Update greyscale.cu

// Color to Greyscale Conversion

//A common way to represent color images is known as RGBA - the color
//is specified by how much Red, Grean and Blue is in it.
//The 'A' stands for Alpha and is used for transparency, it will be
//ignored in this homework.

//Each channel Red, Blue, Green and Alpha is represented by one byte.
//Since we are using one byte for each color there are 256 different
//possible values for each color.  This means we use 4 bytes per pixel.

//Greyscale images are represented by a single intensity value per pixel
//which is one byte in size.

//To convert an image from color to grayscale one simple method is to
//set the intensity to the average of the RGB channels.  But we will
//use a more sophisticated method that takes into account how the eye 
//perceives color and weights the channels unequally.

//The eye responds most strongly to green followed by red and then blue.
//The NTSC (National Television System Committee) recommends the following
//formula for color to greyscale conversion:

//I = .299f * R + .587f * G + .114f * B

//Notice the trailing f's on the numbers which indicate that they are 
//single precision floating point constants and not double precision
//constants.