Add exif reading of LinearResponseLimit

The electronic circuit of a camera and/or the sensor might not work in a linear/correct way
above certain values.
This information might be provided in vendor-tags or `Exif.Image.LinearResponseLimit`.

We can't make use of that info in modules like temperature or demosaic as there is no generic
correction algorithm but highlights reconstruction could use it as a threshold.

The single float value is now available in dt_image_t defaulting to 1.0 if there is no exif tag.

Author: jenshannoschwalm

src/common/exif.cc

@@ -858,6 +858,25 @@ static bool _check_usercrop(Exiv2::ExifData &exifData,
   return FALSE;
 }
 
+static void _check_linear_response_limit(Exiv2::ExifData &exifData,
+                                         dt_image_t *img)
+{
+  bool found_one = false;
+
+  Exiv2::ExifData::const_iterator linr =
+    exifData.findKey(Exiv2::ExifKey("Exif.Image.LinearResponseLimit"));
+  if(linr != exifData.end() && linr->count() == 1)
+  {
+    img->linear_response_limit = linr->toFloat();
+    found_one = true;
+  }
+
+  // currently this only reads the dng tag, could also be used for other raws
+  if(found_one)
+    dt_print(DT_DEBUG_IMAGEIO, "[exif] `%s` has LinearResponseLimit %.4f",
+      img->filename, img->linear_response_limit);
+}
+
 static gboolean _check_dng_opcodes(Exiv2::ExifData &exifData,
                                    dt_image_t *img)
 {
@@ -1074,13 +1093,14 @@ void dt_exif_img_check_additional_tags(dt_image_t *img,
       _check_usercrop(exifData, img);
       _check_dng_opcodes(exifData, img);
       _check_lens_correction_data(exifData, img);
+      _check_linear_response_limit(exifData, img);
     }
     return;
   }
   catch(Exiv2::AnyError &e)
   {
     const char *errstring = e.what();
-    dt_print(DT_DEBUG_IMAGEIO, "[exiv2 reading DefaultUserCrop] %s: %s", filename, errstring);
+    dt_print(DT_DEBUG_IMAGEIO, "[exiv2 reading additional exif tags] %s: %s", filename, errstring);
     return;
   }
 }

src/common/image.c

@@ -2132,6 +2132,7 @@ void dt_image_init(dt_image_t *img)
   img->colorspace = DT_IMAGE_COLORSPACE_NONE;
   img->fuji_rotation_pos = 0;
   img->pixel_aspect_ratio = 1.0f;
+  img->linear_response_limit = 1.0f;
   img->wb_coeffs[0] = NAN;
   img->wb_coeffs[1] = NAN;
   img->wb_coeffs[2] = NAN;

src/common/image.h

@@ -336,6 +336,9 @@ typedef struct dt_image_t
   uint32_t fuji_rotation_pos;
   float pixel_aspect_ratio;
 
+  /* might help to improve highlights reconstruction module */
+  float linear_response_limit;
+
   /* White balance coeffs from the raw */
   dt_aligned_pixel_t wb_coeffs;