How to Work with Underlays in BricsCAD

Quick answer: A PDF underlay is linked to a drawing rather than being part of it, much like a raster image or Xref. It’s a different concept from PDF import (covered elsewhere in this hub), which actually converts PDF content into drawing entities, an underlay stays a linked reference instead.

What a PDF Underlay Is

The path to an underlay’s PDF file is saved in the parent drawing and can be edited. By default, BricsCAD searches the parent drawing’s own folder first, then the saved path folder; if it’s found in neither, it reports “Missing or invalid reference” in the drawing.

Two things worth knowing: underlays are only visible when the visual style is set to 2dWireframe, and hypertext links from the original PDF convert to plain text rather than staying clickable.

PDF Underlay Caching

A multi-resolution persistent image cache displays attached PDF underlays, enabling very fast zoom and pan, cached up to 5000 by 5000 pixels. Zoom in closer than that and the display gets pixelated, so a hybrid mode is available that switches to real-time generation for a crisp display when you zoom in very close. Building the initial cache takes a few seconds, but processing stays fast afterward, including in later sessions.

PDFCACHE controls this: 0 never uses the cache, always generating in real time (the old default in earlier BricsCAD versions); 1 uses the cache but switches to real-time generation when you zoom in very close; 2 always uses the cache (the current default).

Attaching a PDF Underlay

1. Open the Select PDF Underlay File dialog: click Attach PDF on the Insert toolbar or Insert/PDFs ribbon panel, click New on the Details toolbar of the Drawing Explorer’s PDF Underlays dialog, or type PDFattach and press Enter.
2. Select the file, then double-click it or click Open; the Attach PDF Underlay dialog appears.
3. For a multi-page PDF, click the down arrow in the Name field and choose the page you want.
4. Choose a Path option: Full path, Relative path, or No path, each searches its own location first, falls back to the parent drawing’s folder, and reports “Missing or invalid reference” if the file still isn’t found.
5. Choose whether to specify the Insertion Point, Scale, and Rotation angle on screen or not.
6. Click OK; the dialog closes.
7. Follow whatever prompts your choices in step 5 trigger.

You can also drag and drop underlays directly into the current drawing.

Inserting Another Instance or Page

1. Launch EXPPDFS. The Drawing Explorer’s PDF Underlays dialog opens.
2. Select the PDF underlay.
3. Right-click and choose Insert, or choose Insert from the dialog’s Edit menu; the dialog closes.
4. Continue from step 3 of the attaching procedure above.

Controlling PDF Underlay Display

1. Click the underlay’s frame; it highlights, and its properties appear in the Properties panel.
2. Under Misc, set Show underlay to Yes or No.

Setting Layer Display Within a PDF

1. Launch PDFLAYERS. You’re prompted to select the PDF underlay.
2. Click its frame; it highlights, and the Underlay Layers dialog opens.
3. The icon in the On column shows each layer’s current state.
4. Click that icon to toggle a layer’s display.
5. Optionally use the Search for layer field, with ? or * wildcards, to filter the list.
6. Optionally click Reference name to switch to a different PDF underlay.
7. Click OK to finish.

Clipping a PDF Underlay

1. Launch PDFCLIP. You’re prompted to select the PDF underlay.
2. Click its frame; it highlights, and you’re prompted for a clipping option (On, Off, Delete, or New).
3. Press Enter to accept New. You’re prompted for a clipping type.
4. Press Enter for Rectangular, or type P for Polygonal.
5. Define the clipping boundary. Points must lie inside the PDF frame; click outside it, and the point gets placed on the frame’s edge instead.

Toggling the Clipped Display

1. Click the underlay’s frame; it highlights, and its properties appear in the Properties panel.
2. Under Misc, set Show clipped to Yes or No.

Deleting a Clipping Boundary

1. Launch PDFCLIP. You’re prompted to select the PDF underlay.
2. Click its frame; it highlights, and you’re prompted for a clipping option.
3. Type D and press Enter; the boundary is deleted and the full PDF displays again.

Adjusting Fade, Contrast, and Monochrome

PDFADJUST adjusts these settings at the Command line, or select the underlay and adjust them directly under Underlay Adjust in the Properties panel.

Related System Variables

PDFOSNAP, when on, lets you snap to geometry within the PDF underlay. PDFNOTIFY controls whether warnings appear for missing or modified PDF underlays, and XNOTIFYTIME sets how often BricsCAD checks for those modifications, the same variables covered for Xrefs elsewhere in this hub, since they govern Xrefs, images, and PDF attachments together. If FRAME is set to 3, PDFFRAME controls PDF frame visibility: 0 hides them, 1 displays and plots them, 2 displays but doesn’t plot them.

Frequently Asked Questions

Can I see a PDF underlay in every visual style? No, underlays are only visible when the visual style is set to 2dWireframe.

What happens if I click outside the PDF frame while defining a clipping boundary? The point still gets placed, but clamped onto the PDF underlay’s frame edge rather than wherever you actually clicked.

Do hypertext links in an attached PDF stay clickable? No, they convert to plain text once attached as an underlay.

What’s the difference between a PDF underlay and PDF import? An underlay stays a linked external reference to the PDF file, similar to an Xref. PDF import, covered elsewhere in this hub, actually converts the PDF’s content into native drawing entities instead.