Rotating a .bmp image by a multiple of 90 in C

I am trying to rotate a bmp image by 90(-270), 180(-180), 270(-90) degrees. But I am not sure how to go about it. I know I have to manipulate the rows and columns for the bitmap. But I am unsure how to. I don't want the answer, just some guidance so I can get there ... Here is what I have:

Code:

---

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


  int c,r;
  *newrows = cols;
  *newcols = rows;
  PIXEL *o;
  PIXEL *n;
  //for 90 or -270 degrees
  if(rotation == 90 || rotation == -270 )
  {


        for(r = 0; r < rows; r++)
        {
                for(c = 0; c < cols; c++ )
                {
                    //not sure what to do




                }//end of for
        }//end of for
  }//end of if




  return 0;
}

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Well, think about it. When you rotate by 90 degrees, what happens to the columns and rows? The transformation is:

Code:

---

rotCol = col*cos(theta) - row*sin(theta)
rotRow = col*sin(theta) + row*cos(theta)

---

which for 90 degrees simplifies to:

Code:

---

rotCol = -row
rotRow = col

---

You can do similar derivations for the other three rotations too.

Makes sense, Okay Ill try it now. Thanks for the help by the way. People usually dont like to help much these days.

I tried implementing your approach but it didnt quite work :-/. I really don't know why.

Code:

---

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


  int c,r;
  //for 90 or -270 degrees
  if(rotation == 90 || rotation == -270 )
  {


        for(r = 0; r < rows; r++)
        {
                for(c = 0; c < cols; c++ )
                {
                        *newcols = -r;
                        *newrows = c;


                }//end of for
        }//end of for
  }//end of if




  return 0;
}

---

I was able to get the rotation of 90 degrees and -90 degrees. Cant figure out the 180 yet...

Code:

---

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


  int c,r;//counters for rows and columns
  *newrows = cols;
  *newcols = rows;
  //allocating space for the new rotated image
  *new = (PIXEL*)malloc(rows*cols*sizeof(PIXEL));
  PIXEL *o;
  PIXEL *n;
  //rotating -90 or 270 degrees (they are the same)
  if(rotation == -90 || rotation == 270 )
  {
        for(r = 0; r < rows; r++)
        {
                for(c = 0; c < cols; c++ )
                {
                        n = (*new) + (c * rows) + (rows - r - 1 );
                        o = ((original) + (r * cols) + c);
                        *n = *o;
                }//end of for
        }//end of for
  }else if(rotation == 90 || rotation == -270)//rotating 90 or -270
  {
        for(r = 0; r < rows; r++)
        {
                for(c = 0; c < cols; c++ )
                {
                        n = (*new) + (c * rows) + (rows + r + 1 );
                        o = ((original) + (r * cols) + c);
                        *n = *o;
                }//end of for
        }//end of for
  }else if(rotation == 180 || rotation == -180)//rotating 180 or -180
  {
        for(r = 0; r < rows; r++)
        {
                for(c = 0; c < cols; c++ )
                {
                        n = (*new) + (c * rows) + (rows + r + 1);
                        o = ((original) + (r * cols) + c);
                        *n = *o;
                }//end of for
        }//end of for


  }




  return 0;
}

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My bad, when I gave you that formula, I assumed that you count the center of the image as the origin... which almost nobody does. In that case, the formula should be altered like this:

Code:

---

rotCol = (col - width/2)*cos(theta) - (row - height/2)*sin(theta) + width/2
rotRow = (col - width/2)*sin(theta) + (row - height/2)*cos(theta) + height/2

---

Which for 90 degrees simplifies to:

Code:

---

rotCol = (height + width)/2 - row
rotRow = (height - width)/2 + col

---

for 180 degrees:

Code:

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rotCol = width - col
rotRow = height - row

---

and for 270 degrees:

Code:

---

rotCol = (width - height)/2 + row
rotRow = (width + height)/2 - col

---