Some size, image and filter and blur combinations causes artifacts or even make image completely scrambled. As far, as I see, it happens with blur values smaller than 0.25 (sometimes less) and goes worse to the point of 0 - black image. Sometimes only some values gives artifacts, like Hanning with my test image: 0.0 blur is fine, 0.1 produces artefacts.
Affected filters are e.g. Catrom, sinc, cubic, quadratic, while unaffected are e.g. Lanczos and Hanning. The problem seems to be the domain of the filter algorithms, not IMagick implementation. This image shows three filters: http://i.imgur.com/HcdwoUS.jpg
Sometimes test image could look fine, but other not, so if you are using affected filters, the 0.5 value should be safe.
This test script takes example image (you can download and use http://i.imgur.com/KsTJpFr.jpg which is affected) and creates resized images in the same directory for every filter and one of four blur values, with naming like "test.jpg.sinc.0.1.jpg" for "test.jpg" input.
<?php
$imgPath = 'imgtest/test.jpg'; // set your image file
$testBlurs = [0, 0.1, 0.2, 0.5]; // test these blur values
$im = new IMagick();
$im->readImage($imgPath);
foreach ((new ReflectionClass('IMagick'))->getConstants() as $n => $f) {
if (strncmp($n, 'FILTER_', 7) === 0) { // get available IMagick filters
$filterName = strtolower(substr($n, 7)); // extract filter name from constant
foreach ($testBlurs as $blur) {
$imSize = clone $im;
$imSize->resizeImage(500, 500, $f, $blur, true);
$imSize->writeImage(sprintf("%s-%s-%.1f.jpg", $imgPath, $filterName, $blur));
$imSize->destroy();
}
}
}
?>