CoreGraphics.h

#define M_E         2.71828182845904523536028747135266250   e
#define M_LOG2E     1.44269504088896340735992468100189214   log 2e
#define M_LOG10E    0.434294481903251827651128918916605082  log 10e
#define M_LN2       0.693147180559945309417232121458176568  log e2
#define M_LN10      2.30258509299404568401799145468436421   log e10
#define M_PI        3.14159265358979323846264338327950288   pi
#define M_PI_2      1.57079632679489661923132169163975144   pi/2
#define M_PI_4      0.785398163397448309615660845819875721  pi/4
#define M_1_PI      0.318309886183790671537767526745028724  1/pi
#define M_2_PI      0.636619772367581343075535053490057448  2/pi
#define M_2_SQRTPI  1.12837916709551257389615890312154517   2/sqrt(pi) #define M_SQRT2     1.41421356237309504880168872420969808   sqrt(2)
#define M_SQRT1_2   0.707106781186547524400844362104849039  1/sqrt(2)

CGAffineTransformMakeTranslation(width, 0.0);是改变位置的，

CGAffineTransformRotate(transform, M_PI);是旋转的。

CGAffineTransformMakeRotation(-M_PI);也是旋转的

transform = CGAffineTransformScale(transform, -1.0, 1.0);是缩放的。

view.transform = CGAffineTransformIdentity;线性代数里面讲的矩阵变换，这个是恒等变换   当 你改变过一个view.transform属性或者view.layer.transform的时候需要恢复默认状态的话，记得先把他们重置可以使用view.transform = CGAffineTransformIdentity，或者view.layer.transform = CATransform3DIdentity。

Quartz转换实现的原理:Quartz把绘图分成两个部分，
用户空间，即和设备无关，
设备空间，

Quartz提供的3大功能

void CGContextDrawImage (
CGContextRef c,
CGRect rect,
CGImageRef p_w_picpath
);

CGContextTranslateCTM (myContext, 100, 50);

include <math.h>
static inline double radians (double degrees) {return degrees * M_PI/180;}

CGContextScaleCTM (myContext, .5, .75);

CGContextTranslateCTM (myContext, w,h);