Background Images

The reference raster: the imported P&ID image (or any reference image) that sits behind your drawing. How to move, resize, fade, lock and remove it.

What the background image is

When you import a P&ID — or add a reference image from the ribbon's Import/Export group (Reference Image) — the picture is placed centred on the sheet as a dedicated background layer. It is not a drawing element: it renders behind everything, and recognition and manual tracing both work on top of it.

Sizes on the sheet are real-world millimetres: at 100% scale, one image pixel equals one millimetre.

Selecting the image

The image must be unlocked and unfrozen to be selectable (see below). Then either:

• click it on the canvas, or
• open the Layer panel (Layers button in the toolbar) and click the Background Image row's name — the row reads "Selected — drag to move, corners to resize" when active.

Once selected, drag the image to move it and drag its corner handles to resize. Mouse-wheel zoom keeps working over the image.

The Background image properties panel

With the image selected, the right panel shows Background image properties:

Dimensions

• Unit dropdown — switch every length field between Millimetres (mm), Centimetres (cm) and Inches (in).
• Width / Height — the image's size on the sheet, in the chosen unit.
• Aspect locked / Aspect unlocked — the padlock toggle beneath the size fields. Locked (default), editing width or height keeps the proportions; unlocked, each axis scales independently.
• Source resolution and Scale — read-only: the image's pixel dimensions and the current scale percentage.

Position

X / Y — the image's position on the sheet, in the chosen unit.

Appearance

• Opacity — a slider from 10–100%. Fade the raster so your placed symbols stand out over it.
• Visible — show or hide the image without removing it.
• Lock to canvas (moves with pan/zoom) — pins the image to the sheet so it cannot be dragged independently.
• Freeze (prevent select / move / resize) — makes the image completely inert: it cannot be selected, moved or resized until unfrozen. Freeze it once it is positioned correctly, so tracing clicks never grab the raster by accident.

Actions

• Reset to original size — restores the size the image was imported at, uniform and re-centred.
• Fit to sheet — scales the image to fit the drawing sheet.
• Remove image — deletes the background image (with confirmation from the Layer panel).

The Layer panel row

The Layer panel shows a highlighted Background Image row whenever a raster is loaded, with quick toggles:

Control	Action
Eye	Show / hide the image
Padlock	Lock / unlock to canvas
Snowflake	Freeze / unfreeze (tinted when frozen)
Name	Click to select the image for moving/resizing (tooltip explains if it is locked or frozen: "Unlock and unfreeze to select")
Bin	Remove the image (asks for confirmation)

Reference overlay: PDF or image with a two-point scale

For brownfield work with no CAD source, Insert → Reference image… (or Trace over a scan instead… in the CAD import dialog) puts a scan behind the drawing to trace over.

1. Drop a PDF, PNG, JPG or WebP. For a PDF, choose the page and press Load page; the page is rendered in your browser.
2. Under Scale Mode choose Two known points, click two points on the preview whose real distance you know (two nozzles, a dimensioned run) and enter that distance in millimetres. The image is scaled so the two points are exactly that far apart.
3. Set Opacity, leave Lock to canvas on once aligned, then Apply Background.

The overlay is not a symbol, so it never appears in the Bill of Materials. It is stored with the drawing (one reference image per drawing; a new one replaces it), so it is still there after a reload. If the upload fails, an error message shows the reason and the image will not survive a reload; the overlay stays visible for this session and you can apply it again.

Placement is saved

The image's position, size, opacity and lock/freeze state are saved with the drawing — reopening the drawing restores the raster exactly where you left it.