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; } 

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

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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];            }