MS RFC 97: Dynamic creation of higher zoom levels tiles

Author:Thomas Bonfort
Version:MapCache 1.2

1. Overview

When tiling raster data, it is common for the native resolution of the original data to be somewhat/much lower than the resolutions of the higher zoom levels of the standard GoogleMapsCompatible or WGS84 grids. Users whishing to serve low resolution data at high resolutions are currently forced to use one of these approaches:

  • setting a <grid maxzoom=”15”> attribute in their tilesets, meaning that all zoom levels higher than the maximum configured one are considered unavailable. Upsampled tiles or images for the higher zoom levels can then be obtained through WMS calls only, tile requests will fail.
  • not setting a maxzoom parameter, and therefore caching all tiles at all zoom levels, which is inefficient both in terms of storage on the mapcache instance, and in terms of load set on the original WMS server when initially requesting images before splitting them into tiles.

This RFC proposes to add the concept of “maximum cached level” for a given tileset and a given grid. In practice, tiles above the configured maximum cached level are still transparently available for an end user or when vertically merging multiple tiles, however they are dynamically upscaled from the maximum cached level instead of being cached on the mapcache instance (after having been requested once from the source WMS service).

2. Proposed solution

In order to be available for all tile operations, the mapcache_tileset_tile_get function will detect calls that are over the maximum cached level and fall back to a new function who’s responsibility is to treat this special case. Two configurations will be available for this “over the maximum cached zoom” scenario:

  • Reconstruct the requested tile from the lower zoom level, by:
    • determining the one or four tiles of the maximum cached that interect the extent of the requested tile.
    • calling mapcache_tileset_tile_get once again, this time with (x,y,z) adapted to have z=max-cached-level (to avoid infinite recursion).
    • decoding the image data of the lower zoom level tile(s).
    • upscaling the decoded image data into the final tile.
    • the upscaled image data will not be encoded straight away, as one of the common use-cases for this functionality will be when overlaying multiple tilesets (i.e. access to the raw pixel data will happen)
  • Proxying the requested tile to the source WMS server, thus bypassing mapcache’s caching mechanism entirely (this will not be initially implemented):
    • construct the extent of the tile, eventually account for the metabuffer (but not the metatile size).
    • send to the source wms.
    • eventually decode data if using metabuffers or if the requested tile is to be a vertical assembly of multiple tilesets.

Activating this functionality in the configuration file is done for each grid referenced by a tileset:

  <!-- ... -->
  <grid max-cached-zoom="12" out-of-zoom-strategy="reassemble">WGS84</grid>
  <grid max-cached-zoom="12" out-of-zoom-strategy="proxy">g</grid> <!-- not implemented -->
  <!-- ... -->

3. Implementation Details

Two new functions will be added, and will produce a tile’s image by either upscaling lower level tiles or directly querying the source wms. They will be called from the mapcache_tileset_tile_get() function when such a behavior has been requested in the configuration file.

3.1 Files affected

  • mapcache.h
  • configuration_xml.c
  • tileset.c
  • mapcache_seed.c

3.2 Backwards Compatibility Issues

None expected, new functionality

3.3 Performance implications

When upscaling tiles, each tile request will imply at least one image decompression and one image compression, along with bilinear resampling on the image data. Servicing upsampled tiles will therefore be orders of magnitude slower than directly serving them from a cache.

It should be possible to deactivate bilinear resampling for users wanting better performance at the cost of degraded output quality. This could be addressed in a later phase should that need arise.

3.4 Caveats

  • Bilinear resampling fails inside pixman for extreme resampling scales (i.e. when a requested tile spans only a couple of pixels in the max-cached-zoom tile). See For these cases, we switch to nearest-neighbour resampling, i.e. we set the tile to a uniform color.
  • Bilinear resampling will produce slightly visible artifacts at the edge of the max-cached-zoom tile. This is due to the fact that data from adjacent tiles would be needed in order to produce a seamless upscaled version.

5. Bug ID

6. Voting history

+1 from ThomasB, StephanM, MikeS, TomK, JeffM, DanielM, PerryN and SteveW