Thanks for the replies.

I suspect the first post hit the nail on the head, I use a fairly old toughbook in the lab as it has a bigger screen than the newer ones which is easier on the eyes. I re-installed on a newer laptop and the supplied one works.

The bad news is that the supplied astcenc is doing exactly the same.

I'm suspecting the problem is that this application is for custom dials so solid lines and text with sharp edges and high contrast. Not ideal for compressed graphics but there are 2.2MB in ARGB555 format. I was assuming I could display direct from flash as stated in the data sheet however when searching forums to solve the issues it has become apparent this is not the case. There should be a warning in the data sheet than only a few small graphics can be displayed direct from flash.

The workaround at the moment is to display the one causing major issues, a large needle with rotation applied, in ARGB555 which takes up 600K and then use higher compression on the remainder and crop the bottoms of the images to hide the artifacts. The customer is complaining that the graphics are looking rather fuzzy like this though.

Are there any suggested optimisation switches for astcenc to make it more suitable for this type of image ?

The only other route I can try is uncompressed / palleted to reduce the image size since there aren't a great deal of colours. Not a route I want to go as it will be more work and restrictive for future changes.

I'm under an NDA so can't post the images on here. It may be I can create something similar myself which shows the same problem.