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Apr 99 Tips

Volume Number: 15 (1999)
Issue Number: 4
Column Tag: Tips & Tidbits

Apr 99 Tips

by Jeff Clites <>

PixMap Rotation

CopyBits is one of the most powerful and ubiquitous of the Mac OS toolbox calls. It can scale, convert colors, and mask in a variety of ways. About the only thing it can't do is rotate an image. Similarly, we all know how to use DrawString to put text on the screen - but when we need to draw rotated text (say, for labeling a graph), we find no toolbox call up to the task. The routines below serve both needs.

RotatePixMap takes pointers to two PixMaps, and rotates the first into the second, rotating through a specified number of degrees (counter-clockwise). DrawRotatedString uses RotatePixMap to draw a string rotated to any angle, and then moves the graphics cursor accordingly. It also illustrates how to create a pair of pixel maps, draw into one, rotate into the other, and copy to the screen; a similar technique was used to rotate Clarus the dogcow in the figure below.

The code here only rotates pixel maps with 8-bit color depth or higher; it will return -1 if given a 1-bit image. It uses GWorlds, so requires at least System 7, but will work on both 68k and PowerPC machines. These functions form a valuable addition to the standard toolbox.

#include <Errors.h>
#include <fp.h>
#include "Rotate.h"

static void GetTrigs(float degrees, double_t *sinang, 
   double_t *cosang)
   // compute sine and cosine of the angle given in degrees;
   // check for special cases to avoid any rounding error
   if (degrees == 0) {
      *sinang = 0; *cosang = 1;
   } else if (degrees == 90 || degrees == -270) {
      *sinang = -1; *cosang = 0;
   } else if (degrees == 180 || degrees == -180) {
      *sinang = 0; *cosang = -1;
   } else if (degrees == 270 || degrees == -90) {
      *sinang = 1; *cosang = 0;
   } else {
      double_t radians = -degrees * pi / 180.0;
      *cosang = cos(radians);
      *sinang = sin(radians);

OSErr RotatePixMap(PixMapPtr srcPm,
                                  PixMapPtr destPm,
   float degrees)
   double_t          cosang, sinang;
   short                x,y,x2,y2,i;
   char                *srcbyte, *destbyte;
   short                pixsize = srcPm->pixelSize / 8;
   unsigned char    blankByte = pixsize == 1 ? 0 : 255;

   short srcRowBytes = srcPm->rowBytes & 0x7FFF;
   short destRowBytes = destPm->rowBytes
                                           & 0x7FFF;

   short srcwidth = srcPm->bounds.right
                                           - srcPm->bounds.left,
      destwidth = destPm->bounds.right
                                           - destPm->bounds.left,
      srcheight = srcPm->bounds.bottom
                                           - srcPm->,
      destheight = destPm->bounds.bottom
                                           - destPm->;

   short srcCx = srcwidth/2,
                                           srcCy = srcheight/2;
   short destCx = destwidth/2,
                                           destCy = destheight/2;

   if (destPm->pixelSize/8
                                  != pixsize || pixsize < 1)
      return -1;   // requires 8 bit pixmap or higher
   GetTrigs(degrees, &sinang, &cosang);
   // loop over each element in dest, copying from source or          // setting to white
   for (y=0; y<destheight; y++) {
      destbyte = destPm->baseAddr +       y*destRowBytes;
      for (x=0; x<destwidth; x++) {
         x2 = srcCx + cosang*(x-destCx) +    sinang*(y-destCy);
         y2 = srcCy + cosang*(y-destCy) -    sinang*(x-destCx);
         if (x2 >= 0 && x2 < srcwidth
          && y2 >= 0 && y2 < srcheight) {
            srcbyte = srcPm->baseAddr +   y2*srcRowBytes
                  + x2*pixsize;
            for (i=0; i<pixsize; i++) {
               *destbyte++ = *srcbyte++;   // copy color
         } else {
            for (i=0; i<pixsize; i++) {
               *destbyte++ = blankByte;   // fill in blank

   return noErr;

OSErr DrawRotatedString(Str255 s, float degrees)
   FontInfo   fi;
   short            cheight, cwidth, bufsize;
   Rect               srcR, destR, screenR;
   GWorldPtr      srcGW, destGW;
   PixMapHandle   srcPmH, destPmH;
   PixMapPtr      srcPm, destPm;
   CGrafPtr         origPort;
   GDHandle         origDevice;
   OSErr            err=noErr;
   double_t         cosang, sinang;

   // find the string size
   GetFontInfo( &fi );
   cheight = fi.ascent + fi.descent +   fi.leading;
   cwidth = StringWidth(s);
   // set buffer size (allow room for rotation)
   bufsize = (cheight > cwidth ?
                                        cheight*2 : cwidth*2);
   // create an offscreen GWorld in which to draw the string,
   // and another in which to draw the rotated version
   SetRect( &srcR, 0, 0, bufsize, cheight*2 );
   err = NewGWorld( &srcGW, 0, &srcR,
                                        NULL, NULL, 0 );
   if (err) return err;

   SetRect( &destR, 0, 0, bufsize, bufsize );
   err = NewGWorld( &destGW, 0, &destR,
                                        NULL, NULL, 0 );
   if (err) { DisposeGWorld( srcGW ); return err; }

   srcPmH = GetGWorldPixMap( srcGW );
   LockPixels( srcPmH );
   srcPm = *srcPmH;

   destPmH = GetGWorldPixMap( destGW );
   LockPixels( destPmH );
   destPm = *destPmH;

   // store original port, and draw string to source GWorld
   GetGWorld( &origPort, &origDevice );

   SetGWorld( srcGW, NULL );
   TextFont( origPort->txFont );
   TextSize( origPort->txSize );
   TextFace( origPort->txFace );
   EraseRect( &srcR );
   MoveTo( bufsize/2, cheight );
   DrawString( s );

   // rotate into the dest buffer
   RotatePixMap( srcPm, destPm, degrees );

   // copy to screen in OR mode, to avoid clobbering background
   SetGWorld( origPort, origDevice );
   SetRect( &screenR, 0, 0, bufsize, bufsize );
   OffsetRect( &screenR, 
                           origPort->pnLoc.h - bufsize/2,
         origPort->pnLoc.v - bufsize/2 );
         &destR, &screenR, srcOr, NULL );
   // release memory
   UnlockPixels( srcPmH );
   DisposeGWorld( srcGW );
   UnlockPixels( destPmH );
   DisposeGWorld( destGW );
   // finally, move the pen by the proper amount and direction
   GetTrigs(degrees, &sinang, &cosang);
   Move(cosang*cwidth + 0.5, sinang*cwidth+0.5);
   return err;

Figure 1. Clarus the dogcow is rotated through 30 degrees using RotatePixMap. DrawRotatedString is used to surround him with moofs.

Joseph J. Strout <>

You can download a demo of this Tip from


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