3.11. Convolve (Blur/Sharpen)
Convolve tool icon in the Toolbox
The Convolve tool uses the current brush to locally blur or
sharpen your image. Blurring with it can be useful if some element
of your image stands out too much, and you would like to soften
it. If you want to blur a whole layer, or a large part of one,
you will probably be better off using one of the Blur Filters. The direction of a
brushstroke has no effect: if you want directional blurring, use
the Smudge tool.
In "Sharpen" mode, the tool works by increasing the contrast where
the brush is applied. A little bit of this may be useful, but
overapplication will produce noise. Some of the Enhancement Filters, particularly
the Unsharp Mask, do a
much cleaner job of sharpening areas of a layer.
You can create a more sophisticated sharpening brush using the
Clone tool. To do this, start by duplicating the layer you want
to work on, and run a sharpening filter, such as Unsharp Mask,
on the copy. Then activate the Clone tool, and in its Tool
Options set Source to “Image source” and Alignment to
“Registered”. Set the Opacity to a modest value, such as
10. Then Ctrl-click on the copy to make it the source image. If you
now paint on the original layer, you will mix together, where
the brush is applied, the sharpened version with the unsharpened
Both blurring and sharpening work incrementally: moving the brush
repeatedly over an area will increase the effect with each
additional pass. The Rate control allows you to determine how
quickly the modifications accumulate. The Opacity control,
however, can be used to limit the amount of blurring that can be
produced by a single brushstroke, regardless of how many passes
are made with it.
The Convolve tool can be activated from an image menu as
or from the keyboard using the shortcut V.
-> -> ;
from the Toolbox by clicking on the tool icon
See the Brush Tools
Overview for a description of key modifiers that have the
same effect on all brush tools.
Tool Options for the Convolve tool
See the Brush Tools Overview
for a description of tool options that apply to many or all brush
mode causes each pixel affected by the brush to be blended with
neighboring pixels, thereby increasing the similarity of pixels
inside the brushstroke area. Sharpen
mode causes each pixel to become more different from its neighbors
than it previously was: it increases contrast inside the
brushstroke area. Whatever setting you choose here, you can
reverse it on-the-fly by holding down the Ctrl key.
slider sets the strength of the convolve effect.