[Issue with Channel selection and mask]

Hi NotMyFault, apologies for not responding sooner - only just realised anyone had responded; had given up all hope!

Your suggestion to deactivate OpenCL worked! Thanks very much for the suggestion and the help. It's much appreciated!

 

[Issue with Channel selection and mask]

Hi,

I've having trouble getting my channel selection to make a nice mask. I've created a spare channel and got a decent selection of the cat I want but when it comes to masking it, it looks terrible. I load this as a pixel selection:

and it looks okay as a quick mask:

but then as soon as I come to mask it, it comes out like this:

What's going on here/ what am I doing wrong?

Thanks in advance, I'm sure it's me doing something stupid, so any help would be much appreciated.