Weird behavior of multiple shadow-casting spot lights

[jwwalker]

I have a test program with 3 shadow-casting spot lights, identically configured except for intensity. If I have spotlight 1 set to high intensity (1000000 lumens) and spotlights 2 and 3 turned way down to 0.1 lumens, I get a good shadow. Same if spotlight 2 is bright and 1 and 3 are dim. But if spotlight 3 is bright and 1 and 2 are dim, it casts no shadow. At that point, one might think there is something wrong with spotlight 3. But if I then remove spotlight 1 and/or spotlight 2 from the scene, there's a shadow. Does this make any sense to anyone? If not, I'll try to construct a proper bug report.

[romainguy]

There is a limit to the number of spot lights that can cast shadows at the same time in the frustum. @bejado can give you the details. It might just be that we cast shadows for the first 2 spotlights in the scene so 3, even when bright, doesn't get one. We've been thinking of giving priority to lights closer to the camera or the brighter lights (it should really be a combo of both).

[jwwalker]

Ouch. If there can be only 2 shadow-casting spot lights at the same time in the frustum, that would kill my evil plot to simulate a shadow-casting point light using 4 shadow-casting spot lights.

[jwwalker]

@bejado Is that correct that you can't have more than 2 shadow-casting spot lights visible at a time? I was hoping to use Filament to replace OpenGL in my product, but this shadow limitation might be a deal-breaker.

[bejado]

You can also try building the bjd/spot-shadow-varyings branch, which removes all varyings related to spot shadows. If it still doesn't work for you, then something else must be going on.

[jwwalker]

@bejado Thanks for checking. I built the spot-shadow-varyings branch and still had problems. I guess it's time for me to start trying to build a minimal example.

[bejado]

You could also try to reproduce in our gltf_viewer sample, or use it as a starting point.

[jwwalker]

@bejado OK, I figured out what I was doing wrong. Although I was linking with libraries built with CONFIG_MAX_SHADOW_CASTING_SPOTS = 6, I was baking the material using an old build of matc.

If the number of shadows could be made more configurable, that would be great. Even 6 isn't really enough for my purposes. I suppose I could consider a multi-pass approach where I render to textures with different batches of lights, and then add the images together, but that would be ugly.

[bejado]

Yes, one of our goals is to make the number of allowed spot shadows configurable.