image re-sizing quality
I found these notes in the image_functions.php library on the quality variable:
// Optional "quality" parameter (defaults is 3). Fractional values are allowed, for example 1.5. Must be greater than zero.
// Between 0 and 1 = Fast, but mosaic results, closer to 0 increases the mosaic effect.
// 1 = Up to 350 times faster. Poor results, looks very similar to imagecopyresized.
// 2 = Up to 95 times faster. Images appear a little sharp, some prefer this over a quality of 3.
// 3 = Up to 60 times faster. Will give high quality smooth results very close to imagecopyresampled, just faster.
// 4 = Up to 25 times faster. Almost identical to imagecopyresampled for most images.
// 5 = No speedup. Just uses imagecopyresampled, no advantage over imagecopyresampled.
I'd try updating the image $quality variable on line 64 of image_functions.php to 5 and see if that resolves the issue.
What file format are the images being uploaded in? I'd recommend trying different file types as well, for example large solid blocks of color are often rendered better in PNG format, while images with a lot of color gradients and contrast often look better in JPG format.
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