What Remove Background does
Remove Background strips out the non-symbol parts of the drawing — coloured fills, shaded zones, image bleed, the kind of thing that looks like decoration to a human but confuses template matching.
When you turn it on, the canvas redraws with backgrounds removed (or at least suppressed). The original PDF on disk isn't changed — only the working image fed to the detection pipeline.
Use it when:
- Stage 1 returns very few matches on a drawing that obviously has plenty of symbols.
- The drawing has a coloured base layer (shaded zones, hatched fills) that overlaps the symbols.
- Manual detection with Auto Count Symbol misses what your eye can clearly see.
Where the toggle lives
Remove Background shares a dropdown with Ignore Colours, accessed from the sidebar's marker list toolbar.
- 1
Open the Ignore Colours & Remove Background dropdown at the top of the marker list.
- 2
Inside the dropdown, find the Remove Background toggle.
- 3
Switch it on. The canvas re-processes — you'll see a brief loading indicator.
What gets removed
Remove Background is conservative — it targets large flat-coloured regions and gentle shading. It doesn't touch:
- The drawing's linework.
- Symbol outlines.
- Text.
- Annotations.
So you should see the same drawing minus the wash of background colour.
If specific colours are giving you trouble, you can also target them individually with Ignore Colours — those two features are designed to be used together.
When to apply it
There's no penalty for turning Remove Background on — try it as a first pass on drawings that aren't behaving. If it makes the drawing look worse, turn it off.
A common sequence:
- 1
Open the drawing. Glance at it — does it have a coloured base layer or shading?
- 2
If yes, turn on Remove Background before drawing AOIs and pressing Submit in Stage 1.
- 3
Submit and let detection run on the cleaned drawing.
- 4
If results are still poor, add specific colours to the Ignore Colours list.
Performance impact
Remove Background runs locally in your browser using OpenCV. On big drawings it can take a few seconds the first time. Subsequent toggling is faster because the cleaned image is cached.
If you find the canvas feels sluggish, turn Remove Background off — it's a perfectly reasonable trade-off on lighter drawings that don't need it.