Enhancing Libexif: Missing C Interfaces For Tag Name Extraction

Enhancing libexif: Missing C Interfaces for Tag Name Extraction

Hey there, fellow C developers and libexif enthusiasts! 👋 I've been diving deep into the world of libexif, the fantastic library for handling EXIF data, particularly in the context of developing a PostgreSQL extension. During my exploration, I've bumped into a few areas where the C interfaces could be even more streamlined, especially when it comes to extracting specific tag values directly. I'm excited to share my insights and open a discussion to see how we can collectively enhance this powerful library. This is the kind of stuff that gets me pumped, you know? Let's get into it!

Specifically, I've noticed a bit of a gap in how we can efficiently extract values using the unified tag names like GPSLatitude. The approach seems to involve several steps, as I've outlined below, which work, but could potentially be improved for speed and simplicity.

Here's the usual process for extracting a tag value:

1. ExifTag tag = exif_tag_from_name(tagname);
2. content = exifdata->ifd[???????]
3. ee = exif_content_get_entry(content, tag);
4. exif_entry_get_value(ee, buf0, sizeof(buf0));

The main challenge lies in step 2: figuring out the correct ifd (Image File Directory) index. For those unfamiliar, the ifd essentially organizes the EXIF data into different sections. To get the GPSLatitude, you need to know which ifd it resides in. Currently, there isn't a straightforward function within libexif to directly map a tag name to its corresponding ifd. This requires either manual lookup or a more involved process, like parsing the entire EXIF data to then extract the ifd info. This is a great example of a problem that is definitely worth taking the time to find a solution to.

Current Limitations and Proposed Solutions

As a PostgreSQL extension developer, I am trying to create functions that can extract specific EXIF tag values from images stored as bytea. My primary goal is to provide users with a fast and easy way to query and access EXIF metadata. The current library design requires a bit more work than I'd like. While I can extract values using the full JSON output, as shown in my test case, I am unable to make it faster, specifically:

--Testcase 20:
SELECT id,
       bytea_get_exif_json(img) ->> 'GPSLatitude' lat,
       bytea_get_exif_json(img) ->> 'GPSLongitude' lon
FROM img;

This works by first getting the full JSON output of all EXIF data, and then parsing the JSON to get the specific fields. This works, but it's not optimal for performance. Ideally, I'd like a function that directly extracts the value of a given tag, such as:

--Testcase 19:
SELECT id,
       bytea_get_exif_tag_value(img, 'GPSLatitude') lat,
       bytea_get_exif_tag_value(img, 'GPSLongitude') lon
FROM img;

Here, the function bytea_get_exif_tag_value would directly get the tag value without having to parse the full JSON. This kind of direct access could greatly enhance the speed and efficiency of my extension. However, with the current libexif interfaces, there's no easy way to implement this without first fetching the full JSON. This is where I feel there is an opportunity to simplify the library's API.

The ExifTagTable is a good resource, but it's not structured in a way that immediately tells me which ifd a tag belongs to. It seems to be a cartesian product of ifd and ExifDataType, and I'm not quite sure how to use it to get the ifd of a given tag. The question is: Can we add functionality to directly look up the ifd for a given tag name?

The Core Issue

The main missing piece seems to be a simple way to determine the ifd (Image File Directory) that a specific tag name belongs to. Without this information, implementing a function like bytea_get_exif_tag_value(img, 'GPSLongitude') directly, without parsing the full JSON output, becomes unnecessarily complex. The lack of this simple mapping prevents more efficient, partial data extraction and leads to using workarounds that can be slow.

For instance, knowing that GPSLatitude is typically found in the GPS IFD would allow a function to directly access that section of the EXIF data, which would be much faster than parsing the whole structure.

Potential Solutions and Discussion Points

1. Adding a Function to Map Tag Names to IFDs: The most straightforward solution would be to add a function like ifd_of_tag(char *tagname). This function would take a tag name as input (e.g., "GPSLatitude") and return the corresponding ifd index. This would significantly simplify the process of retrieving specific tag values.
2. Improving the ExifTagTable: Another approach could involve restructuring or augmenting the ExifTagTable to include ifd information directly. This might involve adding an ifd field to the ExifTag structure or providing a separate lookup table.
3. Providing Helper Functions: Perhaps, instead of a direct ifd_of_tag function, the library could offer helper functions that streamline the process of extracting values. These could combine steps 1-4 into a single function call, taking the tag name as input and returning the value directly.
4. Performance Considerations: I know that the performance impact of any change is important. We need to be careful to ensure that any new interfaces don't introduce performance bottlenecks.
5. Error Handling: We need to consider how to handle cases where a tag name is not found or is located in multiple IFDs. Clear error-handling mechanisms are essential.

Data Example & Real-World Impact

I've included a sample JSON output from my PostgreSQL extension to illustrate the kind of data we're dealing with. This data is less than 1.5 KB and shows the typical structure of EXIF data, including tags like GPSLatitude, GPSLongitude, and other metadata from my Nikon D90. This shows how the current extraction method works. The function works great, but it's not as efficient as it could be.

By improving the ability to extract specific EXIF data, we can unlock the following benefits:

• Faster Queries: By providing direct access to specific tag values, we can drastically speed up queries for EXIF data, especially for those interested in a few particular fields.
• Reduced Resource Usage: Parsing the entire EXIF data structure is resource-intensive. With the ability to extract only the necessary fields, we reduce memory consumption and CPU usage.
• Simplified Development: Simplifying the extraction process would make it easier for developers to integrate EXIF data into their applications. This would lead to increased adoption and more interesting use cases.

I’m super curious to hear your thoughts, guys! Are there any existing methods within libexif that I might have missed? Do you think these proposed changes would be beneficial? How can we best integrate these improvements while ensuring backward compatibility? I’m eager to learn from your expertise and collaborate on ways to make libexif even more awesome. Let's make this a great discussion, and let's build some cool stuff together! 🚀