Try setting the blur option of ImageMagick(1) directly with the SCALE
object(2):
20 = SCALE
20 {
params = -gaussian-blur 3
}
Tomas Mrozek
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[1] http://www.imagemagick.org/script/command-line-options.php#gaussian-blur
[2]
http://typo3.org/documentation/document-library/references/doc_core_tsref/4.2.0/view/1/9/#id4431622