RFC - Viewport Support for Infovis

• Author: Ib Green
• Date: Dec 7, 2016
• Status: Approved and Implemented

Notes:

• Implemented (with a different class hierarchy) as part of deck.gl v4.
• This was the first deck.gl RFC.

Background

Problems with Current Viewport

• The current Viewport class used in deck.gl v3 is mapping/geospatial focused and not directly suitable for generic infovis.
• Current Viewport always expects longitude and latitude arguments, which make no sense for non-map related visualizations
• Current Viewport view must be created with lng,lat,zoom,pitch,bearing,altitude parameters, which are natural when a “ground” plane is clearly defined by a map, but may be restrictive for general 3D visualizations (e.g. embeddings as 3D point clouds). Makes it harder to write interfaces (event handling).
• Current Viewport assumes that positions (i.e x,y,z coordinates in deck.gl layers) are in mercator coordinates or meter offsets. Disregarding complications of the initial non-linear part of the mercator transformation, this currently builds in assumptions about “world size” (512 * zoom ^ 2) that just doesn’t fit a generic data set.

Other Design Considerations

• When designing the viewport, it is tempting to just pick a coordinate system that is most elegant or natural (e.g. 0-1). But we should at least sanity check floating point precision implications of such a choice in the shaders.

Proposal: Create a Viewports class Hierarchy

A possibility is to split current viewport class into 3 classes. The idea is that any of these viewports can be used with deck.gl (and they will share some code), but they provide different interfaces to the application:

• BaseViewport - completely generic and linear, works with matrices
• ScaledViewport - generic and linear, but calculates view and projection matrices for center/zoom/...
• MercatorViewport - understands lng/lat/zoom/pitch/bearing

BaseViewport

// BaseViewport works in a [0,1] world system. Any standard matrices can be used.

new BaseViewport({
    // Window width/height in pixels (for pixel projection)
    width,
    height,

    // Defines the coordinate system
    worldMatrix,

    projectionMatrix,
    viewMatrix,

})

ScaledViewport

// ScaledViewport takes center, zoom, pitch, bearing etc parameters
// Calculates ViewMatrix and ProjectionMatrix from these
// Still allows app to set worldMatrix and modelMatrix

new ScaledViewport({
    // Window width/height in pixels (for pixel projection)
    width,
    height,

    worldMatrix,

    // Current center (in 0-1 coordinates? Or worldMatrix coordinates)
    centerX,
    centerY,
    zoom,
    altitude,
    pitch,
    bearing,
})

MercatorViewport

MercatorViewport adds non-linear WebMercator project/unproject.

new MercatorViewport({
  lng, lat, zoom // converted to centerX, centerY, scale
})

Other considerations

Model Matrix support

Proposal : Add modelMatrix parameter to the BaseViewport.getMatrices call. The viewport will right multiply in the modelMatrix into the combined matrices. This would work for all Viewport classes.

Implications on Shaders

The viewport interacts with the deck.gl shaders mainly through uniforms.

References

viewport-mercator-project