3.18WorldWind Android 绘图过程解析

因为WorldWindAndroid版是利用Opengl ES实现绘图的，所以大体的框架还是确定的，那就是实现GLSurfaceView.Renderer接口下的三个方法：OnSurfaceCreated，OnDrawFrame和OnSurfaceChanged，而这一实现是在WorldWindow类中！

 @Override
    public void onSurfaceCreated(GL10 unused, EGLConfig config) {
        // Specify the default World Wind OpenGL state.
        GLES20.glEnable(GLES20.GL_BLEND);
        GLES20.glEnable(GLES20.GL_CULL_FACE);
        GLES20.glEnable(GLES20.GL_DEPTH_TEST);
        GLES20.glEnableVertexAttribArray(0);
        GLES20.glDisable(GLES20.GL_DITHER);
        GLES20.glBlendFunc(GLES20.GL_ONE, GLES20.GL_ONE_MINUS_SRC_ALPHA);
        GLES20.glDepthFunc(GLES20.GL_LEQUAL);

        // Clear any cached OpenGL resources and state, which are now invalid.
        this.dc.contextLost();

        // Set the World Window's depth bits.
        int[] depthBits = new int[1];
        GLES20.glGetIntegerv(GLES20.GL_DEPTH_BITS, depthBits, 0);
        this.mainThreadHandler.sendMessage(
            Message.obtain(this.mainThreadHandler, MSG_ID_SET_DEPTH_BITS /*msg.what*/, depthBits[0] /*msg.obj*/));

        // Clear the render resource cache on the main thread.
        this.mainThreadHandler.sendEmptyMessage(MSG_ID_CLEAR_CACHE /*msg.what*/);
    }

    /**
     * Implements the GLSurfaceView.Renderer.onSurfaceChanged interface which is called on the GLThread when the window
     * size changes.
     */
    @Override
    public void onSurfaceChanged(GL10 unused, int width, int height) {
        GLES20.glViewport(0, 0, width, height);

        // Set the World Window's new viewport dimensions.
        Viewport newViewport = new Viewport(0, 0, width, height);
        this.mainThreadHandler.sendMessage(
            Message.obtain(this.mainThreadHandler, MSG_ID_SET_VIEWPORT /*msg.what*/, newViewport /*msg.obj*/));

        // Redraw this World Window with the new viewport.
        this.mainThreadHandler.sendEmptyMessage(MSG_ID_REQUEST_REDRAW /*msg.what*/);
    }

    /**
     * Implements the GLSurfaceView.Renderer.onDrawFrame interface which is called on the GLThread when rendering is
     * requested.
     */
    @Override
    public void onDrawFrame(GL10 unused) {
        // Remove and process pick the frame from the front of the pick queue, recycling it back into the pool. Continue
        // requesting frames on the OpenGL thread until the pick queue is empty. This is critical for correct operation.
        // All frames must be processed or threads waiting on a frame to finish may block indefinitely.
        Frame pickFrame = this.pickQueue.poll();
        if (pickFrame != null) {
            try {
                this.drawFrame(pickFrame);
            } catch (Exception e) {
                Logger.logMessage(Logger.ERROR, "WorldWindow", "onDrawFrame",
                    "Exception while processing pick in OpenGL thread", e);
            } finally {
                pickFrame.signalDone();
                pickFrame.recycle();
                super.requestRender();
            }
        }

        // Remove and switch to to the frame at the front of the frame queue, recycling the previous frame back into the
        // pool. Continue requesting frames on the OpenGL thread until the frame queue is empty.
        Frame nextFrame = this.frameQueue.poll();
        if (nextFrame != null) {
            if (this.currentFrame != null) {
                this.currentFrame.recycle();
            }
            this.currentFrame = nextFrame;
            super.requestRender();
        }

        // Process and display the Drawables accumulated in the last frame taken from the front of the queue. This frame
        // may be drawn multiple times if the OpenGL thread executes more often than the World Window enqueues frames.
        try {
            if (this.currentFrame != null) {
                this.drawFrame(this.currentFrame);
            }
        } catch (Exception e) {
            Logger.logMessage(Logger.ERROR, "WorldWindow", "onDrawFrame",
                "Exception while drawing frame in OpenGL thread", e);
        }
    }

而要使用WorldWind来完成绘图，需要自定义一个Fragment子类，如开发示例BasicGlobleFragment.java。

public class BasicGlobeFragment extends Fragment {

    private WorldWindow wwd;

    public BasicGlobeFragment() {
    }

    /**
     * Creates a new WorldWindow (GLSurfaceView) object.
     */
    public WorldWindow createWorldWindow() {
        // Create the WorldWindow (a GLSurfaceView) which displays the globe.
        this.wwd = new WorldWindow(getContext());
        // Setup the WorldWindow's layers.
        this.wwd.getLayers().addLayer(new BackgroundLayer());
        this.wwd.getLayers().addLayer(new BlueMarbleLandsatLayer());
        return this.wwd;
    }

    /**
     * Gets the WorldWindow (GLSurfaceView) object.
     */
    public WorldWindow getWorldWindow() {
        return this.wwd;
    }

    /**
     * Adds the WorldWindow to this Fragment's layout.
     */
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_globe, container, false);
        FrameLayout globeLayout = (FrameLayout) rootView.findViewById(R.id.globe);

        // Add the WorldWindow view object to the layout that was reserved for the globe.
        globeLayout.addView(this.createWorldWindow());

        return rootView;
    }
    /**
     * Resumes the WorldWindow's rendering thread
     */
    @Override
    public void onResume() {
        super.onResume();
        this.wwd.onResume(); // resumes a paused rendering thread
    }

    /**
     * Pauses the WorldWindow's rendering thread
     */
    @Override
    public void onPause() {
        super.onPause();
        this.wwd.onPause(); // pauses the rendering thread
    }
}

关于Fragment可以参考： 菜鸟教程：Fragment基本概述

对于图层的渲染在BasicFrameController.java

    @Override
    public void renderFrame(RenderContext rc) {
        rc.terrainTessellator.tessellate(rc);

        if (rc.pickMode) {
            this.renderTerrainPickedObject(rc);
        }

        rc.layers.render(rc);
        rc.sortDrawables();
    }