How to open LinearDNG in ObjectiveC/C++?

Question

I've trying to open LinearDNG file in my MacOS app. As far as I've investigated, that's not possible with CoreImage.

So, the questions:

1. Is it possible to open LinearDNG file as 16-bit/8-bit RGBA data with CoreImage?
2. Is it possible to open LinearDNG file as 16-bit/8-bit RGBA data with dcraw?
3. Do you know the other libs to open LinearDNG file as 16-bit/8-bit RGBA data?

My LinearDNG files are 16-bit. The relevant EXIF data:

File Type                       : DNG
File Type Extension             : dng
MIME Type                       : image/x-adobe-dng

Bits Per Sample                 : 16 16 16
Compression                     : Uncompressed
Photometric Interpretation      : Linear Raw

Format                          : image/jpeg

I've read that CIContext can convert an image to TIFF/JPEG, but code with this particular LinearDNG file doesn't work. All JPEG images are black, all TIFF images are transparent. I guess CIImage just doesn't understand this encoding.

My testing code:

[self convertLinearDNGToJPEG:@"super_jpeg.jpeg"];
        
[self convertLinearDNGToTIFF:kCIFormatRGBA8 fileName:@"super_kCIFormatRGBA8.tiff"];
[self convertLinearDNGToTIFF:kCIFormatRGBA16 fileName:@"super_kCIFormatRGBA16.tiff"];
[self convertLinearDNGToTIFF:kCIFormatARGB8 fileName:@"super_kCIFormatARGB8.tiff"];
[self convertLinearDNGToTIFF:kCIFormatRGBAh fileName:@"super_kCIFormatRGBAh.tiff"];

The corresponding functions (convertLinearDNGToTIFF/convertLinearDNGToJPEG ):

- (void)convertLinearDNGToJPEG:(NSString*)fileName
{
    CIImage* inputImage = [self createInputImage];
    
    NSError* superError = nil;
    CIContext* ciContext = [self createCiContext];
    
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    
    NSURL* superURL = [NSURL fileURLWithPath: [documentsDirectory stringByAppendingPathComponent:fileName] ];
    CGColorSpaceRef superColorSpace = CGColorSpaceCreateDeviceRGB();
    [ciContext writeJPEGRepresentationOfImage:inputImage
                                        toURL:superURL
                                   colorSpace:superColorSpace
                                      options:@{} // CIImageRepresentationOption
                                        error:&superError];
}
- (void)convertLinearDNGToTIFF:(CIFormat)format fileName:(NSString*)fileName
{
    CIImage* inputImage = [self createInputImage];
    
    NSError* superError = nil;
    CIContext* ciContext = [self createCiContext];
    
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    
    NSURL* superURL = [NSURL fileURLWithPath: [documentsDirectory stringByAppendingPathComponent:fileName] ];
    CGColorSpaceRef superColorSpace = CGColorSpaceCreateDeviceRGB();
    [ciContext writeTIFFRepresentationOfImage:inputImage
                                        toURL:superURL
                                       format:format
                                   colorSpace:superColorSpace
                                      options:@{} // CIImageRepresentationOption
                                        error:&superError];
}

Answer 1

Well, I've tried LibRaw: https://github.com/LibRaw/LibRaw. It's able to export my LinearDNG to TIFF. And I can open TIFF with CoreImage. The white balance is wrong though. I assume this is because libraw reads metadata wrong ( file samples/raw-identify.cpp ).

The code for exporting to TIFF is in file samples/simple_dcraw.cpp

/* -*- C++ -*-
 * File: simple_dcraw.cpp
 * Copyright 2008-2020 LibRaw LLC (info@libraw.org)
 * Created: Sat Mar  8, 2008
 *
 * LibRaw simple C++ API:  emulates call to "dcraw  [-D]  [-T] [-v] [-e] [-4]"

LibRaw is free software; you can redistribute it and/or modify
it under the terms of the one of two licenses as you choose:

1. GNU LESSER GENERAL PUBLIC LICENSE version 2.1
   (See file LICENSE.LGPL provided in LibRaw distribution archive for details).

2. COMMON DEVELOPMENT AND DISTRIBUTION LICENSE (CDDL) Version 1.0
   (See file LICENSE.CDDL provided in LibRaw distribution archive for details).


 */
#include <stdio.h>
#include <string.h>
#include <math.h>

#include "libraw/libraw.h"

#ifndef LIBRAW_WIN32_CALLS
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/mman.h>
#endif

#ifdef LIBRAW_WIN32_CALLS
#define snprintf _snprintf
#endif

int my_progress_callback(void *unused_data, enum LibRaw_progress state,
                         int iter, int expected)
{
  if (iter == 0)
    printf("CB: state=%x, expected %d iterations\n", state, expected);
  return 0;
}

char *customCameras[] = {
    (char *)"43704960,4080,5356, 0, 0, 0, 0,0,148,0,0, Dalsa, FTF4052C Full,0",
    (char *)"42837504,4008,5344, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF4052C 3:4",
    (char *)"32128128,4008,4008, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF4052C 1:1",
    (char *)"24096096,4008,3006, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF4052C 4:3",
    (char *)"18068064,4008,2254, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF4052C 16:9",
    (char *)"67686894,5049,6703, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF5066C Full",
    (char *)"66573312,4992,6668, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF5066C 3:4",
    (char *)"49840128,4992,4992, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF5066C 1:1",
    (char *)"37400064,4992,3746, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF5066C 4:3",
    (char *)"28035072,4992,2808, 0, 0, 0, 0,0,148,0,0,Dalsa, FTF5066C 16:9",
    NULL};

int main(int ac, char *av[])
{
  int i, ret, verbose = 0, output_thumbs = 0;

