How to get the RGB values for a pixel on an image on the iphone

Question

I am writing an iPhone application and need to essentially implement something equivalent to the 'eyedropper' tool in photoshop, where you can touch a point on the image and capture the RGB values for the pixel in question to determine and match its color. Getting the UIImage is the easy part, but is there a way to convert the UIImage data into a bitmap representation in which I could extract this information for a given pixel? A working code sample would be most appreciated, and note that I am not concerned with the alpha value.

Answer 1

A little more detail...

I posted earlier this evening with a consolidation and small addition to what had been said on this page - that can be found at the bottom of this post. I am editing the post at this point, however, to post what I propose is (at least for my requirements, which include modifying pixel data) a better method, as it provides writable data (whereas, as I understand it, the method provided by previous posts and at the bottom of this post provides a read-only reference to data).

Method 1: Writable Pixel Information

1. I defined constants

   #define RGBA        4
   #define RGBA_8_BIT  8
   

2. In my UIImage subclass I declared instance variables:

   size_t bytesPerRow;
   size_t byteCount;
   size_t pixelCount;
   
   CGContextRef context;
   CGColorSpaceRef colorSpace;
   
   UInt8 *pixelByteData;
   // A pointer to an array of RGBA bytes in memory
   RPVW_RGBAPixel *pixelData;
   

3. The pixel struct (with alpha in this version)

   typedef struct RGBAPixel {
       byte red;
       byte green;
       byte blue;
       byte alpha;
   } RGBAPixel;
   

4. Bitmap function (returns pre-calculated RGBA; divide RGB by A to get unmodified RGB):

   -(RGBAPixel*) bitmap {
       NSLog( @"Returning bitmap representation of UIImage." );
       // 8 bits each of red, green, blue, and alpha.
       [self setBytesPerRow:self.size.width * RGBA];
       [self setByteCount:bytesPerRow * self.size.height];
       [self setPixelCount:self.size.width * self.size.height];
   
       // Create RGB color space
       [self setColorSpace:CGColorSpaceCreateDeviceRGB()];
   
       if (!colorSpace)
       {
           NSLog(@"Error allocating color space.");
           return nil;
       }
   
       [self setPixelData:malloc(byteCount)];
   
       if (!pixelData)
       {
           NSLog(@"Error allocating bitmap memory. Releasing color space.");
           CGColorSpaceRelease(colorSpace);
   
           return nil;
       }
   
       // Create the bitmap context. 
       // Pre-multiplied RGBA, 8-bits per component. 
       // The source image format will be converted to the format specified here by CGBitmapContextCreate.
       [self setContext:CGBitmapContextCreate(
                                              (void*)pixelData,
                                              self.size.width,
                                              self.size.height,
                                              RGBA_8_BIT,
                                              bytesPerRow,
                                              colorSpace,
                                              kCGImageAlphaPremultipliedLast
                                              )];
   
       // Make sure we have our context
       if (!context)   {
           free(pixelData);
           NSLog(@"Context not created!");
       }
   
       // Draw the image to the bitmap context. 
       // The memory allocated for the context for rendering will then contain the raw image pixelData in the specified color space.
       CGRect rect = { { 0 , 0 }, { self.size.width, self.size.height } };
   
       CGContextDrawImage( context, rect, self.CGImage );
   
       // Now we can get a pointer to the image pixelData associated with the bitmap context.
       pixelData = (RGBAPixel*) CGBitmapContextGetData(context);
   
       return pixelData;
   }
   

---

Read-Only Data (Previous information) - method 2:

---

Step 1. I declared a type for byte:

 typedef unsigned char byte;

Step 2. I declared a struct to correspond to a pixel:

 typedef struct RGBPixel{
    byte red;
    byte green;
    byte blue;  
    }   
RGBPixel;

Step 3. I subclassed UIImageView and declared (with corresponding synthesized properties):

//  Reference to Quartz CGImage for receiver (self)  
CFDataRef bitmapData;   

//  Buffer holding raw pixel data copied from Quartz CGImage held in receiver (self)    
UInt8* pixelByteData;

//  A pointer to the first pixel element in an array    
RGBPixel* pixelData;

Step 4. Subclass code I put in a method named bitmap (to return the bitmap pixel data):

//Get the bitmap data from the receiver's CGImage (see UIImage docs)  
[self setBitmapData: CGDataProviderCopyData(CGImageGetDataProvider([self CGImage]))];

//Create a buffer to store bitmap data (unitialized memory as long as the data)    
[self setPixelBitData:malloc(CFDataGetLength(bitmapData))];

//Copy image data into allocated buffer    
CFDataGetBytes(bitmapData,CFRangeMake(0,CFDataGetLength(bitmapData)),pixelByteData);

//Cast a pointer to the first element of pixelByteData    
//Essentially what we're doing is making a second pointer that divides the byteData's units differently - instead of dividing each unit as 1 byte we will divide each unit as 3 bytes (1 pixel).    
pixelData = (RGBPixel*) pixelByteData;

//Now you can access pixels by index: pixelData[ index ]    
NSLog(@"Pixel data one red (%i), green (%i), blue (%i).", pixelData[0].red, pixelData[0].green, pixelData[0].blue);

//You can determine the desired index by multiplying row * column.    
return pixelData;

Step 5. I made an accessor method:

-(RGBPixel*)pixelDataForRow:(int)row column:(int)column{
    //Return a pointer to the pixel data
    return &pixelData[row * column];           
}