How to cut out an illustration whose lines are the same colour as the background
Tell Vizard Agent that the drawing's own lines share the background colour, and ask it to select the background by what connects to the edge of the frame rather than by colour. A flood fill inward from the border removes the empty field and cannot reach the dark strokes inside the artwork, which is exactly the separation a threshold cannot make.
What do you type into Vizard Agent?
Tell Vizard Agent why a threshold is going to fail. Without that sentence the obvious approach is a colour key, and a colour key is the one method that cannot work on this image. Name the method you want, and ask for a preview on a light checkerboard, where leftover background actually shows.
Prompt
What does Vizard Agent actually do?
It stops asking which pixels are black and asks which black pixels are reachable from outside the drawing. The ones inside a closed outline never are. The session was a real illustration that had to be delivered as a transparent overlay, and a threshold was tried first and found to be a compromise either way. In the session behind this article it:
- Measured the background, tried threshold levels that might keep the darkest strokes, and previewed the result with the background made transparent.
- Checked the transparent edge against a light background and built a preview on a light checkerboard, where every leftover pixel of black is obvious.
- Tuned the threshold and found it still lost detail.
- Built the mask from the black region that touches the frame edge, then checked that it only removed empty field.
- Separated large empty areas from small dark details in the artwork, since a small isolated patch inside the drawing is shading, then rebuilt the mask keeping the details and clearing the field.
When does this not work well?
Connectivity fails where the artwork is open. If the outline has gaps, the background flows through them and takes the interior with it, so the gaps have to be closed or the region handled separately. Soft or feathered edges leave no clean boundary to stop at, a gradient background defeats both methods, and artwork touching the frame edge is connected to the outside.
Common questions
Why does a colour key fail here? Because the drawing and the background are the same colour. Vizard Agent selects by position instead.
What if the outline has gaps? The fill leaks. Vizard Agent finds the gap and closes it first.
Why a checkerboard preview? Leftovers are invisible against the colour you removed, so Vizard Agent shows the cutout on a light field.