Filter Construction: Blur Lore

If you find yourself doing something similar to the above example, where the three components have similar radius settings, then think twice, because High Pass and Sharpen can be constructed from the already present Blur component, given the fact that both of these internally performs blurring. The difference lies in the way the blurred result is combined with the source.

In other words, if you already have Blur present, you can get a High Pass and Sharpen additionally at almost no extra computational cost.



In the above example we see how a High Pass is constructed from a blurred source. The inverted blurred source is blended 50% on the source using Normal blend mode. To get a blur result from a High Pass component, you can blend the inverted High Pass onto the source using Linear Light blendmode at 100%.

A blur radius of 2.5 in this setup resembles the output of a High Pass component with the settings Contrast = 0, Radius = 10 and Monochrome = unchecked.



If we instead want a Sharpen-like result from the blurred source, we simply blend the previous High Pass result onto the source using Linear Light blend mode at 100%.

A blur radius of 2.5 in this setup resembles the output of a Sharpen component with the settings Amount = 20, Radius = 50 and Preserve Color = unchecked.

-- Sphinx.