Creating Image from pixel data with CGBitmapContextCreate

Question

I am trying to write code that can crop an existing image down to some specified size/region. I am working with DICOM images, and the API I am using allows me to get pixel values directly. I've placed pixel values of the area of interest within the image into an array of floats (dstImage, below).

Where I'm encountering trouble is with the actual construction/creation of the new, cropped image file using this pixel data. The source image is grayscale, however all of the examples I have found online (like this one) have been for RGB images. I tried to follow the example in that link, adjusting for grayscale and trying numerous different values, but I continue to get errors on the CGBitmapContextCreate line of code and still do not clearly understand what those values are supposed to be.

My intensity values for the source image go above 255, so my impression is that this is not 8-bit Grayscale, but 16-bit Grayscale.

Here is my code:

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();

CGContextRef context;
context = CGBitmapContextCreate(dstImage, // pixel data from the region of interest
                                dstWidth, // width of the region of interest
                                dstHeight, // height of the region of interest
                                16, // bits per component
                                2 * dstWidth, // bytes per row
                                colorSpace,
                                kCGImageAlphaNoneSkipLast);

CFRelease(colorSpace);

CGImageRef cgImage = CGBitmapContextCreateImage(context);
CFURLRef url = CFURLCreateWithFileSystemPath(kCFAllocatorDefault,
                                             CFSTR("test.png"),
                                             kCFURLPOSIXPathStyle,
                                             false);
CFStringRef type = kUTTypePNG;
CGImageDestinationRef dest = CGImageDestinationCreateWithURL(url,
                                                             type,
                                                             1,
                                                             0);
CGImageDestinationAddImage(dest,
                           cgImage,
                           0);
CFRelease(cgImage);
CFRelease(context);
CGImageDestinationFinalize(dest);
free(dstImage);

The error I keep receiving is:

CGBitmapContextCreate: unsupported parameter combination: 16 integer bits/component; 32 bits/pixel; 1-component color space; kCGImageAlphaNoneSkipLast; 42 bytes/row.

The ultimate goal is to create an image file from the pixel data in dstImage and save it to the hard drive. Help on this would be greatly appreciated as would insight into how to determine what values I should be using in the CGBitmapContextCreate call.

Thank you

Ken Thomases Ken Thomases · Accepted Answer · 2014-04-23T15:36:13

First, you should familiarize yourself with the "Supported Pixel Formats" section of Quartz 2D Programming Guide: Graphics Contexts.

If your image data is in an array of float values, then it's 32-bits-per-component, not 16. Therefore, you have to use kCGImageAlphaNone | kCGBitmapFloatComponents.

However, I believe that Core Graphics will interpret floating-point components as being between 0.0 and 1.0. If your values are outside of that, you may need to convert them using something like (value - minimumValue) / (maximumValue - minimumValue). An alternative may be to use CGColorSpaceCreateCalibratedGray() or to create a CGImage using CGImageCreate() and specifying an appropriate decode parameter and then create a bitmap context from that using CGBitmapContextCreateImage().

In fact, if you're not drawing into your bitmap context, you should just be creating a CGImage instead, anyway.

Creating Image from pixel data with CGBitmapContextCreate

1 Answers