AFAIK , A YUV image from Gray8 image is just copy the Gray8 image data to the Y-component. Meaning that the crominances are 0 value.
Based on this
https://en.wikipedia.org/wiki/YUV#/media/File:Yuv420.svg
So if I have a gray8 Mat (heightwidth) my YUV Mat result is a Mat(1.5height * width) So I just create a Mat with the first (heightwidth) bytes to Y and the rest of bytes just to 0 values.
But when I tried to use cvtColor Conversion; First Gray->BGR and then BGR->YUV420 (cvtcolor does not have direct conversion) my result Mat is a Mat(1.5height * width), but the last values of each frame (the crominances) are 128 not 0-values...
I don't understand this difference but it seems the second one (created with cvtcolor) is the correct one. My own created Mat from the gray8 data results on a Green Image...
Any hints??