> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/cadd/backgroundimage-cadd.md).

# Background Image

A **Background Image** places a raster image or PDF page as a reference underlay in a CAD document. Use it for:

* Tracing geometry from a scanned existing-conditions drawing or as-built sheet.
* Overlaying a survey site plan or aerial ortho beneath the proposed bridge layout.
* Showing a reference detail from another sheet without duplicating it as native geometry.
* Comparing a proposed alignment against a topographic map.

The background image is purely visual — it doesn't snap, dimension, or participate in 3D model geometry. It exists to give you something to draw against.

## Source

**Document:** Path or reference to the image file (PNG, JPG, TIFF) or PDF.

**Image Type:** `image` for raster images, `pdf` for PDF pages.

**Page:** PDF page number (PDF source only). Page 1 is the first page.

**Max Render Size:** Maximum pixel resolution at which the image is rendered. Default 3000 is right for most uses; reduce for faster screen drawing on slower machines, increase only if you need to zoom in to fine detail in the image.

## Placement

**Center X / Center Y:** Where the image's center sits in the CAD document's coordinate system.

**Width / Height:** Display size of the image in real units. Set to match the image's true scale — if the image is a sheet that printed at 1"=20', set Width and Height to the real-world dimensions of what's shown.

**Rotation:** Rotation about the center.

**Scale:** Additional scale factor on top of Width / Height. Use to fine-tune the size against a known dimension on the image (for example, set the width approximately, then scale until a known distance on the image matches a known distance in the drawing).

## Aligning a background to your drawing

The most common task with a background image is getting it positioned correctly:

1. Drop the image into the CAD document.
2. Set Width and Height to approximate real-world dimensions of what's pictured (a survey sheet that's 22" × 34" at 1"=50' would be roughly 1100' × 1700').
3. Use Rotation to orient north-up if needed.
4. Move the image until a known feature (a survey monument, a property corner, a pier location) lines up with the same feature in your drawing.
5. Adjust Scale and Rotation iteratively until two or three reference points all match.
6. Lock the image's level (if your project supports level locking) so it doesn't move accidentally as you draft over it.

## Notes

Background images don't participate in section views — CADD From Section won't pick up an image as if it were geometry, and dimensions can't measure pixels. The image is a visual reference only.

For PDFs, only one page renders at a time; if the source PDF has multiple sheets, drop in multiple Background Image objects with different Page numbers.

Very large or high-resolution images can slow down redraw. Set Max Render Size below 3000 if you find scrolling sluggish.


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