Java Magazine - May/June 2012

//mobile and embedded /

entries, each with a structure that provides the data the directories hold. Finally, the 8 bytes at the end link to the next IFD, if there is one, or they contain all zeros, if there are no more IFDs. Typically, this would mark the end of the EXIF container as well.

Each directory entry follows the same structure. The first 4 bytes (TTTT) represent a tag number that is relevant to the type of IFD. Because we are concentrating on GPS IFDs, the first tag would be 0x0000, which represents the GPS version number. Thus, what we are saying is that this directory entry contains the GPS version number.

The next 4 bytes (FFFF) tell us the format of this data. There are 12 different types of data formats, from ASCII, to byte, to double float. Each type has its own number, for example, ASCII is represented by the number 2 and unsigned rational is represented by the number 5. Thus, if this were the directory structure for the GPS version number, these 4 bytes would contain the value 0x02 0x00.

The next 8 bytes are NNNNNNNN, and they tell us how many components are available for this value. Each data value might have multiple components that together make up the data. Because the GPS version number contains only a single component, we would just use the value 0x01 0x00

0x00 0x00.

Finally, the last 8 bytes (DDDDDDDD) are where the data is stored or they are the offset to where the data is stored. For the GPS version number, this is typically 0x32 0x32 0x00 0x00 (representing GPS version 2. 2.0.0).

LISTING 6 LISTING 7 LISTING 8 LISTING 9 LISTING 10

imageArray = videoControl.getSnapshot("encoding=jpeg"); // read it ByteArrayInputStream bis = new ByteArrayInputStream(imageArray); // for writing the modified array ByteArrayOutputStream bos = new ByteArrayOutputStream();

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Modifying the Image Data

In the prior sections, we gathered the location data and took a snapshot. We will gather this data in a byte array and proceed to modify it, as shown in Listing 6.

I haven’t shown it here, but we are doing this process (including the taking of the snapshot) in a separate thread so we don’t block the main application. (See the complete source code.)

Now we will go through this array one byte at a time and modify it accordingly, as shown in Listing 7.

The EXIF format uses the marker 0xE1. The image that we process using the Sun Java Wireless Toolkit has the marker 0xE0 to represent a pure JPEG image. We replace that with the E1 tag and then write the total size (which we calculate manually).

Then we write the familiar Exif string to identify that we are now writing an EXIF section. This is followed by the mandatory TIFF header. See Listing 8.

We then have the first IFD, the GPS IFD, as shown in Listing 9. Note that the GPS

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Geotagging is not enabled by default in GPS-enabled devices, so you need to write your own special application to add the metadata.

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IFD doesn’t contain the actual data, but an offset to the location of the actual GPS IFD structure, which starts with an indication of how many entries it contains (three: version, latitude, and longitude) followed by the first entry (the GPS version), as shown in Listing 10.