how to convert image to 1D signal?

I want to downsample the image & I'm not getting any direct function

Answer 1

[Image Analyst]

Like your other question - so little information that it's unanswerable. To convert to a 1D signal, you could take the mean or sum along rows or columns, or you could call the radon() function. To downsample to a 1D vector, you could use linear indexing and do something like image1D = grayImage(1:42:numel(grayImage)). But that's enough guessing - you need to specify what you want.