多张UIImage图片合成一张

 NSAutoreleasePool *pool = [[ NSAutoreleasePool alloc ] init ];

    UIGraphicsBeginImageContext(mainImg .size );

    [mainImg drawInRect :CGRectMake( 0 , 0 , mainImg .size .width , mainImg .size .height )];
    int i = 0 ;
    for ( UIImage *img in imgArray) {
        [img drawInRect :CGRectMake([[imgPointArray objectAtIndex :i] floatValue ],
                                   [[imgPointArray objectAtIndex :i+ 1 ] floatValue ],
                                   img .size .width ,
                                   img .size .height )];

        i+= 2 ;
    }

    CGImageRef NewMergeImg = CGImageCreateWithImageInRect(UIGraphicsGetImageFromCurrentImageContext() .CGImage ,
                                                          CGRectMake( 0 , 0 , mainImg .size .width , mainImg .size .height ));

    UIGraphicsEndImageContext();
    [pool release ];

    if ( NewMergeImg == nil ) {
        return NO ;
    }
    else {
        UIImageWriteToSavedPhotosAlbum([ UIImage imageWithCGImage :NewMergeImg], self , nil , nil );
        return YES ;
    }