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

I don't like Apples way of doing it but I would guess no one would answer since they have signed an NDA in which they agree not to talk about their programming with anyone.

Answer 2

You can't access the bitmap data of a UIImage directly.

You need to get the CGImage representation of the UIImage. Then get the CGImage's data provider, from that a CFData representation of the bitmap. Make sure to release the CFData when done.

CGImageRef cgImage = [image CGImage];
CGDataProviderRef provider = CGImageGetDataProvider(cgImage);
CFDataRef bitmapData = CGDataProviderCopyData(provider);

You will probably want to look at the bitmap info of the CGImage to get pixel order, image dimensions, etc.

Answer 3

To do something similar in my application, I created a small off-screen CGImageContext, and then rendered the UIImage into it. This allowed me a fast way to extract a number of pixels at once. This means that you can set up the target bitmap in a format you find easy to parse, and let CoreGraphics do the hard work of converting between color models or bitmap formats.

Answer 4

Lajos's answer worked for me. To get the pixel data as an array of bytes, I did this:

UInt8* data = CFDataGetBytePtr(bitmapData);

More info: CFDataRef documentation.

Also, remember to include CoreGraphics.framework

Answer 5

Hello I am also trying to accomplish this

my problem is reading the pixel data correctly once i have the pointer

so i have an image that takes up the whole iphone screen that has no alpha channel and i want to find out the color of a pixel (24 bits per pixel, 8 bits per component, 960 bytes per row)

i have the pointer to the data

UInt8 *data = CFDataGetBytePtr(bitmapData);

but now i am not sure how to index into the data correctly given an X,Y coordinate, any help on this would be appreciated

-jeff

Answer 6

I dont know how to index into image data correctly based on given X,Y cordination. Does anyone know?

Answer 7

I dont know how to index into image data correctly based on given X,Y cordination. Does anyone know?

pixelPosition = (x+(y*((imagewidth)*BytesPerPixel)));

// pitch isn't an issue with this device as far as I know and can be let zero... // ( or pulled out of the math ).

Answer 8

---

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).

---

Writable Pixel Information - method 1:

---

Step 1. I defined constants

#define RGBA    4
#define RGBA_8_BIT   8

Step 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;

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

typedef struct RGBAPixel {

byte red;
byte green;
byte blue;
byte alpha;

} RGBAPixel;

Step 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 ];  

}

Answer 9

For those that couldn't get the above to work (me) there is this useful post: http://www.markj.net/iphone-uiimage-pixel-color/

You can see the whole implementation there.