iphone - Retrieving a pixel alpha value for a UIImage -

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I am currently trying to get the alpha value of pixels in a UIImageView. I have received CGI image from [UIImageView image] and created an RGBA byte array from this. Alpha is preplanned. 

  CGIJJRF image = uiImage. CGImage; Width of NSUINTER = CGMJetWidth (image); NSUInteger height = CIggate height (image); CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB (); RawData = malloc (height * width * 4); BytePrintPixel = 4; Bytespiro = bytepixel * width; NSUInteger BitsStechnicPoint = 8; CGContextRef Reference = CGBitmapContextCreate (raw data, width, height, bitsprinting, byteper, colorspace, KCGImAlphapremTiltlivedList; KCGITMAPOder32Big); CGColorSpaceRelease (colorspace); CGContextDrawImage (Reference, CGRectMake (0, 0, width, height), image); CGContextRelease (reference);

I then calculate the array index for the given alpha channel using coordinates from UIImageView. 

  int byteIndex = (Bitespiro * uiViewPoint.y) + UiViewPoint.x * BytePixel; Unsigned four characters = raw data [byteindex + 3];

However, I do not get the values ​​that I expect. For the completely black transparent area of ​​the image, I get zero-zero values ​​for the alpha channel. Do I need to translate the coordination between UIKit and core graphics - that is: Is the y-axis inverted? Or have I already understood alpha values ​​wrong? . However, after setting the blend mode my alpha values ​​were what I expected: 

  CGContextSetBlendMode (reference, kCGBlendModeCopy); Yes, points to CGContexts in the UIKit pointing y-axis. .  
  Edit the code after reading:   
 You also want to set the blend mode to change before dragging the image, since you will see the image Let's assume alpha which was in the first reference buffer:  
  CGContactTatbandmod (reference, KCGb and MODCopy);  
 
  Edit after thinking:  
 
 You can make the least possible CGBitmapContext more efficient than building  too  (1x1 pixels? Maybe 8x8? Is a try) and translate it from the context before highlighting your desired position: 
 
  cgiontacttranslatecmtm (reference, xoffet, ouafet);

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