Quality loss on PNG resize

[ThierryGD]

Hi there,

I've been resizing some .png and got some pretty visible loss of quality. I attached those for comparison - around the Titanic (I know, they're big).

I tried to find similar threads but couldn't find anything that helped - at least it made me install Horsts IMagick Image Sizer.  

          foreach($detailrp->detail_repeater_bild as $pRetinaBild) {
            if ($pRetinaBild->width > $pRetinaBild->height) { // 3320px wide
              $pHighBild = $pRetinaBild->size(1660, 0);
              $pLowBild = $pRetinaBild->size(830, 0);
            }
            else { // 3220px high
              $pHighBild = $pRetinaBild->size(0, 1610);
              $pLowBild = $pRetinaBild->size(0, 805);
            }

Anyone has any idea what causes this and how to solve it? Or should I just go with .jpg? Thx!

[JAMES77]

Hey there,

 So you've been resizing some .png and got some pretty visible loss of quality? And now you want to solve this problem by installing Horsts IMagick Image Sizer? I think that such extensions can help you. Reviews are quite positive. I personally have Mac and edit photos via Movavi Photo Editor for Mac. I've never faced any problems with quality loss. It also helps to fix a low-quality areas. 

[horst]

Just to clarify: the png format has no quality loss. But the GD lib has massive issues in resizing dark image parts. 

With such dark images, you must set the imagesizer defaultGamma option to -1 !!

If your server supports imagick, you can activate the ImageSizerEngineIMagick, that is part of processwires core. (Under modules core).

[neophron]

Recently I discovered this plugin: https://github.com/BlowbackDesign/TinyPNG
I found it on this website: https://tinypng.com

I don't know, what and how they are doing, but the size reductions is sometimes more then 50%. 

[horst]

,@neophron, I think this module and the service behind it has nothing to do with what was called "quality loss" in the above posts. It is a good method to compress your (final) png image variations to get smaller filesizes. But maybe nowadays you can use WebP format directly in PWs core, instead to stick with png output, and than having to invoke a remote service?