Algorithm to rotate an image 90 degrees in place? (No extra memory)

Question

In an embedded C app, I have a large image that I\'d like to rotate by 90 degrees. Currently I use the well-known simple algorithm to do this. However, this algorithm requi

Answer 1

This might help: In-place matrix transposition.

(You might also have to do some mirroring after the transposition, as rlbond mentions).

Answer 2

Here is my attempt for matrix 90 deg rotation which is a 2 step solution in C.
First transpose the matrix in place and then swap the cols.

#define ROWS        5
#define COLS        5

void print_matrix_b(int B[][COLS], int rows, int cols) 
{
    for (int i = 0; i <= rows; i++) {
        for (int j = 0; j <=cols; j++) {
            printf("%d ", B[i][j]);
        }
        printf("\n");
    }
}

void swap_columns(int B[][COLS], int l, int r, int rows)
{
    int tmp;
    for (int i = 0; i <= rows; i++) {
        tmp = B[i][l];
        B[i][l] = B[i][r];
        B[i][r] = tmp;
    }
}


void matrix_2d_rotation(int B[][COLS], int rows, int cols)
{
    int tmp;
    // Transpose the matrix first
    for (int i = 0; i <= rows; i++) {
        for (int j = i; j <=cols; j++) {
            tmp = B[i][j];
            B[i][j] = B[j][i];
            B[j][i] = tmp;
        }
    }
    // Swap the first and last col and continue until
    // the middle.
    for (int i = 0; i < (cols / 2); i++)
        swap_columns(B, i, cols - i, rows);
}



int _tmain(int argc, _TCHAR* argv[])
{
    int B[ROWS][COLS] = { 
                  {1, 2, 3, 4, 5}, 
                      {6, 7, 8, 9, 10},
                          {11, 12, 13, 14, 15},
                          {16, 17, 18, 19, 20},
                          {21, 22, 23, 24, 25}
                        };

    matrix_2d_rotation(B, ROWS - 1, COLS - 1);

    print_matrix_b(B, ROWS - 1, COLS -1);
    return 0;
}

Answer 3

Not sure what processing you will do after the rotation, but you can leave it alone and use another function to read rotated pixel from the original memory.

uint16_t getPixel90(Image *img, int x, int y) 
{
    return img->data[(img->height - x) * img->width + y];
}

Where input parameter x and y has swapped dimension from original