在Camera2 API中，你可以通过创建一个CameraCaptureSession绑定到一个SurfaceView或TextureView来显示相机预览。 不幸的是，一些参数（如对比度和伽玛）只对JPEG格式的图像生效，而不适用于预览。

解决方法是使用RenderScript或OpenGL ES处理捕获的帧，然后将结果显示在预览界面上。 这有助于我们在捕获帧后应用不同的滤镜。 例如，您可以使用RenderScript的ColorMatrix来应用对比度/伽马校正，如下所示：

private void createCameraPreviewSession() {
    try {
        SurfaceTexture texture = mTextureView.getSurfaceTexture();
        texture.setDefaultBufferSize(mPreviewSize.getWidth(), mPreviewSize.getHeight());
        Surface surface = new Surface(texture);
        mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        mPreviewRequestBuilder.addTarget(surface);
        mCameraDevice.createCaptureSession(Arrays.asList(surface),
                new CameraCaptureSession.StateCallback() {

                    @Override
                    public void onConfigured(CameraCaptureSession cameraCaptureSession) {
                        // When the session is ready, we start displaying the preview.
                        mCaptureSession = cameraCaptureSession;
                        try {
                            // Auto focus should be continuous for camera preview.
                            mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE,
                                    CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
                            // Set the filter for contrast and gamma correction.
                            RenderScript rs = RenderScript.create(getContext());
                            byte[] colorMatrix = {
                                    vp, 0, 0, 0, 0,    // Red
                                    0, vp, 0, 0, 0,    // Green
                                    0, 0, vp, 0, 0,    // Blue
                                    0, 0, 0, 1, 0};    // Alpha
                            ScriptIntrinsicColorMatrix matrix = ScriptIntrinsicColorMatrix.create(rs);
                            matrix.setColorMatrix(new Matrix4f(colorMatrix));
                            matrix.setAdd(5f/255f);
                            matrix.forEach(new Allocation(rs, Allocation.USAGE_SCRIPT), new Allocation(rs, Allocation.USAGE_SCRIPT));
                            rs.finish();
                            mPreviewRequestBuilder.set(CaptureRequest.CONTROL_VIDEO_STABILIZATION_MODE, CaptureRequest.CONTROL_VIDEO_STABILIZATION_MODE_ON);
                            // Update auto-exposure with FRAME_CONTROL.
                            mPreviewRequestBuilder.setFrameDuration(250000);
                            cameraCaptureSession.setRepeatingRequest(mPreviewRequestBuilder