PHP: How to detect if a jpeg is completely black (no image)?

Question

I built a community website where the users upload their photo and they can crop the thumbnail, ala Facebook to be clear.

But for some reason some of them generate a blank (actually black jpeg thumbnail).

I tried different solutions but it sounds like this is happening with big images or maybe in computers where JS is not enabled? I don't know...

Bottom line, since the number of users with this is very tiny, I was thinking of creating a patch: detect when the user generates a blank jpeg.Then I would be able to warn them.

Do you know how to do it?

Answer 1

Resize it to a 1x1 PX image. Then check if the one pixel is black. Not perfect, but if there is a significant amount of non black in the original, that 1px will not be black. The benefit is that it will be fast.