How to deal with RGB to YUV conversion

Question

The formula says:

Y = 0.299 * R + 0.587 * G + 0.114 * B;

U = -0.14713 * R - 0.28886 * G + 0.436 * B;

V = 0.615 * R - 0.51499 * G - 0.10001 * B;

What if, for example, the U variable becomes negative?

U = -0.14713 * R - 0.28886 * G + 0.436 * B;

Assume maximum values for R and G (ones) and B = 0 So, I am interested in implementing this convetion function in OpenCV, So, how to deal with negative values? Using float image? anyway please explain me, may be I don't understand something..

score 14 · Answer 1 · edited Feb 08 '17 at 14:26

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Y, U and V are all allowed to be negative when represented by decimals, according to the YUV color plane.

YUV Color space

edited Feb 08 '17 at 14:26

Community

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answered May 25 '10 at 14:38

Dolph

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score 13 · Accepted Answer · answered May 25 '10 at 15:57

13

You can convert RGB<->YUV in OpenCV with cvtColor using the code CV_YCrCb2RGB for YUV->RGB and CV_RGBYCrCb for RGB->YUV.

void cvCvtColor(const CvArr* src, CvArr* dst, int code)

Converts an image from one color space to another.

answered May 25 '10 at 15:57

Jacob

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Thank you!! I searched for such a function and I thought that YCrCb is something different than YUV... – maximus May 25 '10 at 16:39
No problem. Note that you can convert 3-channel images as well. If you need to merge the Y,U and Vcomponents, you can do that with `merge` which takes a `vector` object. – Jacob May 25 '10 at 17:20
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@maximus - strictly speaking it is. YUV is for analogue TV standard YCrCb is digital - the color space is different. But only very slightly – Martin Beckett Jan 17 '12 at 19:34
In C++: cv::cvtColor(image, yuv_image, CV_RGB2YCrCb); – Lucas Walter Mar 14 '15 at 22:47
Keep in mind that there is currently a bug in opencv 2.4.*, which causes incorrect convertation from RGB to YUV. *Red* and *Blue* channels are *swapped* in the implemented formula. http://code.opencv.org/issues/4227 – Temak Apr 27 '15 at 08:11
Look at detailed post about the bug here http://stackoverflow.com/a/29890941/1351629 – Temak Apr 27 '15 at 08:43

score 2 · Answer 3 · answered Aug 02 '13 at 20:44

for planar formats OpenCV is not the right tool for the job. Instead you are better off using ffmpeg. for example

static void rgbToYuv(byte* src, byte* dst, int width,int height)
{

    byte* src_planes[3] = {src,src + width*height, src+ (width*height*3/2)};
    int src_stride[3] = {width, width / 2, width / 2};
    byte* dest_planes[3] = {dst,NULL,NULL};
    int dest_stride[3] = {width*4,0,0};
    struct SwsContext *img_convert_ctx = sws_getContext(
        width,height,
        PIX_FMT_YUV420P,width,height,PIX_FMT_RGB32,SWS_POINT,NULL,NULL,NULL);
        sws_scale(img_convert_ctx, src_planes,src_stride,0,height,dest_planes,dest_stride); 
    sws_freeContext(img_convert_ctx);
}

will convert a YUV420 image to RGB32

How to deal with RGB to YUV conversion

3 Answers3

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