iOS CGBitMap 小计

创建context

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CGBitmapContextCreate(<#void * _Nullable data#>,  //指向要渲染的绘制内存的地址
                       <#size_t width#>,     //width * scale
                       <#size_t height#>,    //height *scale
                       <#size_t bitsPerComponent#>,   //内存中像素的每个组件的位数 对于32位像素格式和RGB 颜色空间，你应该将这个值设为8
                       <#size_t bytesPerRow#>,   //每一行在内存所占的比特数
                       <#CGColorSpaceRef  _Nullable space#>,     //上下文使用的颜色空间
                       <#uint32_t bitmapInfo#>  //指定bitmap是否包含alpha通道，像素中alpha通道的相对位置，像素组件是整形还是浮点型等信息的字符串
                      )

针对bitmapInfo字段做一些备注

1. CGBitmapInfo

• 这个主要说一下kCGBitmapByteOrder32Little / kCGBitmapByteOrder32Big
• kCGBitmapByteOrder32Little 生成的信息位置为: 倒序
• kCGBitmapByteOrder32Big 生成的信息位置为: 顺序

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  typedef CF_OPTIONS(uint32_t, CGBitmapInfo) { kCGBitmapAlphaInfoMask = 0x1F, kCGBitmapFloatInfoMask = 0xF00, kCGBitmapFloatComponents = (1 << 8), kCGBitmapByteOrderMask = kCGImageByteOrderMask, kCGBitmapByteOrderDefault = kCGImageByteOrderDefault, kCGBitmapByteOrder16Little = kCGImageByteOrder16Little, kCGBitmapByteOrder32Little = kCGImageByteOrder32Little, kCGBitmapByteOrder16Big = kCGImageByteOrder16Big, kCGBitmapByteOrder32Big = kCGImageByteOrder32Big } CG_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);

2. CGImageAlphaInfo

• 这个主要说一下 kCGImageAlphaPremultipliedLast / kCGImageAlphaPremultipliedFirst
• kCGImageAlphaPremultipliedLast >>>> R G B A
• kCGImageAlphaPremultipliedFirst >>>> A R G B

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  typedef CF_ENUM(uint32_t, CGImageAlphaInfo) { kCGImageAlphaNone, /* For example, RGB. */ kCGImageAlphaPremultipliedLast, /* For example, premultiplied RGBA */ kCGImageAlphaPremultipliedFirst, /* For example, premultiplied ARGB */ kCGImageAlphaLast, /* For example, non-premultiplied RGBA */ kCGImageAlphaFirst, /* For example, non-premultiplied ARGB */ kCGImageAlphaNoneSkipLast, /* For example, RBGX. */ kCGImageAlphaNoneSkipFirst, /* For example, XRGB. */ kCGImageAlphaOnly /* No color data, alpha data only */ };

说了这么多 举个栗子

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//顺序  +   argb  = argb
kCGImageByteOrder32Big | kCGImageAlphaPremultipliedFirst  
const int RED = 1;
const int GREEN = 2;
const int BLUE = 3;
const int ALPHA = 0;

//顺序  +   rgba  = rgba
kCGImageByteOrder32Big | kCGImageAlphaPremultipliedLast 
const int RED = 0;
const int GREEN =1;
const int BLUE = 2;
const int ALPHA = 3;


//倒序   +  rgba   = abgr
kCGImageByteOrder32Little | kCGImageAlphaPremultipliedLast
const int RED = 3;
const int GREEN = 2;
const int BLUE = 1;
const int ALPHA = 0;

//倒序 + argb = bgra;
//kCGImageByteOrder32Little | kCGImageAlphaPremultipliedFirst
const int RED = 2;
const int GREEN = 1;
const int BLUE = 0;
const int ALPHA = 3;