When I was using Image Processing and Design toolbox IDP 8.0 in Scilab environment I came over what's called "3d matrix". This 3d matrix is called RGB matrix also. Each pixel in any image requires to be defined 3 information: pixel row, pixel column, and the RGB values of this pixel. So, this matrix defines pixel location and color components. The size of this RGB matrix is M x N x 3 where M is image height in pixels, and N is image width in pixels.