概要

osmdroidのMapViewでは真上からの表示にしか対応していない。
そこで、MapViewを改造して鳥瞰図表示に改造しました。
計算負荷などは考慮してないのでそこは注意！

コード

package dev.booyah.carmeterdisplay.view;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.os.Handler;
import android.util.AttributeSet;
import android.util.Log;

import org.osmdroid.api.IMapView;
import org.osmdroid.config.Configuration;
import org.osmdroid.tileprovider.MapTileProviderBase;
import org.osmdroid.views.MapView;

public class BirdEyeMapView extends MapView {

    private boolean birdView = true;

    public BirdEyeMapView(Context context, MapTileProviderBase tileProvider, Handler tileRequestCompleteHandler, AttributeSet attrs) {
        super(context, tileProvider, tileRequestCompleteHandler, attrs);
    }

    public BirdEyeMapView(Context context, MapTileProviderBase tileProvider, Handler tileRequestCompleteHandler, AttributeSet attrs, boolean hardwareAccelerated) {
        super(context, tileProvider, tileRequestCompleteHandler, attrs, hardwareAccelerated);
    }

    public BirdEyeMapView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public BirdEyeMapView(Context context) {
        super(context);
    }

    public BirdEyeMapView(Context context, MapTileProviderBase aTileProvider) {
        super(context, aTileProvider);
    }

    public BirdEyeMapView(Context context, MapTileProviderBase aTileProvider, Handler tileRequestCompleteHandler) {
        super(context, aTileProvider, tileRequestCompleteHandler);
    }

    public void setBirdView(boolean birdView) {
        this.birdView = birdView;
    }


    @Override
    protected void dispatchDraw(Canvas c) {
        final long startMs = System.currentTimeMillis();

        this.setProjection(null);

        if (birdView) {
            Matrix perspectiveMatrix = new Matrix();
            perspectiveMatrix.setPolyToPoly(new float[]{
                            0, 0,   // Top-left
                            getWidth(), 0,   // Top-right
                            getWidth() * 0.7f, getHeight()*1.1f,  // Bottom-right
                            getWidth() * 0.3f, getHeight()*1.1f}, // Bottom-left
                    0, new float[]{
                            0, 0,   // Top-left
                            getWidth(), 0,   // Top-right
                            getWidth(), getHeight(),  // Bottom-right
                            0, getHeight()}, // Bottom-left
                    0, 4);
            c.concat(perspectiveMatrix);
        }



        /* Draw background */
        try {
            /* Draw all Overlays. */
            this.getOverlayManager().onDraw(c, this);
            if (this.getZoomController() != null) {
                this.getZoomController().draw(c);
            }
            super.dispatchDraw(c);
        } catch (Exception ex) {
            //for edit mode
            Log.e(IMapView.LOGTAG, "error dispatchDraw, probably in edit mode", ex);
        }
        if (Configuration.getInstance().isDebugMapView()) {
            final long endMs = System.currentTimeMillis();
            Log.d(IMapView.LOGTAG, "Rendering overall: " + (endMs - startMs) + "ms");
        }

    }

}

解説

ミソとなるのは、dispatchDrawメソッドにてMatrixを使い台形に変換しています。