Gradient & Bucket Fill

Gradient and Bucket Fill are the two Paint tools that are not brushes: the Gradient is a drag and the Bucket Fill is a click. Pick either from the Paint mode's tool row. While one of them is the tool, the sidebar shows a Gradient or Bucket Fill accordion of its own, below Auto-Masking. Both write to every ticked channel in the sidebar's channel list, and both put what they paint into a new layer.

Laying a gradient

With the Gradient tool picked, drag across the model. Where you press is the start of the gradient and where you let go is the end; hold Shift to snap the line to 45 degrees. The line is measured on a plane facing you, so the gradient you get matches the drag you see on screen from any viewing angle. The gradient is laid down on every ticked channel, and Ctrl+Z undoes it.

The Gradient accordion in the sidebar sets how it is laid down:

Control	What it does
Gradient bar	Shows the gradient the tool lays down. Click it to edit it in the Gradient Editor.
Shape	How the gradient spreads from the line you drag: Linear runs along it, Radial in rings round its start, Angle sweeps round its start, Reflected is mirrored at its start, and Diamond spreads in a diamond.
Past the ends	What the gradient does beyond the end of the drag: Hold ends keeps the end colour, Repeat starts again, and Mirror runs back and forth.
Reverse	Lays the gradient down end to start, without changing the stops.
Edit Gradient	Opens or closes the Gradient Editor.

The Gradient Editor

Open the Gradient Editor with Edit Gradient or by clicking the gradient bar in the sidebar. At its top is the gradient itself, drawn over a checkerboard so transparency shows, with a track of stops on each side: opacity stops above the bar and colour stops below it.

• Add a stop — click an empty place on a track. The new stop takes the gradient's current colour or opacity at that point, so adding one changes nothing until you edit it.
• Move a stop — drag it along its track.
• Remove a stop — drag it well away from its track and let go (it disappears while a release would remove it), press Delete or Backspace, or click Delete stop. A track always keeps at least one stop.

Click a stop to select it; the rows beneath the bar edit the selected stop:

Control	What it does
Location	Where the stop sits along the gradient, 0–100%. Double-click to type a value.
Colour	A colour stop's colour.
Use brush colour	Sets the colour stop to the Paint sidebar's Color / Albedo.
Opacity	How opaque the gradient is at an opacity stop. Double-click to type a value.
Delete stop	Removes the selected stop. Unavailable when it is the last stop on its track.

Below those, the controls that apply to the whole gradient:

Control	What it does
Blend	How colours mix between stops. Perceptual (Oklab) keeps the middle of a blend as bright and even as its ends; Linear is the classic straight mix; Constant makes hard steps at each stop.
Shape, Past the ends, Reverse	The same settings as in the sidebar's Gradient accordion — change them in either place.
Flip stops	Mirrors every stop, so the gradient itself runs the other way. Unlike Reverse, this changes the stops.
Distribute	Spaces the stops of each track evenly from end to end.

Presets

The Presets section lists gradients as swatches; hover one to see its name and click it to load it into the editor. A set of built-in presets is always there — among them Black to White, Black to Transparent, White to Transparent, Spectrum, Sunset, Fire, Ocean, Copper, Chrome, Viridis and Grime.

To save your own, type a name into New preset name and click Save. Saving under a name you have already used replaces that preset rather than adding a second one. Right-click one of your saved presets and choose Delete preset to remove it; the built-in presets cannot be deleted.

The current gradient and your presets are saved automatically to gradients.txt beside the app as soon as you finish an edit, so they are still there next time you start Vertexix.

Bucket Fill

With the Bucket Fill tool picked, click the model. The area of the colour under the cursor is filled with the ticked channels. The area is found on what you see — the material's combined layers — and the fill stops at the edge of a UV island rather than running out across the gaps between islands.

Control	What it does	Range	Default
Fill a Drawn Shape	Instead of filling a colour you click, drag an outline over the model. When you let go, everything inside it that faces you is filled with the ticked channels.	—	—
Tolerance	How different a colour may be from the one you click and still be filled, per channel on a 0–255 scale. 0 fills only that exact colour. Double-click to type a value.	0–255	32
Contiguous	On: fill only the matching area joined to where you click. Off: every matching texel of that material's image, wherever it is.	—	—

Tolerance and Contiguous are hidden while Fill a Drawn Shape is on — a drawn shape fills everything inside the outline, whatever its colour.

Fills and layers

Every gradient and every fill goes into a new layer of its own — named Gradient, Bucket Fill or Shape Fill — on each material it reaches, rather than being written into the image underneath. To take one off later, delete its layer; to soften it, lower the layer's opacity. Straight after the fill, Ctrl+Z removes it in a single step.

Both tools respect the brush's Auto-Masking settings. If nothing appears, check the channel list: a channel has to be ticked, and the material needs an image for that channel already — the status bar tells you when it has none.