How do reflections work?

[jwwalker]

To do dynamic reflective surfaces with Filament, possibly including planar mirrors, does one need to do anything besides set the appropriate material properties? And can you give a quick and dirty explanation of how it works?

Of course I know that rendering reflections can be accomplished using ray tracing, but Filament doesn't do that. In my previous OpenGL work, I've implemented planar mirrors by doing an extra render from the point of view of a camera reflected across the mirror, so the more mirrors are in the scene, the more renders need to be done. I'm guessing that Filament does something much smarter than that.

[romainguy]

Filament currently only offers built-in support for cubemap based reflections (via IndirectLight). We do plan on adding screen space reflections at some point. If you want to do planar reflections you can do it exactly as you described and I believe we have a sample or two showing how to do it. CC @prideout

[prideout]

@ast3rX, That's strange because the colors look the same for me, modulo the custom border effect (line 31 in mirror.mat). I've attached a screenshot that has this effect removed.

@romainguy I convinced myself to keep post processing for the offscreen view, because it might need to use features like SSAO. Let me know if that doesn't make sense.

[prideout]

Apologies, there is a color shift. I didn't see it in my screenshot until I used a tool. I think this expected since the LUT isn't perfectly invertible. For a perfect match you can disable post-processing as Romain suggested.

[romainguy]

SSAO is not part of the post processing step. Post-processing is color grading, tonemapping, dithering and bloom. You don't want to color grade/tonemap/bloom twice :p

[prideout]

I fixed the demo (#3500) but it exposes a bug, depth testing seems to be incorrect in the offscreen view.

[ast3rX]

Thanks! I'm getting this assertion which seems to confirm your assumption:

validateDepthStencilState:4140: failed assertion MTLDepthStencilDescriptor sets depth test but MTLRenderPassDescriptor has a nil depthAttachment texture