  // don't use fixed size buffers in real apps!
  char outfn[1024], thumbfn[1024];

  LibRaw* RawProcessor = new LibRaw;
  RawProcessor->imgdata.params.custom_camera_strings = customCameras;
  if (ac < 2)
  {
    printf("simple_dcraw - LibRaw %s sample. Emulates dcraw [-D] [-T] [-v] "
           "[-e] [-E]\n"
           " %d cameras supported\n"
           "Usage: %s [-D] [-T] [-v] [-e] raw-files....\n"
           "\t-4 - 16-bit mode\n"
           "\t-L - list supported cameras and exit\n"
           "\t-v - verbose output\n"
           "\t-T - output TIFF files instead of .pgm/ppm\n"
           "\t-e - extract thumbnails (same as dcraw -e in separate run)\n",
           LibRaw::version(), LibRaw::cameraCount(), av[0]);
    delete RawProcessor;
    return 0;
  }

  putenv((char *)"TZ=UTC"); // dcraw compatibility, affects TIFF datestamp field

#define P1 RawProcessor->imgdata.idata
#define S RawProcessor->imgdata.sizes
#define C RawProcessor->imgdata.color
#define T RawProcessor->imgdata.thumbnail
#define P2 RawProcessor->imgdata.other
#define OUT RawProcessor->imgdata.params

  for (i = 1; i < ac; i++)
  {
    if (av[i][0] == '-')
    {
      if (av[i][1] == 'T' && av[i][2] == 0)
        OUT.output_tiff = 1;
      if (av[i][1] == 'v' && av[i][2] == 0)
        verbose++;
      if (av[i][1] == 'e' && av[i][2] == 0)
        output_thumbs++;
      if (av[i][1] == '4' && av[i][2] == 0)
        OUT.output_bps = 16;
      if (av[i][1] == 'C' && av[i][2] == 0)
        RawProcessor->set_progress_handler(my_progress_callback, NULL);
      if (av[i][1] == 'L' && av[i][2] == 0)
      {
        const char **clist = LibRaw::cameraList();
        const char **cc = clist;
        while (*cc)
        {
          printf("%s\n", *cc);
          cc++;
        }
        delete RawProcessor;
        exit(0);
      }
      continue;
    }

    if (verbose)
      printf("Processing file %s\n", av[i]);

    if ((ret = RawProcessor->open_file(av[i])) != LIBRAW_SUCCESS)
    {
      fprintf(stderr, "Cannot open_file %s: %s\n", av[i], libraw_strerror(ret));
      continue; // no recycle b/c open file will recycle itself
    }

    if (!output_thumbs) // No unpack for thumb extraction
      if ((ret = RawProcessor->unpack()) != LIBRAW_SUCCESS)
      {
        fprintf(stderr, "Cannot unpack %s: %s\n", av[i], libraw_strerror(ret));
        continue;
      }

    // thumbnail unpacking and output in the middle of main
    // image processing - for test purposes!
    if (output_thumbs)
    {
      if ((ret = RawProcessor->unpack_thumb()) != LIBRAW_SUCCESS)
      {
        fprintf(stderr, "Cannot unpack_thumb %s: %s\n", av[i],
                libraw_strerror(ret));
        if (LIBRAW_FATAL_ERROR(ret))
          continue; // skip to next file
      }
      else
      {
        snprintf(thumbfn, sizeof(thumbfn), "%s.%s", av[i],
                 T.tformat == LIBRAW_THUMBNAIL_JPEG ? "thumb.jpg"
                                                    : "thumb.ppm");

        if (verbose)
          printf("Writing thumbnail file %s\n", thumbfn);
        if (LIBRAW_SUCCESS != (ret = RawProcessor->dcraw_thumb_writer(thumbfn)))
        {
          fprintf(stderr, "Cannot write %s: %s\n", thumbfn,
                  libraw_strerror(ret));
          if (LIBRAW_FATAL_ERROR(ret))
            continue;
        }
      }
      continue;
    }

    ret = RawProcessor->dcraw_process();

    if (LIBRAW_SUCCESS != ret)
    {
      fprintf(stderr, "Cannot do postpocessing on %s: %s\n", av[i],
              libraw_strerror(ret));
      if (LIBRAW_FATAL_ERROR(ret))
        continue;
    }
    snprintf(outfn, sizeof(outfn), "%s.%s", av[i],
             OUT.output_tiff ? "tiff" : (P1.colors > 1 ? "ppm" : "pgm"));

    if (verbose)
      printf("Writing file %s\n", outfn);

    if (LIBRAW_SUCCESS != (ret = RawProcessor->dcraw_ppm_tiff_writer(outfn)))
      fprintf(stderr, "Cannot write %s: %s\n", outfn, libraw_strerror(ret));

    RawProcessor->recycle(); // just for show this call
  }
  
  delete RawProcessor;
  return 0;
}