9patch AAPT: error: file failed to compile

Question

I'm trying to work with a 9patch image format but I get this error when I try to build the app:

AAPT: error: file failed to compile

Any reasons why this could happen?

Answer 1

The reason that the 9patch failed to compile was 9patch bad format - the 9patch indicating pixels wasn't fully black (#000000) but some of them was grey, fixing those pixels to pure black fixed the problem

Answer 2

Having opened the 9patch in an image editor while editing it at the same time in Android Studio can also cause this error, I guess its since AS can't write the 9patch file when compiling.

Answer 3

For me this was because the top-left pixel was black instead of transparent.