... dialog.5.1
  Actually, the GIMP preferences can be set so that each painting tool has individual access to the Mode menu. This is set by toggling off the Use Global Paint Options checkbox in the Tool Options branch of the `Preferences` dialog. Doing this makes the Mode menu available in the `Tool Options` dialog for each painting tool.

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... 20.8.1
  Gaussian Blur (IIR) and Gaussian Blur (RLE) produce exactly the same results. However, Gaussian Blur (RLE) runs faster on images that have broad constant-valued pixel regions because it exploits a compression technique known as run length encoding. For photographs of real world scenes, Gaussian Blur (RLE) provides no advantage and might even run slower than Gaussian Blur (IIR).

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