I imagine the shading points for progressive photon mapping would be stored in a kd-tree, analogously to the photons in standard photon mapping? Looping through a linear-format framebuffer for each splat to find relevant shading points seems like it would be a performance disaster.
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Yes--a spatial data structure is definitely in order!
The approach that many folks seem to have settled on is to store the visible points in a 3D grid; then each photon just needs to use the grid to find the points that it may be applicable to.
Check out the Lux render implementation for details:
A few things to note from that one:
They're able to have a fairly memory-efficient grid, even with small voxels, via a hash table--they hash grid coordinates (x,y,z) and then only store data for the voxels that are occupied.
Points are stored by expanding their bounding boxes by the maximum photon search radius and then adding them to all of the voxels that they overlap. Then, given an incoming photon, the photon just needs to be hashed to a single voxel for it to find all of the candidate hit points. Since there will in general be many more photons than visible points, this approach is a bit more efficient...