Made separate rendering of the GUI and the frame for Oculus Rift (so HDR, FXAA and so did not affect the GUI). But because the GUI texture resolution is bigger than its image on the final frame, there is undersampling of texture obtained, resulting in fine details flickering. Look at the lines of GUI window frame:
The same is true for the main frame texture (space) - Oculus warp shader compresses it on the edges. So, in theory, frame and GUI texture must be rendered with mipmapping enabled. I implemented generation of mipmap levels, and... got the soap ... I tried different filters, the simplest Box gives the best results, but still very blurry:
Although Lanczos made better smoothing of broken lines. But fonts are horror... The best solution for them - Nearest Neighbor "filter" (almost equivalent to disabling mipmapping as in the first screenshot). But I can't render GUI windows and fonts separate!
Using mipmapping on the frame texture is not much affected final image quality, because first mipmap level visible only at the edges - in Oculus you must strongly squint your eyes to see this area.