Summary of December 10th – Get pixel value of a picture

Logbook noting:

How to get pixel data from a UIImage (Cocoa Touch) or CGImage (Core Graphics)?

+ (NSArray*)getRGBAsFromImage:(UIImage*)image atX:(int)x andY:(int)y count:(int)count

{

    NSMutableArray *result = [NSMutableArray arrayWithCapacity:count]; 

    // First get the image into your data buffer

    CGImageRef imageRef = [image CGImage];

    NSUInteger width = CGImageGetWidth(imageRef);

    NSUInteger height = CGImageGetHeight(imageRef);    

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = (unsigned char*) calloc(height * width * 4, sizeof(unsigned char));

    NSUInteger bytesPerPixel = 4;

    NSUInteger bytesPerRow = bytesPerPixel * width;

    NSUInteger bitsPerComponent = 8;

    CGContextRef context = CGBitmapContextCreate(rawData, width, height,

                    bitsPerComponent, bytesPerRow, colorSpace,

                    kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);

    CGColorSpaceRelease(colorSpace);

    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);

    CGContextRelease(context); 

    // Now your rawData contains the image data in the RGBA8888 pixel format.

    NSUInteger byteIndex = (bytesPerRow * y) + x * bytesPerPixel;

    for (int i = 0 ; i < count ; ++i)

    {

        CGFloat alpha = ((CGFloat) rawData[byteIndex + 3] ) / 255.0f;

        CGFloat red   = ((CGFloat) rawData[byteIndex]     ) / alpha;

        CGFloat green = ((CGFloat) rawData[byteIndex + 1] ) / alpha;

        CGFloat blue  = ((CGFloat) rawData[byteIndex + 2] ) / alpha;

        byteIndex += bytesPerPixel;

        UIColor *acolor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];

        [result addObject:acolor];

    }

  free(rawData); 

  return result;

}

What is CGImageRef

CGImageRef CGImageCreate ( size_t width, size_t height, size_t bitsPerComponent, size_t bitsPerPixel,size_t bytesPerRow, CGColorSpaceRef space, CGBitmapInfo bitmapInfo, CGDataProviderRef provider, constCGFloat *decode, bool shouldInterpolate, CGColorRenderingIntent intent );

How to use CGContextDrawImage?

I need some help using the CGContextDrawImage. I have the following code which will create a Bitmap context and convert the pixel data to CGImageRef. Now I need to display that image using CGContextDrawImage. I'm not very clear on how I'm supposed to use that. The following is my code:

- (void)drawBufferWidth:(int)width height:(int)height pixels:(unsigned char*)pixels

    {

     const int area = width *height;

     const int componentsPerPixel = 4;

     unsigned char pixelData[area * componentsPerPixel];

     for(int i = 0; i<area; i++)

     {

          const int offset = i * componentsPerPixel;

          pixelData[offset] = pixels[0];

          pixelData[offset+1] = pixels[1];

          pixelData[offset+2] = pixels[2];

          pixelData[offset+3] = pixels[3];

     }

     const size_t BitsPerComponent = 8;

     const size_t BytesPerRow=((BitsPerComponent * width) / 8) * componentsPerPixel;

     CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

     CGContextRef gtx = CGBitmapContextCreate(pixelData, width, height, BitsPerComponent, BytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast);

     CGImageRef myimage = CGBitmapContextCreateImage(gtx);

     //What code should go here to display the image?

     CGContextRelease(gtx);

     CGImageRelease(myimage);

}

Comments:

These two blocks of codes are all from http://stackoverflow.com/. If you are trying to get RGB value of a picture, the second block of codes is very helpful.

The basic idea is to set an array which contains 4 numbers representing Red, Green, Blue and Alpha.  And then display a huge matrix containing thousands of arrays.