viewport.md

Viewport

Read the article detailing deck.gl's Views and Projections system.

A deck.gl Viewport is essentially a geospatially enabled camera, and combines a number of responsibilities, which can project and unproject 3D coordinates to the screen.

Viewport classes are focused on mathematical operations such as coordinate projection/unprojection, and calculation of view and projection matrices and other uniforms needed by the WebGL vertex shaders. The basic Viewport class is a generic geospatially enabled version of the typical 3D "camera" class you would find in most 3D/WebGL/OpenGL library.

While the Viewport class can certainly be used directly if you need and are able to calculate your own projection matrices, you typically do not directly create Viewport instances. Instead, Viewport classes are created using the View class descriptors and the current viewState.

Overview of Viewport Classes

Viewport Class	Description
Viewport	The base viewport has to be supplied view and projection matrices. It is typically only instantiated directly if the application needs to work with viewports that have been supplied from external sources, such as the WebVR API.
WebMercatorViewport	While all Viewport subclasses are geospatially enabled, this class renders from a perspective that matches a typical top-down map and is designed to synchronize perfectly with a mapbox-gl base map (even in 3D enabled perspective mode).

Usage

The Viewport class is normally not instantiated directly. The View class is more commonly used by applications. deck.gl automatically creates Viewports from Views and viewState when needed, using the View.makeViewport method.

Constructor

new Viewport({width: 500, height: 500, viewMatrix, projectionMatrix, ...});

General Parameters

• x (Number, optional) - x position of viewport top-left corner. Default 0.
• y (Number, optional) - y position of viewport top-left corner. Default 0.
• width (Number) - Width of viewport. Default 1.
• height (Number) - Height of viewport. Default 1.

View Matrix Parameters

• viewMatrix (Array[16], optional) - 4x4 view matrix. Defaults to the identity matrix.

Position and Geospatial Anchor Options (Optional)

• latitude (Number, optional) - Center of viewport on map (alternative to center). Must be provided if constructing a geospatial viewport.

• longitude (Number, optional) - Center of viewport on map (alternative to center). Must be provided if constructing a geospatial viewport.

• zoom (Number, optional) - zoom level .

• focalDistance (Number, optional) - modifier of viewport scale if zoom is not supplied. Corresponds to the number of pixels per meter. Default to 1.

• position (Array[3], optional) - Position of viewport camera. Default [0, 0, 0].

• modelMatrix (Array[16], optional) - Optional 4x4 model matrix applied to position.

Projection Matrix Parameters.

• projectionMatrix (Array[16], optional) - 4x4 projection matrix.

If projectionMatrix is not supplied, an attempt is made to build from the remaining parameters. Otherwise the remaining parameters will be ignored.

• fovy (Number, optional) - Field of view covered by camera, in the perspective case. In degrees. Default 75.
• aspect (Number, optional) - Aspect ratio. Defaults to the Viewport's width/height ratio.
• near (Number, optional) - Distance of near clipping plane. Default 0.1. (Note that in geospatial viewports, this actual distance used is scaled by the height of the screen).
• far (Number, optional) - Distance of far clipping plane. Default 1000. (Note that in geospatial viewports, this actual distance used is scaled by the height of the screen).
• orthographic (Boolean, optional) - whether to create an orthographic or perspective projection matrix. Default false (perspective projection).
• orthographicFocalDistance (Number, optional) - Used by orthographic projections only. The distance at which the field-of-view frustum is sampled to extract the extents of the view box. Default 1.

Methods

equals

Parameters:

• viewport (Viewport) - The viewport to compare with.

Returns:

• true if the given viewport is identical to the current one.

project

Projects world coordinates to pixel coordinates on screen.

Parameters:

• coordinates (Array) - [X, Y, Z] in world units. Z is default to 0 if not supplied.
• opts (Object)
  • topLeft (Boolean, optional) - Whether projected coords are top left. Default to true.

Returns:

• [x, y] or [x, y, z] in pixels coordinates. z is pixel depth.
  • If input is [X, Y]: returns [x, y].
  • If input is [X, Y, Z]: returns [x, y, z].

unproject

Unproject pixel coordinates on screen into world coordinates.

Parameters:

• pixels (Array) - [x, y, z] in pixel coordinates. Passing a z is optional.
• opts (Object)
  • topLeft (Boolean, optional) - Whether projected coords are top left. Default to true.
  • targetZ (Number, optional) - If pixel depth z is not specified in pixels, this is used as the elevation plane to unproject onto. Default 0.

Returns:

• [X, Y] or [X, Y, Z] in world coordinates.
  • If input is [x, y] without specifying opts.targetZ: returns [X, Y].
  • If input is [x, y] with opts.targetZ: returns [X, Y, targetZ].
  • If input is [x, y, z]: returns [X, Y, Z].

projectPosition

Projects latitude, longitude (and altitude) to coordinates in the common space.

Parameters:

• coordinates (Array) - [lng, lat, altitude] Passing an altitude is optional.

Returns:

• [x, y, z] in WebMercator coordinates.

unprojectPosition

Projects a coordinate from the common space to latitude, longitude and altitude.

Parameters:

• coordinates (Array) - [x, y, z] in the WebMercator world. z is optional.

Returns:

• [longitude, latitude, altitude]

getBounds

Extracts the axis-aligned bounding box of the current visible area.

• options (Object, optional)
  • options.z (Number, optional) - To calculate a bounding volume for fetching 3D data, this option can be used to get the bounding box at a specific elevation. Default 0.

Returns:

• [minX, minY, maxX, maxY] that defines the smallest orthogonal bounds that encompasses the visible region.

getFrustumPlanes

Extract view frustum planes of the current camera. Each plane is defined by its normal normal and distance from the origin distance (such that point x is on the plane if dot(normal, x) === distance) in the common space.

Returns:

• {near: {normal, distance}, far: {normal, distance}, left: {normal, distance}, right: {normal, distance}, top: {normal, distance}, bottom: {normal, distance}}

import {Vector3} from 'math.gl';

// Culling tests must be done in common space
const commonPosition = new Vector3(viewport.projectPosition(point));

// Extract frustum planes based on current view.
const frustumPlanes = viewport.getFrustumPlanes();
let outDir = null;

// Check position against each plane
for (const dir in frustumPlanes) {
  const plane = frustumPlanes[dir];
  if (commonPosition.dot(plane.normal) > plane.distance) {
    outDir = dir;
    break;
  }
}
if (outDir) {
  console.log(`Point is outside of the ${outDir} plane`);
} else {
  console.log('Point is visible');
}

Remarks

• The Viewport class and its subclasses are perhaps best thought of as geospatially enabled counterparts of the typical Camera classes found in most 3D libraries.
• The Viewport class works together with the project shader module and generates the uniforms that module needs to project correctly in GLSL code.
• Accordingly, a main function of viewports is to generate WebGL compatible view and projection matrices (column-major format).
• Functions (including projection and unprojection of coordinates) are available both in JavaScript and in GLSL, so that layers can do consistent projection calculations in both GLSL and JavaScript.
• To support pixel project/unproject functions (in addition to the clipspace projection that Camera classes typically manage), the Viewport is also aware of the viewport extents.
• In geospatial setups, Viewports can contain geospatial anchors.

Source

modules/core/src/viewports/viewport.js