How TE calculates the zoom level of a map

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In the two-dimensional map, the map is displayed at different levels, and the map elements displayed at each level are not exactly the same. In the three-dimensional map of TE, the terrain data is also displayed at different levels. However, the API of TE does not provide the level to which the current field of view is displayed, only SGWorld.Terrain.Levels is provided, and the total number of levels of 3D terrain can be obtained, probably because in the three-dimensional view. Different places may display different levels of maps.

An example of a hierarchical slice of a 2D map:

However, a ready-made tool is provided in TE to calculate the vector layer loaded in TE and set the optimal block width, according to which the vector can achieve the best performance in TE.

This feature is a web page that is embedded in TE. By analyzing the code in it, the current zoom level of TE can be calculated vaguely. The following is a brief analysis according to the code:

Function init ()

{

/ / the block width value positioned in initialization can be used as an indication of the level, but the numbering order defined below should be reversed. The higher the level, the smaller the width of the block.

RealNumLevels = SGWorld.Terrain.Levels

AddLevel (22, "40075" + SGLang.i18n ("Text7"))

AddLevel (21, "20037" + SGLang.i18n ("Text7"))

AddLevel (20, "10018" + SGLang.i18n ("Text7"))

AddLevel (19, "5009" + SGLang.i18n ("Text7"))

AddLevel (18, "2504" + SGLang.i18n ("Text9"))

AddLevel (17, "1252" + SGLang.i18n ("Text7"))

AddLevel (16,626 + SGLang.i18n ("Text10"))

AddLevel (15,313 + SGLang.i18n ("Text11"))

AddLevel (14,156 + SGLang.i18n ("Text7"))

AddLevel (13, "78" + SGLang.i18n ("Text12"))

AddLevel (12, "39" + SGLang.i18n ("Text13"))

AddLevel (11, "19" + SGLang.i18n ("Text7"))

AddLevel (10, "9783" + SGLang.i18n ("Text14"))

AddLevel (9, "4891" + SGLang.i18n ("Text15"))

AddLevel (8, "2445" + SGLang.i18n ("Text8"))

AddLevel (7, "1222" + SGLang.i18n ("Text16"))

AddLevel (6,611 + SGLang.i18n ("Text17"))

AddLevel (5,305 + SGLang.i18n ("Text8"))

AddLevel (4,152 + SGLang.i18n ("Text18"))

AddLevel (3, "76" + SGLang.i18n ("Text19"))

AddLevel (2, "38" + SGLang.i18n ("Text8"))

AddLevel (1, "19" + SGLang.i18n ("Text8"))

}

Function getBestViewPoint () {

/ / get the spatial coordinates of the center of the current screen. Of course, the field of view cannot point to the sky.

Var worldPointMid = SGWorld.Window.CenterPixelToWorld ()

If (worldPointMid.Type = = 32) {

Alert (SGLang.i18n ("Text20"))

Return null

}

Return worldPointMid.Position

}

/ / calculate the level according to the location, which is the key function in it, including the following functions

Function getPreferredLevel (viewPoint)

{

/ / calculate the distance between the position of the current field of view and the center point of the screen

Var DistCameraToTerrain = SGWorld.Navigate.GetPosition () .DistanceTo (viewPoint)

/ / the true width of the half

Var HalfRealWidth = DistCameraToTerrain * Math.tan ((53.02.0) * Math.PI / 180.0)

Var NumBlocksInViewWidth = 4

/ / convert block width to grade

Var IdealBlockSize = HalfRealWidth * 2.0 / NumBlocksInViewWidth

Var blockLevel = BlockWidthToLevel (IdealBlockSize)

Var LayerType =-1

If (document.all ["layertype"] [0] .minutes)

LayerType = 0; / / "points"

If (document.all ["layertype"] [1] .conversation)

LayerType = 1; / / "lines"

If (document.all ["layertype"] [2] .minutes)

LayerType = 2; / / "polygons"

If (LayerType =-1)

{

Alert (SGLang.i18n ("Text21"))

Return-1

}

Else if (LayerType! = 0)

BlockLevel + = 1

BlockLevel = Math.min (blockLevel, fakeNumLevels-1)

Return blockLevel

}

/ / convert block width to grade

Function BlockWidthToLevel (blockWidth)

{

Var numOfBlocksInLevel = blockWidth / bestLevelWidth

Var deltaFromBestLevel = Math.log (numOfBlocksInLevel) / Math.log (2.0)

Var L1 = bestLevelWidth * Math.pow (2, Math.floor (deltaFromBestLevel))

Var L2 = bestLevelWidth * Math.pow (2, Math.floor (deltaFromBestLevel + 0.5))

If (Math.abs (L1-blockWidth)