How to mapping a 1d array to a 2d array in C?

Question

I am trying to map the RGB pixels of an image to 2D arrays of R, G, B seperately. When the image is read the pixels are stored in a 1D array in the form {r1,g1,b1,r2,g2,b2...}. The length of array is 3*height*width. The 2D arrays will be of width X height dimensions

for(i = 0; i < length; i++) { // length = 3*height*width
    image[i][2] = getc(f); // blue pixel
    image[i][1] = getc(f); // green pixel
    image[i][0] = getc(f); // red pixel

    img[count] = (unsigned char)image[i][0];
    count += 1;

    img[count] = (unsigned char)image[i][1];
    count += 1;

    img[count] = (unsigned char)image[i][2];
    count += 1;

    printf("pixel %d : [%d,%d,%d]\n", i+1, image[i][0], image[i][1], image[i][2]);
}

The RGB values are in img[]. The 2d arrays are red[][], green[][] and blue[][].

Please help!

Your code somehow looks as it's already doing it, along with doing the exact opposite. — librin.so.1

paddy paddy · Accepted Answer · 2013-01-23T21:47:01

As I understand it, you are trying to reconstruct the colour fields. Just reverse your function:

unsigned char * imgptr = img;

for( int y = 0; y < height; y++ ) {
    for( int x = 0; x < width; x++ ) {
        red[y][x] = *imgptr++;
        green[y][x] = *imgptr++;
        blue[y][x] = *imgptr++;
    }
}

To create your arrays dynamically:

unsigned char** CreateColourPlane( int width, int height )
{
    int i;
    unsigned char ** rows;

    const size_t indexSize = height * sizeof(unsigned char*);
    const size_t dataSize = width * height * sizeof(unsigned char);

    // Allocate memory for row index and data segment.  Note, if using C compiler
    // do not cast the return value from malloc.
    rows = (unsigned char**) malloc( indexSize + dataSize );
    if( rows == NULL ) return NULL;

    // Index rows.  Data segment begins after row index array.
    rows[0] = (unsigned char*)rows + height;
    for( i = 1; i < height; i++ ) {
        rows[i] = rows[i-1] + width;
    }

    return rows;
}

Then:

unsigned char ** red = CreateColourPlane( width, height );
unsigned char ** green = CreateColourPlane( width, height );
unsigned char ** blue = CreateColourPlane( width, height );

You can free them easily, but it always pays to wrap the free function if you wrapped the allocator function:

void DeleteColourPlane( unsigned char** p )
{
    free(p);
}

How to mapping a 1d array to a 2d array in C?

1 Answers