expose pixel bounds from TileMap not boundingTiles

frontend/src/Main.elm

@@ -469,7 +469,6 @@ powerView = measureView "power" "#994444" (.power >> Maybe.map toFloat)
 
 eleView = measureView "elevation" "#4444ee" (.pos >> .ele)
 
-
 trackView : Int -> Int -> ZoomLevel -> List Point -> Svg Msg
 trackView leftedge topedge zoom points =
     let plot p =
@@ -518,21 +517,15 @@ ifTrack model content =
 canvas : Coord -> ZoomLevel -> Int -> Int -> Model -> Html Msg
 canvas centre zoom width height model =
     let tm = TileMap centre zoom width height
-        (mintile, maxtile) = boundingTiles tm
-        -- offset is pixel difference between centre (which *should*
-        -- be the middle of the image) and actual middle of the canvas
-        (pixelCentreX,pixelCentreY) = pixelFromCoord centre zoom
-        leftedge = mintile.x * 256
-        topedge = mintile.y * 256
-        offsetX = pixelCentreX - (width // 2) - leftedge
-        offsetY = pixelCentreY - (height // 2) - topedge
-        pixWidth = (1 + maxtile.x - mintile.x) * 256
-        pixHeight = (1 + maxtile.y - mintile.y) * 256
+        mapBounds = TileMap.bounds tm
+        (pixelCentreX, pixelCentreY) = pixelFromCoord centre zoom
+        offsetX = pixelCentreX - (width // 2) - mapBounds.left
+        offsetY = pixelCentreY - (height // 2) - mapBounds.top
         epos e = Tuple.mapBoth floor floor e.pointer.clientPos
-        tv = ifTrack model (trackView leftedge topedge zoom)
+        tv = ifTrack model (trackView mapBounds.left mapBounds.top zoom)
     in  div [style "position" "absolute"
-            ,style "width" (px pixWidth)
-            ,style "height" (px pixHeight)
+            ,style "width" (px mapBounds.width)
+            ,style "height" (px mapBounds.height)
             ,style "left" (px -offsetX)
             ,style "top" (px -offsetY)
             ,style "lineHeight" (px 0)

frontend/src/TileMap.elm

@@ -8,7 +8,7 @@ module TileMap exposing (tiles
                         , translate
                         , translatePixels
                         , incZoom
-                        , boundingTiles
+                        , bounds
                         , pixelsToCoord
                         , pixelFromCoord
                         , zoomStep)
@@ -89,6 +89,17 @@ boundingTiles (TileMap centre z width height) =
     in ((tileCovering minCoord z),
         (translate (tileCovering maxCoord z) (TileNumber 1 1)))
 
+bounds : TileMap -> { left : Int, top : Int, width: Int, height: Int }
+bounds tmap =
+    let (mintile, maxtile) = boundingTiles tmap
+    in {
+        left = mintile.x * 256,
+        top = mintile.y * 256,
+        width = (1 + maxtile.x - mintile.x) * 256,
+        height = (1 + maxtile.y - mintile.y) * 256
+    }
+
+
 tileUrl : TileNumber -> ZoomLevel -> String
 tileUrl {x,y} z =
     String.concat ["https://a.tile.openstreetmap.org",