After many trials I finally got what I wanted for the specularity on the butterfly. It shimmers a hint of the thousands of tiny short hairs that coat a butterfly’s wings. My first try I used the noise filter in Blender as a spec pass; what I didn’t realise was that this animates, making the wings look like they had film grain. When I painted a texture, paying more attention to details, it didn’t really work as the light needed to be quite high for there to be any effect and in the actual scene the wings had a flat specularity. After playing with the setting and discovering that I could map the image to “colour specularity” I got the shimmer I wanted. Plus the bloody thing doesn’t sparkle.
Original noise for spec
Image crafted spec