获取图片某点或区域的颜色 与 CGBitmapContext

CGContextRef CGBitmapContextCreate （

　　void *data，

　　size_t width，

　　size_t height，

　　size_t bitsPerComponent，

　　size_t bytesPerRow，

　　CGColorSpaceRef colorspace，

　　CGBitmapInfo bitmapInfo

　　）;

 

 参数data指向绘图操作被渲染的内存区域，这个内存区域大小应该为（bytesPerRow*height）个字节。如果对绘制操作被渲染的内存区域并无特别的要求，那么可以传递NULL给参数date。

　　 参数width代表被渲染内存区域的宽度。

　　 参数height代表被渲染内存区域的高度。

　　 参数bitsPerComponent被渲染内存区域中组件在屏幕每个像素点上需要使用的bits位，举例来说，如果使用32-bit像素和RGB颜色格式，那么RGBA颜色格式中每个组件在屏幕每个像素点上需要使用的bits位就为32/4=8。

　　 参数bytesPerRow代表被渲染内存区域中每行所使用的bytes位数。

　　 参数colorspace用于被渲染内存区域的“位图上下文”。

　　 参数bitmapInfo指定被渲染内存区域的“视图”是否包含一个alpha（透视）通道以及每个像素相应的位置，除此之外还可以指定组件式是浮点值还是整数值。

 

获取图片中单个点的颜色：

- (UIColor*) getPixelColorAtLocation:(CGPoint)point {
   UIColor* color = nil;
   CGImageRef inImage = self.image.CGImage;
   // Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue
   CGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];
   if (cgctx == NULL) { return nil;  }

   size_t w = CGImageGetWidth(inImage);
   size_t h = CGImageGetHeight(inImage);
   CGRect rect = {{0,0},{w,h}}; 

   // Draw the image to the bitmap context. Once we draw, the memory 
   // allocated for the context for rendering will then contain the 
   // raw image data in the specified color space.
   CGContextDrawImage(cgctx, rect, inImage); 

   // Now we can get a pointer to the image data associated with the bitmap
   // context.
   unsigned char* data = CGBitmapContextGetData (cgctx);
   if (data != NULL) {
       //offset locates the pixel in the data from x,y. 
       //4 for 4 bytes of data per pixel, w is width of one row of data.
       @try {
           int offset = 4*((w*round(point.y))+round(point.x));
           NSLog(@"offset: %d", offset);
           int alpha =  data[offset]; 
           int red = data[offset+1]; 
           int green = data[offset+2]; 
           int blue = data[offset+3]; 
           NSLog(@"offset: %i colors: RGB A %i %i %i  %i",offset,red,green,blue,alpha);
           color = [UIColor colorWithRed:(red/255.0f) green:(green/255.0f) blue:(blue/255.0f) alpha:(alpha/255.0f)];
       }
       @catch (NSException * e) {
           NSLog(@"%@",[e reason]);
       }
       @finally {
       }

   }
   // When finished, release the context
   CGContextRelease(cgctx); 
   // Free image data memory for the context
   if (data) { free(data); }

   return color;
}

 

创建取点图片工作域：

- (CGContextRef) createARGBBitmapContextFromImage:(CGImageRef) inImage {

   CGContextRef    context = NULL;
   CGColorSpaceRef colorSpace;
   void *          bitmapData;
   int             bitmapByteCount;
   int             bitmapBytesPerRow;

   // Get image width, height. We'll use the entire image.
   size_t pixelsWide = CGImageGetWidth(inImage);
   size_t pixelsHigh = CGImageGetHeight(inImage);

   // Declare the number of bytes per row. Each pixel in the bitmap in this
   // example is represented by 4 bytes; 8 bits each of red, green, blue, and
   // alpha.
   bitmapBytesPerRow   = (pixelsWide * 4);
   bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);

   // Use the generic RGB color space.
   colorSpace = CGColorSpaceCreateDeviceRGB();

   if (colorSpace == NULL)
   {
       fprintf(stderr, "Error allocating color spacen");
       return NULL;
   }

   // Allocate memory for image data. This is the destination in memory
   // where any drawing to the bitmap context will be rendered.
   bitmapData = malloc( bitmapByteCount );
   if (bitmapData == NULL) 
   {
       fprintf (stderr, "Memory not allocated!");
       CGColorSpaceRelease( colorSpace );
       return NULL;
   }

   // Create the bitmap context. We want pre-multiplied ARGB, 8-bits 
   // per component. Regardless of what the source image format is 
   // (CMYK, Grayscale, and so on) it will be converted over to the format
   // specified here by CGBitmapContextCreate.
   context = CGBitmapContextCreate (bitmapData,
                                                                    pixelsWide,
                                                                    pixelsHigh,
                                                                    8,      // bits per component
                                                                    bitmapBytesPerRow,
                                                                    colorSpace,
                                                                    kCGImageAlphaPremultipliedFirst);
   if (context == NULL)
   {
       free (bitmapData);
       fprintf (stderr, "Context not created!");
   }
   // Make sure and release colorspace before returning
   CGColorSpaceRelease( colorSpace );
   
   return context;
}