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    <h2>A little more detail...</h2> <p>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). </p> <p>Method 1: Writable Pixel Information</p> <ol> <li><p>I defined constants</p> <pre><code>#define RGBA 4 #define RGBA_8_BIT 8 </code></pre></li> <li><p>In my UIImage subclass I declared instance variables:</p> <pre><code>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; </code></pre></li> <li><p>The pixel struct (with alpha in this version)</p> <pre><code>typedef struct RGBAPixel { byte red; byte green; byte blue; byte alpha; } RGBAPixel; </code></pre></li> <li><p>Bitmap function (returns pre-calculated RGBA; divide RGB by A to get unmodified RGB):</p> <pre><code>-(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; } </code></pre></li> </ol> <hr> <h2>Read-Only Data (Previous information) - method 2:</h2> <hr> <p>Step 1. I declared a type for byte:</p> <pre><code> typedef unsigned char byte; </code></pre> <p>Step 2. I declared a struct to correspond to a pixel: </p> <pre><code> typedef struct RGBPixel{ byte red; byte green; byte blue; } RGBPixel; </code></pre> <p>Step 3. I subclassed UIImageView and declared (with corresponding synthesized properties):</p> <pre><code>// 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; </code></pre> <p>Step 4. Subclass code I put in a method named bitmap (to return the bitmap pixel data):</p> <pre><code>//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; </code></pre> <p>Step 5. I made an accessor method:</p> <pre><code>-(RGBPixel*)pixelDataForRow:(int)row column:(int)column{ //Return a pointer to the pixel data return &amp;pixelData[row * column]; } </code></pre>
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    1. POHow to get the RGB values for a pixel on an image on the iphone
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    1. POHow to get the RGB values for a pixel on an image on the iphone
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    1. COGreat detail but this answer could do with some tidying up. The code needs to be properly marked as code so it presents properly. I'd do it myself but I don't yet have the ability to edit other people's answers.
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    2. COyea, horrible formatting
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    3. COThis needs to be modified for retina displays. The size of a UIImage is in points, not pixels. The size needs to be multiplied by the scaling factor (self.scale).
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      1. USVictor Engel
 

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