iOS Loading 16bit Single Channel Image

Question

I have a 16bit grayscale image. I have tried both .png and .tif. .tif works somewhat. I have the following code:

CGDataProviderRef l_Img_Data_Provider = CGDataProviderCreateWithFilename( [m_Name cStringUsingEncoding:NSASCIIStringEncoding] );
CGImageRef l_CGImageRef = CGImageCreate( m_Width, m_Height, 16, 16, m_Width * 2,
                                        CGColorSpaceCreateDeviceGray(), kCGBitmapByteOrder16Big, l_Img_Data_Provider, NULL, false, kCGRenderingIntentDefault );
test_Image = [[UIImage alloc] initWithCGImage:l_CGImageRef];
[_test_Image_View setImage:test_Image];

This results in the following image: faulty gradient

As you can see, there seems to be an issue at the beginning of the image ( could it be trying to use the byte data from the header? ), and the image is offset by about a fifth ( a little harder to see, look at the left and the right, there is a faint line about a fifth away from the right.

My goal is to convert this to a metal texture and use it from there. Also having issues there. Seem like a byte order issue but maybe we can come back to that.

dave

Answer 1

CGDataProvider doesn't know about the format of the data that it stores. It is just meant for handling generic data:

"The CGDataProvider header file declares a data type that supplies Quartz functions with data. Data provider objects abstract the data-access task and eliminate the need for applications to manage data through a raw memory buffer."

CGDataProvider

Because CGDataProvider is generic you must provide the format of the image data using the CGImageCreate parameters. PNGs and JPGs have their own CGImageCreateWith.. functions for handling encoded data.

The CGImage parameters in your example correctly describe a 16 bit grayscale raw byte format but nothing about TIF encoding so I would guess you are correct in guessing that the corrupted pixels you are see are from the file headers.

There may be other ways to load a 16 bit grayscale image on iOS, but to use that method (or the very similar Metal method) you would need to parse the image bytes from the TIF file and pass that into the function, or create another way to store and parse the image data.