Rotating a PPM image 90 degrees in C

Question

I'm struggling to figure out why this will not work. I read in a PPM image and have to rotate it 90 degrees but nothing I have tried will work. I'm pretty sure most of my problems stem from improper usage of pointers especially in a 1d array iteration context.

/** rotate.c 

    CpSc 2100: homework 2 

    Rotate a PPM image right 90 degrees

**/
#include "image.h"

typedef struct pixel_type
{
   unsigned char red;
   unsigned char green;
   unsigned char blue;
} color_t;


image_t *rotate(image_t *inImage) 
{
   image_t *rotateImage;
   int rows = inImage->rows;
   int cols = inImage->columns;
   color_t *inptr;
   color_t *outptr;
   color_t *pixptr;
    int width = rows;
    int height = cols;
    int i, k;

   /*  malloc an image_t struct for the rotated image   */
    rotateImage = (image_t *) malloc(sizeof(image_t));
    if(rotateImage == NULL)
    {
        fprintf(stderr, "Could not malloc memory for rotateImage. Exiting\n");
    }



   /* malloc memory for the image itself and assign the
      address to the image pointer in the image_t struct  */
    rotateImage -> image = (color_t *) malloc(sizeof(color_t) * rows * cols);
    if(rotateImage -> image == NULL)
    {
        fprintf(stderr, "Could not malloc memory for image. Exiting\n");
    }

   /*  assign the appropriate rows, columns, and brightness
       to the image_t structure created above              */
   rotateImage -> rows = cols;
    rotateImage -> columns = rows;
    rotateImage -> brightness = inImage -> brightness;

    inptr = inImage -> image;
    outptr = rotateImage -> image;

   /*  write the code to create the rotated image         */
    for(i = 0; i<height; i++)
    {
        for(k = 0; k<width; k++)
        {
            outptr[(height * k)+(height-i - 1)] = inptr[(width*i)+k];

        }


    }
   rotateImage -> image = outptr;
   return(rotateImage);
}

Perhaps you could describe what the code does now and how that varies from your expectations. — David Hoelzer, Feb 24 '16 at 01:30
Well i expect for the two loops to iterate through all different possible pixels and assigning them to the rotated location on the image. I expect it to work given that i went through it on paper and it works. — Lars Dunson, Feb 24 '16 at 01:50
[don't cast the result of malloc in C](http://stackoverflow.com/q/605845/995714) — phuclv, Feb 24 '16 at 05:56
So how does its behavior vary from your expectations?? What's not working? Is the image skewed? Does it not rotate? http://stackoverflow.com/help/how-to-ask — David Hoelzer, Feb 24 '16 at 11:57

score 0 · Answer 1 · answered Feb 24 '16 at 05:36

try this:

#define COORD_INDEX(x ,y , w)    (x+ (y*w)) // w for width

...   

   /*  for clarity use x,and y */
    for(y = 0; i < height; i++)
    {
        for(x = 0; x < width; k++)
        {
            outptr[COORD_INDEX(y,x,height) ] = inptr[COORD_INDEX(x,y,width)];

        }
... 

    }

Rotating a PPM image 90 degrees in C

1 Answers1