The scalebar plugin shows incorrect scale after upgraded Mapbox from 8.4.0 to 8.5.1

[ystsoi]

After upgraded Mapbox from 8.4.0 to 8.5.1, the scalebar plugin will show different scales for different phones with different screen resolution/density.

scalebar version: com.mapbox.mapboxsdk:mapbox-android-plugin-scalebar-v8:0.3.0

[LukasPaczos]

I suspect that any perceived difference comes from mapbox/mapbox-gl-native#15742.

@Chaoba could you verify if the behavior remains correct?

[Canivek]

After mapbox/mapbox-gl-native#15742 the distance displayed by the scale bar is incorrect on devices where screen density is different than 1.

Verified by drawing a line of 200 meters and measuring it with the scale bar using following code:

    private void setupScaleBarPlugin(MapView mapView, MapboxMap mapboxMap) {
        ScaleBarPlugin scaleBarPlugin = new ScaleBarPlugin(mapView, mapboxMap);
        ScaleBarOptions scaleBarOptions = new ScaleBarOptions(this)
                .setMetricUnit(true);
        scaleBarPlugin.create(scaleBarOptions);
    }

    private void setupLine(Style style, MapboxMap mapboxMap) {
        GeoJsonSource source = new GeoJsonSource("source-id");
        LineLayer lineLayer = new LineLayer("layer-id", source.getId());

        Point startPoint = Point.fromLngLat(0, 0);
        Point endPoint = TurfMeasurement.destination(startPoint, 200, 90, TurfConstants.UNIT_METERS);

        List<Point> pointList = Arrays.asList(startPoint, endPoint);
        source.setGeoJson(LineString.fromLngLats(pointList));

        style.addSource(source);
        style.addLayer(lineLayer);

        mapboxMap.animateCamera(CameraUpdateFactory.newLatLng(new LatLng(startPoint.latitude(), startPoint.longitude())));
    }

And using following dependencies:

    implementation 'com.mapbox.mapboxsdk:mapbox-android-sdk:8.4.0' // or 8.5.0 for incorrect behavior
    implementation 'com.mapbox.mapboxsdk:mapbox-android-plugin-scalebar-v8:0.3.0'
    implementation 'com.mapbox.mapboxsdk:mapbox-sdk-turf:5.0.0'

Following screenshots are on a device with a 2.25 screen density and the black line is 200m:

With mapbox-android-sdk:8.4.0 result is correct:

With mapbox-android-sdk:8.5.0 (or 8.6.0 which is the last stable version when writing these lines) result is incorrect:

I suspect that Projection#getMetersPerPixelAtLatitude() method is actually returning meters per dp.

[Chaoba]

@Canivek Thanks for your support. I got the same conclusion after my test. @LukasPaczos I believe the returning value of Projection#getMetersPerPixelAtLatitude() is not right.

[LukasPaczos]

@Chaoba feel free to take a stab at it, but also keep in mind the accuracy radius value that we use with the LocationComponent which provoked mapbox/mapbox-gl-native#15742.

[Chaoba]

@Canivek I have add your test method in #1088, it's really a good idea 👍 .

[Canivek]

@Chaoba I have looked at your fix, and it will indeed fix the fact that the scale bar doesn't display the correct distance.
But I do believe that the problem is rather on Projection#getMetersPerPixelAtLatitude() or this method is just poorly named? To me, a method with this name returns meters per screen pixel. And here it seems to return meters per density independent pixel (dp).

[Chaoba]

@Canivek Thanks for your quick response. I agree with you, @LukasPaczos shell we rename Projection#getMetersPerPixelAtLatitude() to Projection#getMetersPerDpAtLatitude() or something else that won't make misunderstanding？

[ystsoi]

MapView allows us to set the pixel ratio, as in:
https://github.com/mapbox/mapbox-gl-native-android/blob/master/MapboxGLAndroidSDKTestApp/src/main/res/layout/activity_camera_test.xml

If I set the pixel ratio to a value different from the screen density, the fix will not work.

For the method name, I think that at least you need to have a better documentation, if not a better name.

[Chaoba]

@ystsoi Good catch. If user set this pixel ratio, scale bar will show wrong value. But Mapview.getPixelRatio() is private, we can't get it from outside. @tobrun Any idea about it?

[tobrun]

Fixing this in mapbox/mapbox-gl-native-android#146