Fill

The Fill tool flood-fills a contiguous area with the primary color, starting from where you click.

Drag to adjust the threshold

The fill lands as soon as you press, but the gesture isn't over until you let go. Keep holding and drag sideways to change how far it spreads: right fills more, left fills less. The area is refilled as you move, so you can settle on the right amount by eye instead of guessing at the Tolerance slider first. The percentage shows on canvas while you drag, the slider follows along, and the value you land on is kept for the next fill.

The whole gesture is one undo step, no matter how much you moved.

Smart Edges

Anti-aliased line art has a band of half-blended pixels along every contour. A plain flood fill stops at the first of them, which leaves the pale fringe you get in most paint programs and no amount of Tolerance fixes it - raise it far enough and the fill jumps the line entirely.

Smart Edges handles that band without any settings. From the edge of the region it walks outward while each step lands further from the color you clicked

• which is exactly the shape of an anti-aliasing ramp - and stops at the ramp's peak, the middle of the line. Crisp outlines end the walk after one pixel, soft ones after several, and it cannot cross to the far side of a line because that is where the ramp starts falling again.

How those edge pixels are painted depends on what they held:

• Clicked on empty canvas - the fill goes in underneath the existing pixels. The line keeps its own anti-aliasing and the fill boundary inherits it, so the edge comes out as smooth as the line you drew.
• Clicked on a solid color - the edge pixels are a mix of that color and the line, so the fill takes over exactly the share that belonged to the color you clicked and leaves the line's share alone. Same blend, new color, no staircase.

Where the line art sits in the layer stack is taken into account too. Line art on the layer you are filling, or on a layer above it, blends the fill for you, so the fill goes in solid and lets it. With the line art below the fill layer there is nothing to do that, so the fill carries the soft edge itself rather than painting over the line.

Options

• Tolerance - how far the fill spreads across neighbouring colors. Low tolerance stays within tight edges, higher tolerance bleeds across softer boundaries. With Smart Edges on it no longer has to be tuned for outlines, only for how much variation counts as "the same color" - and dragging during a fill is usually quicker than setting it here.
• Smart Edges - the edge handling described above. Turn it off for a plain hard-edged flood fill.
• Use all Layers - decide where the fill spreads from the composite of every visible layer, while still painting into the active one. This is what lets you color on a separate layer from your line art.

If a selection is active, the fill is clipped to it - including the edge walk.

A fill lands on the layer the moment it is worked out; the outward sweep you see afterwards is just presentation, and it is skipped while you are dragging the threshold. Undo takes the fill back, and clicking again straight away starts a second fill without disturbing the first.