Flood fill (addition) in a voxel world. The code is ugly but should be somewhat self-explanatory.

VoxelLit.cpp

// ref https://www.seedofandromeda.com/blogs/29-fast-flood-fill-lighting-in-a-blocky-voxel-game-pt-1
// wpos: world position
// nLight: new lighting value
// skipself: chunk updating thing
void ChunkManager::lightPropagateAdd(glm::ivec3 wpos, Light nLight, bool skipself)
{
	// get existing light at the position
	LightPtr L = GetLightPtr(wpos);
	if (L)
	{
		// if there is already light in the spot,
		// combine the two by taking the max values only
		glm::u8vec4 t = glm::max(L->Get(), nLight.Get());
		L->Set(t); //*L = t;
	}
	
	// queue of world positions, rather than chunk + local index (they are equivalent)
	std::queue<glm::ivec3> lightQueue;
	lightQueue.push(wpos);

	while (!lightQueue.empty())
	{
		glm::ivec3 lightp = lightQueue.front(); // light position
		lightQueue.pop();
		Light lightLevel = GetLight(lightp);    // node that will be giving light to others
		constexpr glm::ivec3 dirs[] =
		{
			{ 1, 0, 0 },
			{-1, 0, 0 },
			{ 0, 1, 0 },
			{ 0,-1, 0 },
			{ 0, 0, 1 },
			{ 0, 0,-1 },
		};

		// update each neighbor
		for (const auto& dir : dirs)
		{
			Block block = GetBlock(lightp + dir);       // neighboring block
			LightPtr light = GetLightPtr(lightp + dir); // neighboring light (pointer)

			// add chunk to update queue if it exists
			if (Chunk::chunks[Chunk::worldBlockToLocalPos(lightp + dir).chunk_pos] != nullptr)
				delayed_update_queue_.insert(Chunk::chunks[Chunk::worldBlockToLocalPos(lightp + dir).chunk_pos]);
			
			// invalid light check (should be impossible)
			ASSERT(light != nullptr);

			// if neighbor is solid block, skip dat boi
			if (Block::PropertiesTable[block.GetTypei()].color.a == 1)
				continue;

			// iterate over R, G, B
			bool enqueue = false;
			for (int ci = 0; ci < 3; ci++)
			{
				// skip blocks that are too bright to be affected by this light
				if (light->Get()[ci] + 2 > lightLevel.Get()[ci])
					continue;

				// TODO: light propagation through transparent materials
				// get all light components (R, G, B, Sun) and modify ONE of them,
				// then push the position of that light into the queue
				glm::u8vec4 val = light->Get();
				val[ci] = (lightLevel.Get()[ci] - 1);// *Block::PropertiesTable[block.GetTypei()].color[ci];
				light->Set(val);
				enqueue = true;
			}
			if (enqueue) // enqueue if any lighting component changed
				lightQueue.push(lightp + dir);
		}
	}

	// do not update this chunk again if it contained the placed light
	if (skipself)
		delayed_update_queue_.erase(Chunk::chunks[Chunk::worldBlockToLocalPos(wpos).chunk_pos]);
}