Archive for the ‘Tutorials’ Category

Creating UIImage from raw RGBA data

Friday, March 4th, 2011

Our earlier tutorial, described how one can access RGBA data from a UIImage at any particular pixel coordinate.

The following code illustrates just the opposite – how does one go about constructing a UIImage from raw RGBA data.

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//
//Constructing RGBA data
//
/*height and width are integers denoting the dimensions of the image*/
unsigned char *rawData = malloc(width*height*4);

//populating rawData with pixel colour values and alpha information
for (int i=0; i<width*height;++i)
{
    rawData[4*i] = <red colour value>;
    rawData[4*i+1] = <green colour value>;
    rawData[4*i+2] = <blue colour value>;
    rawData[4*i+3] = 255; //alpha
}

/*
Once we have the raw data, we convert it into a UIImage.
The following code does the required work.
*/

CGDataProviderRef provider = CGDataProviderCreateWithData(NULL,rawData,width*height*4,NULL);

int bitsPerComponent = 8;
int bitsPerPixel = 32;
int bytesPerRow = 4*width;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;

CGImageRef imageRef =
CGImageCreate(width,height,8,32,4*width,colorSpaceRef,bitmapInfo,provider,NULL,NO,renderingIntent);

UIImage *newImage = [UIImage imageWithCGImage:imageRef];

/*
newImage is the final image that is constructed from raw RGBA data.
Do remember to release the allocated parts.
*/

Please feel free to use this code in your projects.

Accessing RGB data from UIImage

Wednesday, March 2nd, 2011

Our iphone application ArtMaestro, reads RGB data from a UIImage.

The following function illustrates how to read RGBA data from a UIImage.

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-(void) readImage:(UIImage*)inputImage
{
    CGImageRef inImage = [inputImage CGImage];
    //height of input image
    int ht = CGImageGetWidth(inImage);
    //width of input image
    int wdth = CGImageGetHeight(inImage);

    //determine colour space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    //Array to store RGB values
    unsigned char *rawData = malloc(ht*wdth*4);
    //input image characteristics
    NSUInteger bytesPerRowImg = CGImageGetBytesPerRow(inImage);
    NSUInteger bitsPerComponentImg = CGImageGetBitsPerComponent(inImage);
    NSUInteger bitsPerPixelImg = CGImageGetBitsPerPixel(inImage);
   
    //create CGContext
 CGContextRefcontext=CGBitmapContextCreate(rawData,ht,wdth,bitsPerComponentImg,bytesPerRowImg,colorSpace,kCGImageAlphaPremultipliedLast| kCGBitmapByteOrder32Big);

    // One might change these parameters -
    // kCGImageAlphaPremultipliedLast| kCGBitmapByteOrder32Big
    // This depends upon RGBA/ ARGB (location of alpha)
    // and other parameters

    //free the colorspace
    CGColorSpaceRelease(colorSpace);
    CGContextDrawImage(context,CGRectMake(0,0,wdth,ht), inImage);
   
    //release CGImage
    CFRelease(inImage);

    //release context
    CGContextRelease(context);

    int byteIndex = 0;
   
    //Copy RGB values into rawData
    for(int i = 0;i < count; ++i)
    {
        Byte redColour = rawData[byteIndex];
        Byte greenColour = rawData[byteIndex+1];
        Byte blueColour = rawData[byteIndex+2];
        CGFloat alphaColour = rawData[byteIndex + 3]/255.0f;
        //would also depend upon CGBitmapInfo of the input image
       
        byteIndex +=4;
    }

    /*
        The array rawData now contains the RGB values
        You might use these values for you image processing
        projects.
        Make sure you free this after your work
    */

    ///
    // Your RGBA manipulation code goes here
    // To access the RGBA data at a particular pixel (xCor,yCor)
    // int redValueAtPixel = rawData[wdth*yCor+xCor];
    // int greenValueAtPixel = rawData[wdth*yCor+xCor+1];
    // int blueValueAtPixel = rawData[wdth*yCor+xCor+2];
    // float alphaAtPixel = rawData[wdth*yCor+xCor+3]x255.0f;
    ///

    free(rawData);
}

Please feel free to use this code in your projects.
Happy Coding :)