Pictures with patterns sometimes display strangely

Question

My site is a clothing store, and my partner has complained about the following issue.

The pictures of clothing with more complex patterns (checkerboard for example) displays like this: instead of this:

I assume the other pictures are also displaying weirdly, but it's just less noticeable. As far as I can tell, it happens most often on Macs.

If anyone has any information about this phenomenon it would be much appreciated.

Answer 1

This is called the Moiré effect. From a Wikipedia article:

In physics, mathematics, and art, a moiré pattern (/mwɑrˈeɪ/; French: [mwaˈʁe]) is a secondary and visually evident superimposed pattern created, for example, when two identical (usually transparent) patterns on a flat or curved surface (such as closely spaced straight lines drawn radiating from a point or taking the form of a grid) are overlaid while displaced or rotated a small amount from one another.

In context of images the overlaying comes from anti-aliased (in case of upsampling) or averaged pixels (for downsampling).

To resize them properly use high-quality resizing such as bi-cubic interpolation based resampling. Most browser has built-in support for this but certain conditions are affecting which stratgey is selected (bi-cubic or bi-linear), for example for performance reason. The latter is more prone to this effect.

It can be reduced using a canvas to scale down the image. I have an article here on this topic and an SO answer here showing a concrete example on how to.