Pointer to 1D Array, Greyscale Image

Problem:  Have an Image of any arbitrary size and want to store it.  You can use a defined 2-D array like
 

                    int  Image[10][10];
 

But this only works if you have a 10x10 image.
 
 

Solution:  You should use the concept of a pointer and dynamically allocate memory to it.  In this case, you will be representing a 2D image by a 1D array.
 

First,  Lets review the use of Pointers  in C/C++.  

• Returning to our problem, we will only find out a run time from the user of our program (or by reading the image file's header information) what the dimensions of the image are.  Hence, we first must declare a pointer for the image data I.
• Suppose we are dealing with a greyscale image.

  We can store each pixel value most efficiently as an unsigned char  and not as an integer which takes up more than one byte of data we need to store a value from 0 to 255.

  Example:     unsigned char *I;
   

• Now we must somehow find out the size of the image (could ask the user) and store this information say in the variables:  Image_Width, Image_Height

• Next we must allocate memory to our point to contain all of our pixels:

  Example:  I = malloc(Image_Width*Image_Height);

    Note:  sizeof(unsigned char) = 1
 
• Now we can fill our pointer I which can be treated like a 1D array.  How can we do this?

  Answer:  Store 1 row of data after the other in the same fashion as we have discussed visiting all the pixels in an image which is called "row scanning".
   

  

  If we had a 2D array  I2[r][c]where r = row and c=column and we wanted to store its pixels in our 1D array, this is the algorithm that would do it:
   

  for(r=0; r<Image_Height; r++)
  for(c=0; c<Image_Width; c++)
      I[r*Image_Width + c] = I2[r][c];
   
   
   

  Looking in more detail, this is how the pixels (r,c) are stored in the 1D array:

Where W = Image_Width

2D Array Greyscale Image

Another possibility is to create a multi-dimensional array I[r][c] dynamically. See See 3240 discussion of arrays.

Also, take a look at the following exercise that has some code for creating such arrays.. it is not complete...it is an exercise.

Pointer to 1D Array , Color Image

Now consider a color image instead of a greyscale image.
 

• we must store for each pixel the red, green and blue values
• Hence now, instead of one grey value, for each pixel we store its red, green , blue values in this order.
• Below is a partial program illustrating how this is done.  Notice for each pixel we store 3 values  ->  Hence the multiplicative factor of 3 througout!
 

unsigned char *I;

I = malloc(Image_Width*Image_Height*3);
 

/*to access Red value for Pixel (r,c) */

red_at_r_c =  I[r*Image_Width*3  + c*3];
 

/*to access Green value for Pixel (r,c) */

green_at_r_c =  I[r*Image_Width*3  + c*3 + 1];
 

/*to access Blue value for Pixel (r,c) */

blue_at_r_c =  I[r*Image_Width*3  + c*3 + 2];
 
 

This can be viewed as follows:

Multidimensional Array, Color Image  

• Case 1: 2D array, I[row][column] where store a data stucture value "color" that contains red, green, blue fields
• Case 2: 3D array, I[row][column][x] where x indicates the color field, if x=0 means red, if x=1 means green and if x=2 means blue.