Resizing Images in Rust, Now with Exif Orientation Support

I could also imagine that the earliest digital cameras wouldn't have had the processing power, or RAM to store the entire image in memory after reading it from the sensor, in order to do such a rotation. Hence EXIF rotation as a cheap alternative.

> why does EXIF transform metadata exist?

Because there are times when it is useful to be able to quickly and losslessly rotate a photo. For example an image viewer might provide quick options to rotate a photo (in case it did end up rotated the wrong way originally).

Probably this should just have been part of the actual image format rather than metadata, but that isn't how it played out.

> why doesn't the camera itself perform a simple rotation or mirror

There could be various reasons. The simplest is that adjusting the rotation naturally happens later in the pipeline than lossy encoding. You don't want to re-encode for quality and performance reasons so just slap on the metadata. The other could be that it is less expensive to avoid rotating. In many cameras the encoding is handled by specialized hardware, adding a way to rotate the raw pixels is likely more expensive than always having the source pixel map to the same input pixel in the encoder. There is no hard reason it couldn't be done, but in many situations it is cheaper to do it this way.

I can only imagine caring when it comes to streaming a very large image serially, and then I still have no idea what preference I would have (other than that I get the metadata first.)

As an analogy, if I have some irreplaceable video that has a bad aspect ratio, I fix it in the metadata, I don't transcode it. The data is a blob, the metadata is what tells me how to display it (even if it's just the mimetype.)

As far as cost:

Applying the right tag: 0

Anything else: not 0

Why not include a faster CPU or more RAM? In an industry where people pick one product over the other because of a $10 price difference, you’re never going to convince management to approve the better CPU when a tag solves the problem.

These are hardware companies. They look at software as a nuisance. “fix it in software” is for the downstream.

Or is there a benefit to generating the thumbnail in the original rotation then preserving the metadata?

For example, if you wanted to blur or grayscale an image, do you really want to rotate it? When you re-save it, you'll have to remove the exif rotation metadata too.

VipsImage *image = vips_image_new_from_file("something.jpg", "autorotate", TRUE, NULL);

and it'll flip it upright for you and remove any EXIF orientation tag. It isn't the default since it has (obviously) a large memory and cpu cost.

For simple resize and crop it's much better to use the exif tag to adjust the resize and crop parameters (the vipsthumbnail command does this).