Transparent property

Transparent returns True when the active page contains any object whose effective alpha is less than 1.0 or which references a graphics state that enables transparency — soft masks, opacity values (CA / ca), non-default blend modes (/BM), or transparency groups (PDF 1.7 spec section 11). It is the same flag PDFium checks internally to decide whether the page must be rendered through its transparent-page rasterizer.

The property is False when Active is False or when the page is purely opaque. Pages that are transparent are slightly more expensive to render — PDFium has to allocate an alpha channel and composite each object against the page background — so detecting transparency up front lets callers pick the right output target (32-bit ARGB TBitmap, PDF watermark overlay, printer’s alpha-aware path).

The value is a per-page property; iterate every page if you need a document-wide “has transparency anywhere?” answer. Combine with the Background color when rendering to a flat bitmap: a transparent page on a clWhite background produces flattened output, whereas rendering on a transparent surface preserves alpha for downstream compositing.

• Highlight annotations, watermarks and stamp overlays commonly trigger transparency on otherwise opaque pages.
• Use this flag to short-circuit alpha-channel handling: opaque pages can be rendered to a 24-bpp bitmap and saved as JPEG without information loss.
• Detecting transparency is O(1) — PDFium stores the flag during content stream parsing, so reading the property is essentially free.