Rotating a BMP image using C

Question

An uncompressed 24-bit .bmp file

I need to rotate a .bmp file given a rotation multiple of 90. For example, i have an image and im given a +90 or -90 rotation factor. my image will rotate 90 degrees to the left or to the right according to the rotation factor. my program works fine when the dimensions of the file are equal, meaning height & width are equal, but when i do it with an image thats not a square i get seg errors.

this is the code that i have so far.

if(rotation == 90 || rotation == -270 )
{
    /* 90 = -270 */
    for(row = 0; row < rows; row++)
    {
        for(col = 0; col < cols; col++ )
        {
            PIXEL* o = original+ (row*cols) + col;
            PIXEL* n = (*new)+((cols-col-1)*cols) + row;
            *n = *o;
        }
    }
    *newcols = cols;
    *newrows = rows;

the header of this method is:

int rotate(PIXEL* original, int rows, int cols, int rotation,
   PIXEL** new, int* newrows, int* newcols)

where PIXEL* original contains the original .bmp file

rows and cols are obtained by a call to the method that read the .bmp file

rotation = is the rotation factor given by the user

If your image is rectangular you need to truncate it. Are you sure you're doing this properly? — Justin
im not too sure @jucestain, thats why im asking to see if someone can show me how to do this properly. Because it shouldnt matter if its a square or a rectangular .bmp, it should rotate it properly — user2913269
When you rotate a 2x3 rectangle by 90 (or -90) degrees, it becomes a 3x2 rectangle... — Scott Mermelstein
You should use a debugger to find out where, exactly, your program segfaults. — Dolda2000

vacawama vacawama · Accepted Answer · 2013-11-05T21:41:40

Here is your problem. You should be multiplying by rows and not cols here:

PIXEL* n = (*new)+((cols-col-1)*rows) + row;

You want to be multiplying by the width of a row in your new image, and that is the same as the number of rows in your original image.

Also, you should be swapping rows and cols here:

*newcols = rows;
*newrows = cols;

To rotate -90:

PIXEL* n = (*new)+(col*rows) + (rows-row-1);
*newcols = rows;
*newrows = cols;

To rotate 180:

PIXEL* n = (*new)+((rows-row-1)*cols) + (cols-col-1);
*newcols = cols;
*newrows = rows;

In general, the formula is:

PIXEL* n = (*new)+(newrow*newcols) + newcol;

You just need to figure out what how to determine newrow, newcols, and newcol from the previous unrotated BMP. Drawing pictures helps.

Rotating a BMP image using C

2 Answers