Number of pixels per area in Rstudio?

Question

Image processing

There is a pre-processed image containing only three colors. It is necessary to load this image into Rstudio, scale it (that is, it is necessary that the size of the image in mm or cm can be displayed along the x and y axes) and determine the number of pixels of each color per image height. I explain, perhaps it is not clear, but it means that in the end you need to get something like at a height of 2 cm there are 15 pixels of red color, and at a height of 5 cm - 20 pixels of red color, something like that.
All the packages that I have looked at involve manipulations with the image from the category to brighten / flip, etc. Maybe there is something I missed? Or is it rather necessary to do this with the help of some function? In general, unfortunately, I am not very familiar with Rstudio, so I will be grateful for your help.