Delphi/C++Builder には PDFium VCL Component のワークフローを、Lazarus/FPC には PDFium LCL Component のワークフローを組み込み、表示、レンダリング、フォーム、印刷、プリフライトレポート、標準対応の検証をソースコード付きコンポーネントで実装できます。
この記事は developers adding viewing aids for users who need contrast, color inversion, or reduced visual strain 向けです。low-vision color filters and reading modes を単なるコンポーネント呼び出しではなく、本番向けのドキュメントエンジニアリングとして扱います。
実務上のリスクは display filters can help users read documents, but they can also mislead review workflows if the application does not explain that the source PDF is unchanged です。そのため、明確な契約、観測可能な診断、実際の顧客ファイルに近い回帰サンプルが必要です。
アーキテクチャ上の判断
Keep visual assistance separate from document editing. available modes such as high contrast, grayscale, inverted color, and warm background / whether filters apply to pages, thumbnails, selection highlights, and annotations
- available modes such as high contrast, grayscale, inverted color, and warm background
- whether filters apply to pages, thumbnails, selection highlights, and annotations
- preference persistence per user, per document, or per application profile
- screen capture, print, and export behavior while filters are active
実装フロー
Apply filters in the rendering pipeline. The order below keeps the workflow reviewable for Delphi and C++Builder teams.
- add filter selection to viewer state rather than PDF modification code
- render representative text, images, forms, and annotations through each mode
- keep selection colors and focus indicators visible under every filter
- show whether printing uses the original PDF appearance or the filtered view
- record mode settings when users report readability issues
検証エビデンス
Usability evidence for low-vision modes. Keep these fields with the output or support record.
- filter mode, contrast ratio spot checks, zoom level, and page rendering backend
- selection, annotation, form-field, and focus visibility under the active mode
- preference storage decision and reset path
- support screenshot that clearly labels filtered display state
A filter is a view, not a document change
Low-vision support should alter the rendered presentation without changing the PDF bytes. Users need predictable toggles, persistent preferences, clear print behavior, and fallback text when a document cannot be read visually.
Decision table for low-vision color filters and reading modes
A decision table keeps product ownership visible when the same workflow is reused by a desktop tool, service job, and support utility.
| Decision | Engineering reason | Evidence |
|---|---|---|
| available modes such as high contrast, grayscale, inverted color, and warm background | add filter selection to viewer state rather than PDF modification code | filter mode, contrast ratio spot checks, zoom level, and page rendering backend |
| whether filters apply to pages, thumbnails, selection highlights, and annotations | render representative text, images, forms, and annotations through each mode | selection, annotation, form-field, and focus visibility under the active mode |
| preference persistence per user, per document, or per application profile | keep selection colors and focus indicators visible under every filter | preference storage decision and reset path |
Engineering review notes for low-vision color filters and reading modes
Use these review notes to make sure the feature has moved beyond a demo and can be defended during release, support, and customer escalation.
- Decision: available modes such as high contrast, grayscale, inverted color, and warm background. Implementation pressure point: render representative text, images, forms, and annotations through each mode. Acceptance evidence: preference storage decision and reset path. Regression trigger: dark-mode application chrome should not reduce document focus visibility
- Decision: whether filters apply to pages, thumbnails, selection highlights, and annotations. Implementation pressure point: keep selection colors and focus indicators visible under every filter. Acceptance evidence: support screenshot that clearly labels filtered display state. Regression trigger: image-heavy PDFs may lose detail under aggressive contrast transforms
- Decision: preference persistence per user, per document, or per application profile. Implementation pressure point: show whether printing uses the original PDF appearance or the filtered view. Acceptance evidence: filter mode, contrast ratio spot checks, zoom level, and page rendering backend. Regression trigger: highlight colors can disappear if filters are applied after overlay painting
- Decision: screen capture, print, and export behavior while filters are active. Implementation pressure point: record mode settings when users report readability issues. Acceptance evidence: selection, annotation, form-field, and focus visibility under the active mode. Regression trigger: printing a filtered view may be desired for accessibility but wrong for legal review
境界ケース
- image-heavy PDFs may lose detail under aggressive contrast transforms
- highlight colors can disappear if filters are applied after overlay painting
- printing a filtered view may be desired for accessibility but wrong for legal review
- dark-mode application chrome should not reduce document focus visibility
Delphi / C++Builder notes
PDFium Component should sit behind a small service boundary that receives files, streams, profiles, and credentials, then returns output paths, warnings, metrics, and validation status. Important terms include color filter, contrast, inversion, reading mode, render pipeline, accessibility.
Delphi コード例
次の Delphi スケッチは、このテーマに対する実用的なサービス境界を示します。ポリシー確認、ログ記録、検証を製品呼び出しの狭い部分の外側に置くと、ワークフローをテストしやすくなります。
procedure TPreviewForm.ApplyReadingTheme(const Theme: TReadingTheme);
begin
FCurrentTheme := Theme;
RenderCurrentPage;
ApplyBitmapColorMatrix(FPageBitmap, Theme.ColorMatrix);
PaintContrastCheckedBitmap(FPageBitmap);
LogAccessibilitySetting(Theme.Name);
end;本番チェックリスト
- Run the workflow on an empty file, a normal customer file, and a worst-case file
- Open the generated PDF with the target viewer, validator, printer, or downstream application
- Log product version, profile version, input hash, output path, elapsed time, and warning count
- Keep passwords, certificates, temporary files, and customer data under explicit retention rules
- Add regression documents when a customer file exposes a new edge case