Quickest way to convert a vec3b to vec3d

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I want to access a pixel, which is in uchar format, in double format, for some floating point operations. I am forced to use at<vec3b> since .at<vec3d> will access wrong elements. Is there a way to quickly convert it to vec3d or should I do it manually ?

Also, I thought about using at<mat> but it crashes without informations. Is this behavior normal, i.e Mat not supported ?