Changing RBG value doesn't seem to work in C

Question

I'm supposed to change the RGB color to gray by getting the average of the 3 and then assigning that average to each of the RBG colors(red, blue, green) however it isn't working.

First I'll post my code for the grayscale part:

#include "helpers.h"

// Convert image to grayscale
void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
   for(int i = 0; i < height; i++)
   {
     for(int j = 0; j < width; j++)
     {
       RGBTRIPLE rgbt = image[i][j];
       BYTE average = (rgbt.rgbtBlue/3)+(rgbt.rgbtGreen/3)+(rgbt.rgbtRed);
       *rgbt.rgbtBlue = *average;
       *rgbt.rgbtGreen = *average;
       *rgbt.rgbtRed = *average;
     }        
   }
   return;
}

And since its important, here where the RGBT values are mentionned: (in another file, mentionned in the header for helpers.h, called bmp.h):

/**
 * RGBTRIPLE
 *
 * This structure describes a color consisting of relative intensities of
 * red, green, and blue.
 *
 * Adapted from http://msdn.microsoft.com/en-us/library/aa922590.aspx.
 */
 typedef struct
 {
   BYTE  rgbtBlue;
   BYTE  rgbtGreen;
   BYTE  rgbtRed;
 } __attribute__((__packed__))
 RGBTRIPLE;

The error message is as follows:

indirection requires pointer operand ('int' invalid)

Thanks for your help (and I know know that cs50 isn't the best course but the Harvard diploma helps.).

Answer 1

I think you want this:

void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
   for(int i = 0; i < height; i++)
   {
     for(int j = 0; j < width; j++)
     {
       RGBTRIPLE rgbt = image[i][j];
       BYTE average = (rgbt.rgbtBlue + rgbt.rgbtGreen + rgbt.rgbtRed) / 3;
       rgbt.rgbtBlue = average;
       rgbt.rgbtGreen = average;
       rgbt.rgbtRed =  average;
       image[i][j] = rgbt;   
     }        
   }
   return;
}

Hint: you can simplyfy this by getting rid of the rgbt variable. You only need one line of code inside the inner for loop.

Problems in your original code:

This is wrong:

BYTE average = (rgbt.rgbtBlue/3)+(rgbt.rgbtGreen/3)+(rgbt.rgbtRed);

because you didn't divide rgbt.rgbtRed by 3, and anyway you should first add all values and then divide the whole thing by 3 which this will prevent roundoff errors:

BYTE average = (rgbt.rgbtBlue + rgbt.rgbtGreen + rgbt.rgbtRed) / 3;

This is wrong, you use * to dereference something that is not a pointer:

 *rgbt.rgbtBlue = *average;

You simply want:

 rgbt.rgbtBlue = average;

And last but not least you still need to modify the original value in the image by adding this:

 image[i][j] = rgbt;