qcam: dng_writer: Record creation time in the EXIF directory

If the EXIF directory is empty due to no metadata being available tools
such as tiffinfo complains that the directory is malformed.

    TIFFFetchDirectory: Sanity check on directory count failed, this is probably not a valid IFD offset.
    TIFFReadCustomDirectory: Failed to read custom directory at offset 0.

Always record the creation time in the EXIF directory instead of adding
complexity to skip creating the EXIF directory if there is no metadata
to record. This ensures there are at least some entries in the EXIF
directory and that makes tiffinfo happy.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/qcam/dng_writer.cpp

@@ -427,6 +427,23 @@ int DNGWriter::write(const char *filename, const Camera *camera,
 	/* Create a new IFD for the EXIF data and fill it. */
 	TIFFCreateEXIFDirectory(tif);
 
+	/* Store creation time. */
+	time_t rawtime;
+	struct tm *timeinfo;
+	char strTime[20];
+
+	time(&rawtime);
+	timeinfo = localtime(&rawtime);
+	strftime(strTime, 20, "%Y:%m:%d %H:%M:%S", timeinfo);
+
+	/*
+	 * \todo Handle timezone information by setting OffsetTimeOriginal and
+	 * OffsetTimeDigitized once libtiff catches up to the specification and
+	 * has EXIFTAG_ defines to handle them.
+	 */
+	TIFFSetField(tif, EXIFTAG_DATETIMEORIGINAL, strTime);
+	TIFFSetField(tif, EXIFTAG_DATETIMEDIGITIZED, strTime);
+
 	if (metadata.contains(controls::AnalogueGain)) {
 		float gain = metadata.get(controls::AnalogueGain);
 		uint16_t iso = std::min(std::max(gain * 100, 0.0f), 65535.0f);