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it is not
img.at<vec3b>(0,0)[0] = -1; but img.at<vec3f>(0,0)[0] = -1;

it is not

 img.at<Vec3b>(0,0)[0] = -1;

but

  img.at<vec3b>(0,0)[0] img.at<Vec3f>(0,0)[0] = -1; -1;

~~but img.at<vec3f>(0,0)[0] = -1;~~

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here is how