RGB2YCBCR image convertion

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eng m
eng m 2021-5-9
评论： Walter Roberson 2021-5-9
i have this code i need to center Y,CB,CR around the center please show me what can i do.is truo?
I = imread('lena.tiff');
J2 = rgb2ycbcr((I));;
J2 =double(J2);
Y =((J2(:,:,1)));
cb=((J2(:,:,2)));
cr=((J2(:,:,3)));
Y=Y-128;
cb=cb-128;
cr=cr-128
T=dctmatx(8);
  2 个评论
DGM
DGM 2021-5-9
What do you mean "center" it? Cb and Cr should be centered about 128, since they're uint8. Also since they're uint8, subtracting 128 will truncate half the data.
eng m
eng m 2021-5-9
mean around to zero ,thanks

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采纳的回答

Walter Roberson
Walter Roberson 2021-5-9
Y=Y-128;
cb=cb-128;
cr=cr-128
If you read the documentation for rgb2ycbcr
Converted YCbCr color values, returned as a numeric array of the same size as the input.
  • If the input is double or single, then Y is in the range [16/255, 235/255] and Cb and Cr are in the range [16/255, 240/255].
  • If the input is uint8, then Y is in the range [16, 235] and Cb and Cr are in the range [16, 240].
  • If the input is uint16, then Y is in the range [4112, 60395] and Cb and Cr are in the range [4112, 61680].
So if you wanted to center Y around 0 you would subtract (235-16)/2 + 16 = 125.5 and for Cb and Cr you would subtract (240-16)/2 + 16 = 128
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eng m
eng m 2021-5-9
thanks mr walter ,
and then can add again in the last code to repeat it ?
like this
Y = Y+125.5;
cb = cb+128;
cr = cr+128;

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更多回答（1 个）

Image Analyst
Image Analyst 2021-5-9
Do you mean like
cb = cb - mean(cb(:));
cr = cr - mean(cr(:));
or possibly rescale so the max is at 127 and the min is at -127?
cb = rescale(cb, -127, 127);
cr = rescale(cr, -127, 127);
Those are two different things of course. They shift and scale the gamut by different amounts. I don't really know what your intent is, or why it's even necessary.
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eng m
eng m 2021-5-9
please @Image Analyst i need to center y cb, cr around to zero
to do best compession and reduse storge .to do jpeg color image
Image Analyst
Image Analyst 2021-5-9
编辑：Image Analyst 2021-5-9
@eng m, as Walter discussed "centered" is not a clear cut definition. You need to describe what centered means. We can think of at least 3 definitions based on range, mean, or median. But it looks like you are all set now (whatever the definition you used was) because you've accepted an answer.

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