COG Browser

A zero-install, in-browser viewer for Cloud-Optimized GeoTIFFs streamed from a URL — no download, no server-side tiling. It renders in the COG’s native planetary projection (any CRS via proj4), streams overviews by HTTP range request, and adds histogram equalization on the raw 16-bit DN. Everything runs client-side; the only requirement is that the COG’s host allows cross-origin reads (CORS).

Open it full-screen: S-polar CTX ↗ · Equatorial CTX belt ↗ · HRSC level-3 global ↗

Choosing a dataset

The viewer is driven entirely by URL query parameters, so a link is a view:

Parameter Meaning
?preset=<name> A built-in preset — robbins_spole (default), robbins_equatorial, or hrsc_level3.
?cog=<url> Any Cloud-Optimized GeoTIFF (repeatable for a multi-COG mosaic sharing one CRS).
&crs=<proj4> The COG’s projection as a proj4 string. Omit for COGs that carry an EPSG code; planetary COGs usually don’t, so pass it (see below).
&nodata=<value> Override the no-data value. Normally read from the COG’s GDAL_NODATA tag.

Extent, native resolution, no-data, and a default 2/98-percentile stretch are all auto-derived from the COG — a preset only needs the URL(s) and, for planetary data, the CRS.

Generating the CRS from planetarypy

Planetary COGs rarely carry an EPSG authority code, so the viewer needs the projection as a proj4 string. Generate it — never hand-write it — from the COG itself or an IAU code:

from pyproj import CRS

# from a registered dataset's IAU authority code …
CRS.from_user_input("IAU_2015:49910").to_proj4()

# … or straight from the COG (works for any body / projection)
import rasterio
with rasterio.open("/vsicurl/https://host/product.tif") as ds:
    proj4 = ds.crs.to_proj4()

This is the same resolution planetarypy.datasets uses to build a viewer link for a registered raster or a STAC item.

Working with STAC collections

A STAC collection is a discovery index, not a single raster: each item points to a COG. So the workflow composes with this viewer —

from planetarypy import datasets

coll = datasets._REGISTRY["usgs.mars.ctx.controlled_dtms"]
item = coll.at(lon=137.4, lat=-4.6, limit=1)[0]   # STAC search → a COG
# item.cog_url is a COG → open it here with ?cog=<item.cog_url>&crs=<proj4>

STAC finds the product; the COG browser shows it.

Notes

  • Auto-restretch. The min/max stretch recomputes from the current viewport as you pan and zoom (2/98 percentile of the on-screen DN), so a dark or bright region always fills the dynamic range. Switch Stretch to Manual to lock a value, or press Restretch to view for a one-shot recompute after tweaking the sliders.
  • CORS is the one hard requirement. A browser can only range-read a COG whose host sends Access-Control-Allow-Origin. The USGS asc-pds-services bucket (the Robbins presets) is open; some hosts are readable server-side (e.g. from rasterio) but not from a browser.
  • First paint can take 20–30 s on a cold start while the decode worker spins up.
  • The bright white streaks in the equatorial sample are outlier columns in the source mosaic (above the stretch max), not a viewer artifact.