Light reflections from transparent blocks are ignored.

[kpreid]

The light ray tracer currently has a structure like

if hit block has a fully opaque face {
    // use light from before the hit block, multiplied by face reflectance
} else {
    // use light occupying the hit block
}

This ignores reflection from the face when the block is transparent. Instead, we should do both simultaneously, weighting the contributions appropriately by the face alpha. The tricky part is doing this without significantly decreasing performance; so far my best attempt still adds +30% time to lighting_bench.

[kpreid]

Fixed in 1453326. I didn't find any substantial improvements to this specific part of the algorithm, but I did spend some time optimizing other aspects first.

Something that belatedly occurred to me is that lighting_bench doesn't just measure lighting calculation throughput: it measures the total time to quiescence, which will be longer when we take more blocks into account. So that 30% number isn't necessarily as bad as it sounds.