Scaling up an image using nearest-neighbor - c

Question

I have been trying to make my program scale up an image. I had some problem to allocate new space for my scaled image, but I think it is fixed. The problem I am having is that the program crashes when I am trying to send back my image from my temporary memory holder.
The loaded image is placed in my struct Image. The pixels are placed in
img->pixels, the height in img->height and the width in img->width. But I have no idea why the program crashes when I transfer the pixels from my tmp2 struct to my img struct while it does not crash when I do the opposite. Here is the code:
void makeBigger(Image *img, int scale) {
Image *tmp2;
tmp2 = (Image*)malloc(sizeof(Image));
tmp2->height = img->height*scale;
tmp2->width = img->width*scale;
tmp2->pixels = (Pixel**)malloc(sizeof(Pixel*)*tmp2->height);
for (unsigned int i = 0; i < img->height; i++)
{
tmp2->pixels[i] = (Pixel*)malloc(sizeof(Pixel)*tmp2->width);
for (unsigned int j = 0; j < img->width; j++)
{
tmp2->pixels[i][j] = img->pixels[i][j];
}
}
free(img->pixels);
//scaling up the struct's height and width
img->height *= scale;
img->width *= scale;
img->pixels = (Pixel**)malloc(sizeof(Pixel*)*img->height);
for (unsigned int i = 0; i < tmp2->height; i++)
{
img->pixels[i] = (Pixel*)malloc(sizeof(Pixel)*img->width);
for (unsigned int j = 0; j < tmp2->width; j++)
{
img->pixels[i][j] = tmp2->pixels[i+i/2][j+j/2];
}
}
}
I would be glad if you have any idea of how to make the nearest-neighbor method to work.
EDIT: I am trying to crop the inner rectangle so I can scale it up (zoom).
Image *tmp = (Image*)malloc(sizeof(Image));
tmp->height = img->height / 2;
tmp->width = img->width / 2;
tmp->pixels = (Pixel**)malloc(sizeof(Pixel*) * tmp->height);
for (unsigned i = img->height / 4 - 1; i < img->height - img->height / 4; i++) {
tmp->pixels[i] = (Pixel*)malloc(sizeof(Pixel) * tmp->width);
for (unsigned j = img->width / 4; j < img->width - img->width / 4; j++) {
tmp->pixels[i][j] = img->pixels[i][j];
}
}
for (unsigned i = 0; i < img->height; i++) {
free(img->pixels[i]);
}
free(img->pixels);
img->height = tmp->height;
img->width = tmp->width;
img->pixels = tmp->pixels;
free(tmp);

Answer 1

I see that you're overcomplicating things (walking over the image twice for example).Here's the code (I am posting the whole program - I made assumptions about Pixel and Image that might not match what you have), but if you copy / paste makeBigger it should work in your code OOTB:
code00.c:
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
typedef uint32_t Pixel;
typedef struct {
uint32_t width, height;
Pixel **pixels;
} Image;
void makeBigger(Image *img, int scale)
{
uint32_t i = 0, j = 0;
Image *tmp = (Image*)malloc(sizeof(Image));
tmp->height = img->height * scale;
tmp->width = img->width * scale;
tmp->pixels = (Pixel**)malloc(sizeof(Pixel*) * tmp->height);
for (i = 0; i < tmp->height; i++) {
tmp->pixels[i] = (Pixel*)malloc(sizeof(Pixel) * tmp->width);
for (j = 0; j < tmp->width; j++) {
tmp->pixels[i][j] = img->pixels[i / scale][j / scale];
}
}
for (i = 0; i < img->height; i++)
free(img->pixels[i]);
free(img->pixels);
img->width = tmp->width;
img->height = tmp->height;
img->pixels = tmp->pixels;
free(tmp);
}
void printImage(Image *img)
{
printf("Width: %d, Height: %d\n", img->width, img->height);
for (uint32_t i = 0; i < img->height; i++) {
for (uint32_t j = 0; j < img->width; j++)
printf("%3d", img->pixels[i][j]);
printf("\n");
}
printf("\n");
}
int main()
{
uint32_t i = 0, j = 0, k = 1;
Image img;
// Initialize the image
img.height = 2;
img.width = 3;
img.pixels = (Pixel**)malloc(sizeof(Pixel*) * img.height);
for (i = 0; i < img.height; i++) {
img.pixels[i] = (Pixel*)malloc(sizeof(Pixel) * img.width);
for (j = 0; j < img.width; j++)
img.pixels[i][j] = k++;
}
printImage(&img);
makeBigger(&img, 2);
printImage(&img);
// Destroy the image
for (i = 0; i < img.height; i++)
free(img.pixels[i]);
free(img.pixels);
printf("\nDone.\n");
return 0;
}
Notes (makeBigger related - designed to replace the content of the image given as argument):
Construct a temporary image that will be the enlarged one
Only traverse the temporary image once (populate its pixels as we allocate them); to maintain scaling to the original image and make sure that the appropriate pixel is "copied" into the new one, simply divide the indexes by the scaling factor: tmp->pixels[i][j] = img->pixels[i / scale][j / scale]
Deallocate the original image content: since each pixel row is malloced, it should also be freed (free(img->pixels); alone will yield memory leaks)
Store the temporary image content (into the original one) and then deallocate it
Output:
[cfati#cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q041861274]> ~/sopr.sh
### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###
[064bit prompt]> ls
code00.c
[064bit prompt]> gcc -o code00.exe code00.c
[064bit prompt]> ./code00.exe
Width: 3, Height: 2
1 2 3
4 5 6
Width: 6, Height: 4
1 1 2 2 3 3
1 1 2 2 3 3
4 4 5 5 6 6
4 4 5 5 6 6
Done.