OpenCV Android - 使用 CameraBridgeViewBase 的颜色问题

OpenCV Android - color issue using CameraBridgeViewBase(OpenCV Android - 使用 CameraBridgeViewBase 的颜色问题)

问题描述

我在使用 Android 模拟器时遇到了一个奇怪的问题.OpenCV CameraBridgeViewBase.

public Mat onCameraFrame(CvCameraViewFrame inputFrame) {
    return inputFrame.rgba();
}

    public Mat rgba() {
        Imgproc.cvtColor(mYuvFrameData, mRgba, Imgproc.COLOR_YUV2RGBA_NV21, 4);
        return mRgba;
    }

正如我们所见，Imgproc.cvtColor(...) 方法用于将帧从 YUV 转换为 RGBA.<代码>NV21 YUV ->RGBA 转换发生在那里.在初始化过程中，我们将格式设置为 NV21，所以这应该是正确的.此外，每台 Android 设备都应该支持 NV21.此外，我们可以使用调试器检查设备是否接受格式:

As we see, Imgproc.cvtColor(...) method is used in order to convert frame from YUV to RGBA. NV21 YUV -> RGBA conversion takes place there. During the initialization process we set the format to NV21, so this should be right. Moreover, every Android device should support NV21. Also, we can check whether device accepted the format using debugger:

protected boolean initializeCamera(int width, int height) {
    ...
    params.setPreviewFormat(ImageFormat.NV21);
    ...
    mCamera.setParameters(params);
    ...
    params = mCamera.getParameters();
    Log.d(TAG, String.format("Actual preview format is 0x%X", params.getPreviewFormat()));
}

据报道，手机(HTC Sensation)和模拟器都在使用 NV21.

Both phone (HTC Sensation) and emulator reported to be using NV21 indeed.

但是，如果我们将 COLOR_YUV2RGBA_NV21 更改为 COLOR_YUV2RGB_I420(YV12 和 I420 是一样的，只是将 Y 和 V 反转；)我们会看到模拟器会得到终于有了合适的色彩空间.在 params.setPreviewFormat(ImageFormat.NV21); 中将 NV21 更改为 YV12 我们会得到类似的结果.看起来 Imgproc.cvtColor 或 Android 中存在错误.

However, if we change COLOR_YUV2RGBA_NV21 to COLOR_YUV2RGB_I420 (YV12 and I420 is the same thing, just with Y and V inverted;) we'll see that emulator will get a proper color space finally. Changing NV21 to YV12 in params.setPreviewFormat(ImageFormat.NV21); we'll get similar results. Looks like there's bug either in Imgproc.cvtColor, or in Android.

解决方案来了.修改public Mat rgba()如下:

    public Mat rgba() {
        if (previewFormat == ImageFormat.NV21) {
            Imgproc.cvtColor(mYuvFrameData, mRgba, Imgproc.COLOR_YUV2RGBA_NV21, 4);
        }
        else if (previewFormat == ImageFormat.YV12) {
            Imgproc.cvtColor(mYuvFrameData, mRgba, Imgproc.COLOR_YUV2RGB_I420, 4);  // COLOR_YUV2RGBA_YV12 produces inverted colors
        }
        return mRgba;
    }

previewFormat 是一个新的 int 变量，它是这样声明的:

previewFormat is a new int variable, it's declared this way:

private int previewFormat = ImageFormat.NV21;

在初始化中添加以下更改:

Add following changes to the initialization:

protected boolean initializeCamera(int width, int height) {
        ...
                params.setPreviewFormat(ImageFormat.NV21);
                // "generic" = android emulator
                if (Build.BRAND.equalsIgnoreCase("generic")) {
                    params.setPreviewFormat(ImageFormat.YV12);
                }
                ...
                mCamera.setParameters(params);
                params = mCamera.getParameters();
                previewFormat = params.getPreviewFormat();
        ...
}

重要提示:
请注意:这只是一个临时解决方案，以使 OpenCV 在我的情况下可与模拟器一起使用.应该做进一步的研究.在 onPreviewFrame 中检查设备是否使用正确的图像格式非常容易.有时间我会回到这个.

Important:
Please note: this is just a temporary solution in order to make OpenCV usable with emulator in my case. Further research should be done. It's quite easy to check whether device uses correct image format in onPreviewFrame. I'll get back to this when I have some time.

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