Gimp fill selection with image

This tool fills a selection with the current foreground color. If you Shift+click and use the Bucket tool, it will use the background color instead. Depending on how the tool options are set, the Bucket Fill tool will either fill the entire selection, or only parts whose colors are similar to the point you click on.

The tool options also affect the way transparency is handled.

Gimp bucket fill gradient

The amount of fill depends on what Fill Threshold you have specified. The fill threshold determines how far the fill will spread (similar to the way in which the magic wand works). The fill starts at the point where you click and spreads outward until the color or alpha value becomes “too different”.

When you fill objects in a transparent layer (such as letters in a text layer) with a different color than before, you may find that a border of the old color still surrounds the objects. This is due to a low fill-threshold in the Bucket Fill options dialog.

With a low threshold, the bucket tool won't fill semi-transparent pixels, and they will stand out against the fill

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