How can I manipulate the pixel values in a CGImageRef in Xcode - ios

Question

I have some
CGImageRef cgImage = "something"
Is there a way to manipulate the pixel values of this cgImage? For example if this image contains values between 0.0001 and 3000 thus when I try to view or release the image this way in an NSImageView (How can I show an image in a NSView using an CGImageRef image)
I get a black image, all pixels are black, I think it has to do with setting the pixel range values in a different color map (I don't know).
I want to be able to manipulate or change the pixel values or just be able to see the image by manipulating the color map range.
I have tried this but obviously it doesn't work:
CGContextDrawImage(ctx, CGRectMake(0,0, CGBitmapContextGetWidth(ctx),CGBitmapContextGetHeight(ctx)),cgImage);
UInt8 *data = CGBitmapContextGetData(ctx);
for (**all pixel values and i++ **) {
data[i] = **change to another value I want depending on the value in data[i]**;
}
Thank you,

Answer 1

In order to manipulate individual pixels in an image
allocate a buffer to hold the pixels
create a memory bitmap context using that buffer
draw the image into the context, which puts the pixels into the
buffer
change the pixels as desired
create a new image from the context
free up resources (note be sure to check for leaks using instruments)
Here's some sample code to get you started. This code will swap the blue and red components of each pixel.
- (CGImageRef)swapBlueAndRedInImage:(CGImageRef)image
{
int x, y;
uint8_t red, green, blue, alpha;
uint8_t *bufptr;
int width = CGImageGetWidth( image );
int height = CGImageGetHeight( image );
// allocate memory for pixels
uint32_t *pixels = calloc( width * height, sizeof(uint32_t) );
// create a context with RGBA pixels
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate( pixels, width, height, 8, width * sizeof(uint32_t), colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedLast );
// draw the image into the context
CGContextDrawImage( context, CGRectMake( 0, 0, width, height ), image );
// manipulate the pixels
bufptr = (uint8_t *)pixels;
for ( y = 0; y < height; y++)
for ( x = 0; x < width; x++ )
{
red = bufptr[3];
green = bufptr[2];
blue = bufptr[1];
alpha = bufptr[0];
bufptr[1] = red; // swaps the red and blue
bufptr[3] = blue; // components of each pixel
bufptr += 4;
}
// create a new CGImage from the context with modified pixels
CGImageRef resultImage = CGBitmapContextCreateImage( context );
// release resources to free up memory
CGContextRelease( context );
CGColorSpaceRelease( colorSpace );
free( pixels );
return( resultImage );
}