Configuring Display Options

Display settings can be configured below the item display.

Geostationary satellite imagery can be displayed in its original projection or with a projection transformation applied to fit the currently selected map projection. The former is referred to here as pixel matrix display, while the latter is referred to as geolocated display. SIFT implements two kinds of geolocated display: the so-called tiled geolocated image display and the simple geolocated image display.

The kind of display used for image data (data with an AreaDefinition in Satpy terms) is controlled by setting the display.image_mode configuration to one of three options:

  • simple_geolocated: In this mode the rasterisation of image data to the screen pixels is done entirely on the GPU. Technically, the image is placed as a texture on a coarser grid (a regular subdivision of a quad). This is then re-projected, resulting in the appropriate distortion of the texture to resemble the map projection. The accuracy of this kind of projection can be controlled by setting the display.grid_cell_width and display.grid_cell_height options, which specify the size in metres of the subdivision grid cell containing the sub-satellite point. The smaller the value, the finer the grid and the more accurate the projection, but the more time and memory consuming the projection will be.

    As long as the projection of the data is exactly the same as the currently selected display projection the result is the same as when using the pixel matrix display.

  • tiled_geolocated: The image is broken into smaller pieces (called tiles), which are resampled to currently 512x512 pixels each before being passed to the GPU, which does the final rasterisation to screen pixels. The advantage of this is that images that exceed the texture size limitations of the GPU can still be rendered.

  • pixel_matrix: The image is only scaled and moved when zooming in and out or panning, but otherwise no map projection is applied. This ensures that the original data can be examined without any resampling artefacts. Changing the current map projection has no effect on the visualisation of the data, so the coastline and latitude/longitude grid overlay would no longer fit the data. Also, loading data with different projections is usually not useful with this setting.

Currently the modes are mutually exclusive and only one mode can be used during the same SIFT session. By default, simple geolocated image display with a subdivision grid resolution of 24000 metres is active, as in this configuration:

display:
  image_mode: simple_geolocated
  grid_cell_width: 24000
  grid_cell_height: 24000