That is a very small reflection/refraction of some kind of strong light source such as the sun. It is a challenging scenario for Corona and other renderers. There are some possible ways of mitigating this, for example:
- enabling highlight clamping (not highlight compression!) - this will clamp the brightest pixels and in turn make them darker, which will not really solve the issue, but will make it less visible -
https://support.chaos.com/hc/en-us/articles/4528471000849-What-is-highlight-clamping-- lowering the light source's intensity, making the light source larger (e.g. lowering sun's intensity, making it bigger, using an overcast HDRI instead)
- rendering to a higher quality (allowing more time per frame)
- increasing the render resolution
- using some kind of post-production anti-flicker / denoiser in 3rd party software
The main issue is that there is some small detail (in this case strong reflection) which has similar size in the image as a single pixel, or even smaller. In one frame, Corona will "catch" the reflection appearing in one place. In another frame, in another place. Hence the flickering. The reflection would stay pretty much the same without any kind of flickering only if we allow a very long rendering time and/or render in a very high resolution.
Another idea (I don't know how well it would work, sorry) would be selecting the flickering object and using the "render selected" feature of Corona to render only this specific object to a higher quality. Then compositing it on top of the whole frame.
