title: Matlab imread return 4 channel matrix
body: <p>I do imread image in Matlab, and it returns a 4-channel image:</p>
  
  <p>im: 1012x972x4 uint8.</p>
  
  <p>Which format is this image? How to check its color format(RGB, CMYK, etc)? I opened it in Gimp and the color profile is simply sRGB built-in</p>