Ideally, you would have a writeable metadata format associated with all images in your database. Therefore, you might find a 20061015_1234.CRW along with a 20061015_1234.THM file in your image directory. The THM and RAW file must have exactly the same name, and any file operation (copy, move, delete) must be done to both in order to maintain the link. THM file is simply a small JPEG preview image that is used to house all of the writeable metadata content that would otherwise be written into the associated RAW file. THM files are the buddy-files associated with Canon RAW files.Ī. Since Canon (and other manufacturers) realize that a) they want to keep their RAW format secret and b) they must allow some means for people to annotate the metadata, the solution is to create a "buddy file" mechanism. Therefore, it is very difficult (or risky) for a third-party program to make modifications to these files, such as the addition of metadata content. CR2 in the case of Canon) are proprietary and not published. Unfortunately RAW file formats (e.g.CRW or. The specification of the JPEG (or THM) file format is openly published, and so inserting in additional metadata into a file is generally no problem. These details include titles, descriptions, keywords and many other fields (even GPS coordinates!). While most of this metadata is written by the camera itself, some items are done by the user by a catalog / photo browser program.
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In addition to the compressed image data, a JPEG file contains a series of markers that carry useful metadata - details about how the photo was taken, camera settings, etc. Similarly, renaming a *.THM file to *.JPG will allow you to double-click and open in a photo viewer. The THM file format is essentially identical to the JPEG File Format ( JFIF) - in fact, renaming a suitable (i.e. THM files are THuMbnails that are automatically generated alongside certain larger photo and video file types, such as Canon's RAW (.
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