[libcamera-devel] [RFC v1 PATCH] android: jpeg: exif: Set timezone information

[Umang Jain]

Get timezone information from the timestamp, although the resolution
for EXIF_TAG_TIME_ZONE_OFFSET is fairly limited (per-hour only).

Experimentation with 'exiftool', commandline utility to read/write
exif metadata on images, resulted in rounding off the hours if the
minutes came out to >= 30. Hence, the behaviour is inspired from
exiftool itself. For e.g.,

Timezone      Tag's value
 +1015     =>     10
 +0945     =>     10
 -1145     =>    -12

The EXIF specification defines three other tags (OffsetTime,
OffsetTimeOriginal, OffsetTimeDigitized), in the EXIF IFD.
These are not supported by libexif yet.

Signed-off-by: Umang Jain <email at uajain.com>
---
 src/android/jpeg/exif.cpp | 31 +++++++++++++++++++++++++++++++
 src/android/jpeg/exif.h   |  1 +
 2 files changed, 32 insertions(+)

---

Hi Laurent/Kieran,

Can you soft review(or test) this patch to see if anything
is wrong with it?
The reason I am asking, because I cannot make it work :(
exiftool with a captured frame shows empty Timezone Offset tag as:
```
Time Zone Offset                : 
```

exif tool shows nothing.

I made sure a valid value being generated for timezone
offset in setTimeStamp().
I also checked the case where it might be preset using
exif_data_fix and not been able to re-set again.
But no, that doesn't seem the problem in my testing.


diff --git a/src/android/jpeg/exif.cpp b/src/android/jpeg/exif.cpp
index c0dbfcc..9c23cfb 100644
--- a/src/android/jpeg/exif.cpp
+++ b/src/android/jpeg/exif.cpp
@@ -7,6 +7,8 @@
 
 #include "exif.h"
 
+#include <stdlib.h>
+
 #include "libcamera/internal/log.h"
 
 using namespace libcamera;
@@ -135,6 +137,16 @@ void Exif::setShort(ExifIfd ifd, ExifTag tag, uint16_t item)
 	exif_entry_unref(entry);
 }
 
+void Exif::setSShort(ExifIfd ifd, ExifTag tag, int16_t item)
+{
+	ExifEntry *entry = createEntry(ifd, tag);
+	if (!entry)
+		return;
+
+	exif_set_sshort(entry->data, EXIF_BYTE_ORDER_INTEL, item);
+	exif_entry_unref(entry);
+}
+
 void Exif::setLong(ExifIfd ifd, ExifTag tag, uint32_t item)
 {
 	ExifEntry *entry = createEntry(ifd, tag);
@@ -196,6 +208,25 @@ void Exif::setTimestamp(time_t timestamp)
 	setString(EXIF_IFD_0, EXIF_TAG_DATE_TIME, EXIF_FORMAT_ASCII, ts);
 	setString(EXIF_IFD_EXIF, EXIF_TAG_DATE_TIME_ORIGINAL, EXIF_FORMAT_ASCII, ts);
 	setString(EXIF_IFD_EXIF, EXIF_TAG_DATE_TIME_DIGITIZED, EXIF_FORMAT_ASCII, ts);
+
+	/*
+	 * If possible, query and set timezone information via
+	 * EXIF_TAG_TIME_ZONE_OFFSET. There is only per-hour resolution for tag
+	 * hence, round up hours if minutes >= 30.
+	 */
+	int r = strftime(str, sizeof(str), "%z", &tm);
+	if (r > 0) {
+		int16_t timezone = atoi(str);
+		int16_t hour = timezone / 100;
+		int16_t min = timezone % 100;;
+
+		if (min <= -30)
+			hour--;
+		else if (min >= 30)
+			hour++;
+
+		setSShort(EXIF_IFD_0, EXIF_TAG_TIME_ZONE_OFFSET, hour);
+	}
 }
 
 void Exif::setOrientation(int orientation)
diff --git a/src/android/jpeg/exif.h b/src/android/jpeg/exif.h
index f04cefc..9c9cc3b 100644
--- a/src/android/jpeg/exif.h
+++ b/src/android/jpeg/exif.h
@@ -37,6 +37,7 @@ private:
 			       unsigned long components, unsigned int size);
 
 	void setShort(ExifIfd ifd, ExifTag tag, uint16_t item);
+	void setSShort(ExifIfd ifd, ExifTag tag, int16_t item);
 	void setLong(ExifIfd ifd, ExifTag tag, uint32_t item);
 	void setString(ExifIfd ifd, ExifTag tag, ExifFormat format,
 		       const std::string &item);
-- 
2.25.1