J2ME 3D 程式設計入門：基於 JSR 184 載入 M3G 模型並渲染動畫

JSR 184 (Mobile 3D Graphics API) 為 MIDP 提供保留模式 3D 渲染。本文示範最小可運行範例：載入 swerve.m3g 場景，設定攝影機視口，啟動線程循環呼叫 World.animate() 並 Graphics3D.render()。

環境配置

• WTK 2.2+，專案設定：Profile MIDP 2.0、Configuration CLDC 1.1、勾選 Mobile 3D Graphics (JSR 184)。
• 將 WTK 安裝目錄 apps/Demo3D/res/com/superscape/m3g/wtksamples/retainedmode/content/swerve.m3g 複製至專案 res/ 目錄。

核心類 First3DCanvas

public class First3DCanvas extends Canvas implements Runnable {
    private World myWorld;
    private Graphics3D g3d = Graphics3D.getInstance();
    private Camera cam;
    private int viewport_x, viewport_y, viewport_width, viewport_height;
    private long worldStartTime;

    public First3DCanvas() {
        new Thread(this).start();
        try {
            myWorld = (World) Loader.load("/swerve.m3g")[0];
            viewport_width = getWidth(); viewport_height = getHeight();
            cam = myWorld.getActiveCamera();
            // 依投影類型調整視口長寬比
            float[] params = new float[4];
            if (cam.getProjection(params) != Camera.GENERIC) {
                float waspect = (float)viewport_width / viewport_height;
                if (waspect < params[1]) {
                    viewport_height = (int)(viewport_width / params[1]);
                    viewport_y = (getHeight() - viewport_height) / 2;
                } else {
                    viewport_width = (int)(viewport_height * params[1]);
                    viewport_x = (getWidth() - viewport_width) / 2;
                }
            }
            worldStartTime = System.currentTimeMillis();
        } catch (Exception e) { e.printStackTrace(); }
    }

    protected void paint(Graphics g) {
        g.setColor(0); g.fillRect(0, 0, getWidth(), getHeight());
        g3d.bindTarget(g);
        g3d.setViewport(viewport_x, viewport_y, viewport_width, viewport_height);
        int validity = myWorld.animate((int)(System.currentTimeMillis() - worldStartTime));
        try { g3d.render(myWorld); } finally { g3d.releaseTarget(); }
    }

    public void run() {
        while (true) {
            repaint(viewport_x, viewport_y, viewport_width, viewport_height);
            try { Thread.sleep(30); } catch (InterruptedException ignored) {}
        }
    }
}

關鍵點解析

• Loader.load() 返回 Object[]，首個元素通常為 World。
• World.animate(time) 更新場景動畫狀態，返回有效時間。
• Graphics3D.bindTarget/releaseTarget 必須成對調用，綁定 2D Graphics 作為渲染目標。
• 視口調整確保 3D 內容不變形，適配不同螢幕比例。
• 渲染線程以固定間隔 repaint()，實際幀率受裝置效能影響。

MIDlet 啟動類

public class First3DMIDlet extends MIDlet {
    private First3DCanvas canvas = new First3DCanvas();
    public void startApp() { Display.getDisplay(this).setCurrent(canvas); }
    public void pauseApp() {}
    public void destroyApp(boolean u) {}
}