滤器选项将栅格图像降低到可配置的 maxImageResolution，默认 192 DPI。共享 PDF 画布按 96 CSS DPI 乘以设备像素比渲染；默认值覆盖像素比 2。文本和矢量图形仍可缩放。JSON 过滤器选项替换隐含过滤器数据时，显式导出书签保留 LibreOfficeKit 的默认行为。
本地 Office 预览决策继续负责转换生命周期、授权、缺失字体展示和 PDF 传输。
考虑过的替代方案
在每个转换 Worker 内索引字体。 这会将发现工作移出 Host 事件循环，并立即识别新安装的字体，但会在不同预览间重复读取完整字体文件并解析元数据。提供方快照消除重复工作，代价是同步初始化，以及字体变化后显式重新加载。
只缓存字体族名称。 样式、语言、字距类型和缺失字符可能选择不同文件。完整请求键保留这些区别，同时仍能命中重复排版请求。
所有预览保持 300 DPI。 更高栅格分辨率保留高像素比及放大时的细节，但增加图像导出工作。可配置的 192 DPI 默认值符合常见显示目标，无需栅格化文本或重新构建引擎。
影响
提供方冷启动包含索引成本，不计入转换时限。共享元数据不保留原始字体缓冲区，各 Worker 仍会读取选中字体并按需解码字形覆盖。安装或替换字体后需要重新加载提供方；索引后发生变化的文件可能使后续转换失败。
降低图像分辨率以高倍缩放下的栅格细节换取更少的导出工作。它不是引擎内存上限：LibreOffice 仍可能解码全分辨率原图、加载大型字体集合，或耗尽构建时设定的 WASM 内存上限。此实现不引入持久化字体缓存、文件系统监听器或跨文档引擎实例。
字体测试覆盖完整请求键、重复缺失字体族观察值、快照隔离和失败导入。提供方测试覆盖快照复用和重新加载。真实引擎测试检查默认及覆盖 DPI 设置时的导出图像尺寸、可选择文本和页数；仅验证 ABI 参数转发无法证明过滤器行为。