Blue sky and 8 bit can easily give you banding. Here is a suggested workflow. I'm assuming you used GoPro Studio to import your footage and convert it to the GoPro cuneiform.
Import the 60fps footage into AE. Select the footage in the project panel and go to footage interpretation (right click or File>Interpret Footage>Main) and set the frame rate to 29.97 (standard for video). Then select the footage in the Project panel and create a new comp from selected.
Now set your Project to 16 or 32 bit and make the adjustments you want to make to the footage. This will preserve the original GoPro footage at the highest quality possible until you are ready to render. The last step would be to use the AME and one of the standard YouTube or Vimeo presets to create your final render for distribution on the net. If the AME adds banding you can try increasing the data rate to the maximum suggested by Vimeo or YouTube, or you can jack it way up if you just want to play the videos on your computer. If you are still having banding problems then I would return to AE and add a little film grain to the go pro footage. The grain will tend to hide the 8 bit banding problems in the sky.
If you are going to need to do further editing or processing of your videos I would suggest that you render a DI (digital intermediate) to a lossless or nearly lossless 10 bit or better codec. You could also render to a 16 or 32 bit image sequence. Keep things 10 bit or better until you are ready to render the final compressed product for delivery to your audience.
Many thanks Rick! I'm afraid I didn't do it right from the start. I never used Gopro Studio, just downloaded mp4's straight from the camera. I'm downloading studio now to try cineform, then I will follow your advice.
It perfectly makes sense to use 16/32bit during editing. I use similar settings in the Photoshop when I edit images - 16bit/Prophoto profile and then save at 8bit/adobeRGB. I see the same logic applies to the AE as well.
Thanks a lot!
I followed your advice and got better results instantly. Definitely some compression was the result of using mp4 files and 8bit editing. Thanks again! However, the banding was less, but still present. I had to add grain effect. Using 0,35 intensity and 0,65 size did the trick. It's a bit noisier but smooth. I tried 4-color gradient method but it didn't help. It produced slight colour cast and didn't reduce banding. Adding grain is a heavyweight for the rendering engine though.
I have created several pieces of stock footage that simulate film grain. It's basically just noise on a black background. The effect is very subtle. Most of the time all I have to do is drop my stock footage into the top of the comp and set the blending mode to screen and make an opacity adjustment. If you have a bunch of this to do I would suggest you try doing the same. It doesn't take long to render out a 5 second loop of grain.
Thanks. Rather than generating the real noise, you used the footage of the noise. It's a good solution.