ThinkGeo Raster Sampling Logic and Behavior Matrix¶

Last Updated: February 24, 2026
Validated Against: thinkgeosdk commit 0918c8b2f

1. v14 Preset Behaviors (v14.4.4 Summary)¶

In v14.4.4, raster sampling followed a mostly path-driven model.

SKFilterQuality.High in v14 was adaptive in practice:

1. Upsample: bicubic-style filtering (Mitchell family behavior).
2. Downsample: engine-level optimization that could apply mip-like behavior.
3. Many map-related render paths were effectively forced to high-quality filtering.

2. v15 DrawingQuality Presets (Current)¶

In current v15 code, DrawingQuality maps to RenderingOptions and only controls anti-aliasing flags.

Sampling is configured independently (Section 3) and is not controlled by DrawingQuality.

3. Current Resampling Architecture¶

The runtime is not a single pipeline. There are two entry paths.

3.1 Layer-Based Path (LayerBase)¶

Used by most async/online/tile/vector layer draw paths:

Layer RasterContentType
  -> LayerBase.GetRasterResamplingMode()
  -> RasterResamplingMode
  -> SkiaSamplingHelper.Resolve(...)
  -> SKSamplingOptions

LayerBase.GetRasterResamplingMode() mapping:

3.2 RasterSource-Based Path (RasterLayer)¶

RasterLayer draw flow uses imageSource.GetResolvedMode() instead of LayerBase.GetRasterResamplingMode().

For GeoTiffRasterSource and SkiaRasterSource:

1. If ResamplingMode != Auto, use it directly.
2. Else if GetRasterContentType() == CategoricalRaster, use NearestNeighbor.
3. Else return Auto.

Important consequence:

• In this path, cartographic content does not automatically switch to CartographicAuto; it remains Auto unless explicitly set.

3.3 WPF Raster Path (WpfRasterSource)¶

WpfRasterSource uses WPF BitmapScalingMode for resize operations:

• Explicit ResamplingMode wins when set.
• If Unspecified, default by content type:
• CategoricalRaster / CartographicTiles -> NearestNeighbor
• Imagery / ContinuousRaster -> Linear
• Unspecified -> Unspecified

This path uses WPF scaling (RenderOptions.SetBitmapScalingMode) rather than Skia sampling options.

3.4 Skia Scale-Adaptive Rules¶

SkiaSamplingHelper.Resolve(...) behavior:

4. How Content Type Is Determined (Current Code)¶

4.1 Feature Layers¶

• FeatureLayer: scans styles; if any HeatStyle exists -> ContinuousRaster; otherwise -> CartographicTiles.

4.2 Internet and Cloud Layers¶

Dynamic Imagery vs CartographicTiles by map/style type:

• GoogleMapsAsyncLayer
• BingMapsAsyncLayer
• HereMapsRasterTileAsyncLayer
• MapBoxStaticTilesAsyncLayer
• ThinkGeoRasterMapsAsyncLayer
• AzureMapsRasterAsyncLayer (also has weather tile sets mapped to ContinuousRaster)

Always cartographic:

• OpenStreetMapAsyncLayer
• ThinkGeoVectorMapsAsyncLayer

4.3 Protocol Layers¶

Default to CartographicTiles:

• WmsAsyncLayer
• WmtsAsyncLayer
• ArcGisServerRestAsyncLayer
• ArcGisServerRestVectorAsyncLayer
• WfsV2AsyncLayer

4.4 Raster Sources¶

• GeoTiffRasterSource, SkiaRasterSource, and WpfRasterSource currently do not override GetRasterContentTypeCore(), so default content type is Unspecified unless subclassed.
• Their effective behavior therefore depends on explicit ResamplingMode and/or source-specific fallback logic.

5. Practical Override Patterns¶

5.1 Override Layer Content Type¶

public class MyFeatureLayer : ShapeFileFeatureLayer
{
    protected override RasterContentType GetRasterContentTypeCore()
        => RasterContentType.CartographicTiles;
}

5.2 Override Raster Source Content Type¶

public class MyGeoTiffSource : GeoTiffRasterSource
{
    public MyGeoTiffSource(string path) : base(path) { }

    protected override RasterContentType GetRasterContentTypeCore()
        => RasterContentType.CategoricalRaster;
}

5.3 Force an Explicit Resampling Mode¶

var layer = new GeoTiffRasterLayer("data.tif");
layer.ResamplingMode = RasterResamplingMode.CartographicAuto;

Use explicit mode when you need deterministic behavior independent of inferred content type.