Question

I am reading in the matlab documentation that rgb2hsv will return an m-by-n-by-3 image array, yet when I call it, I get a 1-by-3 vector. Am I misunderstanding something?
Here is an sample code:

image_hsv = rgb2hsv('filepath')

and as output

image_hsv =

        0.7108    0.3696   92.0000

Answer 1

You cannot call rgb2hsv on a filepath – it must be called on a MATLAB image matrix. Try:

image_rgb = imread('filepath');  % load the image array to MATLAB workspace
image_hsv = rgb2hsv(image_rgb);  % convert this array to hsv

You can see these matrices with:

>> whos image*  % display all variables whose name begins with 'image'
  Name             Size                 Bytes  Class     Attributes

  image_hsv      480x640x3            7372800  double              
  image_rgb      480x640x3             921600  uint8     

What your original code was doing was converting your filepath string to ascii numbers, taking the first three values of this array as RGB values and converting these to HSV.

NOTE: This example highlights dangers with MATLAB’s weak typing system, where data types are silently converted from one type to another. Also maybe a lack of correct input checking to the rgb2hsv function.